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Classical conditioning

Classical conditioning (also respondent conditioning and Pavlovian conditioning) is a behavioral procedure in which a biologically potent stimulus (e.g. food, a puff of air on the eye, a potential rival) is paired with a neutral stimulus (e.g. the sound of a musical triangle). The term classical conditioning refers to the process of an automatic, conditioned response that is paired with a specific stimulus.

The Russian physiologist Ivan Pavlov studied classical conditioning with detailed experiments with dogs, and published the experimental results in 1897. In the study of digestion, Pavlov observed that the experimental dogs salivated when fed red meat. Pavlovian conditioning is distinct from operant conditioning (instrumental conditioning), through which the strength of a voluntary behavior is modified, either by reinforcement or by punishment. However, classical conditioning can affect operant conditioning; classically conditioned stimuli can reinforce operant responses.

Classical conditioning is a basic behavioural mechanism, and its neural substrates are now beginning to be understood. Though it is sometimes hard to distinguish classical conditioning from other forms of associative learning (e.g. instrumental learning and human associative memory), a number of observations differentiate them, especially the contingencies whereby learning occurs.

Together with operant conditioning, classical conditioning became the foundation of behaviorism, a school of psychology which was dominant in the mid-20th century and is still an important influence on the practice of psychological therapy and the study of animal behaviour. Classical conditioning has been applied in other areas as well. For example, it may affect the body's response to psychoactive drugs, the regulation of hunger, research on the neural basis of learning and memory, and in certain social phenomena such as the false consensus effect.

Definition
Classical conditioning occurs when a conditioned stimulus (CS) is paired with an unconditioned stimulus (US). Usually, the conditioned stimulus is a neutral stimulus (e.g., the sound of a tuning fork), the unconditioned stimulus is biologically potent (e.g., the taste of food) and the unconditioned response (UR) to the unconditioned stimulus is an unlearned reflex response (e.g., salivation). After pairing is repeated the organism exhibits a conditioned response (CR) to the conditioned stimulus when the conditioned stimulus is presented alone. (A conditioned response may occur after only one pairing.) Thus, unlike the UR, the CR is acquired through experience, and it is also less permanent than the UR.

Usually the conditioned response is similar to the unconditioned response, but sometimes it is quite different. For this and other reasons, most learning theorists suggest that the conditioned stimulus comes to signal or predict the unconditioned stimulus, and go on to analyse the consequences of this signal. Robert A. Rescorla provided a clear summary of this change in thinking, and its implications, in his 1988 article "Pavlovian conditioning: It's not what you think it is". Despite its widespread acceptance, Rescorla's thesis may not be defensible.

Classical conditioning differs from operant or instrumental conditioning: in classical conditioning, behaviors are modified through the association of stimuli as described above, whereas in operant conditioning behaviors are modified by the effect they produce (i.e., reward or punishment).

Pavlov's research
The best-known and most thorough early work on classical conditioning was done by Ivan Pavlov, although Edwin Twitmyer published some related findings a year earlier. During his research on the physiology of digestion in dogs, Pavlov developed a procedure that enabled him to study the digestive processes of animals over long periods of time. He redirected the animals' digestive fluids outside the body, where they could be measured.

Pavlov noticed that his dogs began to salivate in the presence of the technician who normally fed them, rather than simply salivating in the presence of food. Pavlov called the dogs' anticipatory salivation "psychic secretion". Putting these informal observations to an experimental test, Pavlov presented a stimulus (e.g. the sound of a metronome) and then gave the dog food; after a few repetitions, the dogs started to salivate in response to the stimulus. Pavlov concluded that if a particular stimulus in the dog's surroundings was present when the dog was given food then that stimulus could become associated with food and cause salivation on its own.

Terminology
In Pavlov's experiments the unconditioned stimulus (US) was the food because its effects did not depend on previous experience. The metronome's sound is originally a neutral stimulus (NS) because it does not elicit salivation in the dogs. After conditioning, the metronome's sound becomes the conditioned stimulus (CS) or conditional stimulus; because its effects depend on its association with food. Likewise, the responses of the dog follow the same conditioned-versus-unconditioned arrangement. The conditioned response (CR) is the response to the conditioned stimulus, whereas the unconditioned response (UR) corresponds to the unconditioned stimulus.

Pavlov reported many basic facts about conditioning; for example, he found that learning occurred most rapidly when the interval between the CS and the appearance of the US was relatively short.

As noted earlier, it is often thought that the conditioned response is a replica of the unconditioned response, but Pavlov noted that saliva produced by the CS differs in composition from that produced by the US. In fact, the CR may be any new response to the previously neutral CS that can be clearly linked to experience with the conditional relationship of CS and US. It was also thought that repeated pairings are necessary for conditioning to emerge, but many CRs can be learned with a single trial, especially in fear conditioning and taste aversion learning.

Forward conditioning
Learning is fastest in forward conditioning. During forward conditioning, the onset of the CS precedes the onset of the US in order to signal that the US will follow. Two common forms of forward conditioning are delay and trace conditioning.

 * Delay conditioning: In delay conditioning, the CS is presented and is overlapped by the presentation of the US. For example, if a person hears a buzzer for five seconds, during which time air is puffed into their eye, the person will blink. After several pairings of the buzzer and the puff, the person will blink at the sound of the buzzer alone. This is delay conditioning.
 * Trace conditioning: During trace conditioning, the CS and US do not overlap. Instead, the CS begins and ends before the US is presented. The stimulus-free period is called the trace interval or the conditioning interval. If in the above buzzer example, the puff came a second after the sound of the buzzer stopped, that would be trace conditioning, with a trace or conditioning interval of one second.

Simultaneous conditioning

During simultaneous conditioning, the CS and US are presented and terminated at the same time. For example: If a person hears a bell and has air puffed into their eye at the same time, and repeated pairings like this led to the person blinking when they hear the bell despite the puff of air being absent, this demonstrates that simultaneous conditioning has occurred.

Second-order and higher-order conditioning
Second-order or higher-order conditioning follow a two-step procedure. First a neutral stimulus ("CS1") comes to signal a US through forward conditioning. Then a second neutral stimulus ("CS2") is paired with the first (CS1) and comes to yield its own conditioned response. For example: A bell might be paired with food until the bell elicits salivation. If a light is then paired with the bell, then the light may come to elicit salivation as well. The bell is the CS1 and the food is the US. The light becomes the CS2 once it is paired with the CS1.

Backward conditioning
Backward conditioning occurs when a CS immediately follows a US. Unlike the usual conditioning procedure, in which the CS precedes the US, the conditioned response given to the CS tends to be inhibitory. This presumably happens because the CS serves as a signal that the US has ended, rather than as a signal that the US is about to appear. For example, a puff of air directed at a person's eye could be followed by the sound of a buzzer.

Temporal conditioning
In temporal conditioning, a US is presented at regular intervals, for instance every 10 minutes. Conditioning is said to have occurred when the CR tends to occur shortly before each US. This suggests that animals have a biological clock that can serve as a CS. This method has also been used to study timing ability in animals (see Animal cognition).

The example below shows the temporal conditioning, as US such as food to a hungry mouse is simply delivered on a regular time schedule such as every thirty seconds. After sufficient exposure the mouse will begin to salivate just before the food delivery. This then makes it temporal conditioning as it would appear that the mouse is conditioned to the passage of time.

Zero contingency procedure
In this procedure, the CS is paired with the US, but the US also occurs at other times. If this occurs, it is predicted that the US is likely to happen in the absence of the CS. In other words, the CS does not "predict" the US. In this case, conditioning fails and the CS does not come to elicit a CR. This finding – that prediction rather than CS-US pairing is the key to conditioning – greatly influenced subsequent conditioning research and theory.

Extinction
In the extinction procedure, the CS is presented repeatedly in the absence of a US. This is done after a CS has been conditioned by one of the methods above. When this is done, the CR frequency eventually returns to pre-training levels. However, extinction does not eliminate the effects of the prior conditioning. This is demonstrated by spontaneous recovery – when there is a sudden appearance of the (CR) after extinction occurs – and other related phenomena (see "Recovery from extinction" below). These phenomena can be explained by postulating accumulation of inhibition when a weak stimulus is presented.

Acquisition
During acquisition, the CS and US are paired as described above. The extent of conditioning may be tracked by test trials. In these test trials, the CS is presented alone and the CR is measured. A single CS-US pairing may suffice to yield a CR on a test, but usually a number of pairings are necessary and there is a gradual increase in the conditioned response to the CS. This repeated number of trials increase the strength and/or frequency of the CR gradually. The speed of conditioning depends on a number of factors, such as the nature and strength of both the CS and the US, previous experience and the animal's motivational state. The process slows down as it nears completion.

Extinction
If the CS is presented without the US, and this process is repeated often enough, the CS will eventually stop eliciting a CR. At this point the CR is said to be "extinguished."

External inhibition
External inhibition may be observed if a strong or unfamiliar stimulus is presented just before, or at the same time as, the CS. This causes a reduction in the conditioned response to the CS.

Recovery from extinction
Several procedures lead to the recovery of a CR that had been first conditioned and then extinguished. This illustrates that the extinction procedure does not eliminate the effect of conditioning. These procedures are the following:
 * Reacquisition: If the CS is again paired with the US, a CR is again acquired, but this second acquisition usually happens much faster than the first one.
 * Spontaneous recovery: Spontaneous recovery is defined as the reappearance of a previously extinguished conditioned response after a rest period. That is, if the CS is tested at a later time (for example an hour or a day) after extinction it will again elicit a CR. This renewed CR is usually much weaker than the CR observed prior to extinction.
 * Disinhibition: If the CS is tested just after extinction and an intense but associatively neutral stimulus has occurred, there may be a temporary recovery of the conditioned response to the CS.
 * Reinstatement: If the US used in conditioning is presented to a subject in the same place where conditioning and extinction occurred, but without the CS being present, the CS often elicits a response when it is tested later.
 * Renewal: Renewal is a reemergence of a conditioned response following extinction when an animal is returned to the environment (or similar environment) in which the conditioned response was acquired.

Stimulus generalization
Stimulus generalization is said to occur if, after a particular CS has come to elicit a CR, a similar test stimulus is found to elicit the same CR. Usually the more similar the test stimulus is to the CS the stronger the CR will be to the test stimulus. Conversely, the more the test stimulus differs from the CS, the weaker the CR will be, or the more it will differ from that previously observed.

Stimulus discrimination
One observes stimulus discrimination when one stimulus ("CS1") elicits one CR and another stimulus ("CS2") elicits either another CR or no CR at all. This can be brought about by, for example, pairing CS1 with an effective US and presenting CS2 with no US.

Latent inhibition
Latent inhibition refers to the observation that it takes longer for a familiar stimulus to become a CS than it does for a novel stimulus to become a CS, when the stimulus is paired with an effective US.

Conditioned suppression
This is one of the most common ways to measure the strength of learning in classical conditioning. A typical example of this procedure is as follows: a rat first learns to press a lever through operant conditioning. Then, in a series of trials, the rat is exposed to a CS, a light or a noise, followed by the US, a mild electric shock. An association between the CS and US develops, and the rat slows or stops its lever pressing when the CS comes on. The rate of pressing during the CS measures the strength of classical conditioning; that is, the slower the rat presses, the stronger the association of the CS and the US. (Slow pressing indicates a "fear" conditioned response, and it is an example of a conditioned emotional response; see section below.)

Conditioned inhibition
Typically, three phases of conditioning are used.

Phase 1
A CS (CS+) is paired with a US until asymptotic CR levels are reached.

Phase 2
CS+/US trials are continued, but these are interspersed with trials on which the CS+ is paired with a second CS, (the CS-) but not with the US (i.e. CS+/CS- trials). Typically, organisms show CRs on CS+/US trials, but stop responding on CS+/CS− trials.

Phase 3

 * Summation test for conditioned inhibition: The CS- from phase 2 is presented together with a new CS+ that was conditioned as in phase 1. Conditioned inhibition is found if the response is less to the CS+/CS- pair than it is to the CS+ alone.
 * Retardation test for conditioned inhibition: The CS- from phase 2 is paired with the US. If conditioned inhibition has occurred, the rate of acquisition to the previous CS− should be less than the rate of acquisition that would be found without the phase 2 treatment.

Blocking
This form of classical conditioning involves two phases.

Phase 1
A CS (CS1) is paired with a US.

Phase 2
A compound CS (CS1+CS2) is paired with a US.

Test
A separate test for each CS (CS1 and CS2) is performed. The blocking effect is observed in a lack of conditional response to CS2, suggesting that the first phase of training blocked the acquisition of the second CS.

Data sources
Experiments on theoretical issues in conditioning have mostly been done on vertebrates, especially rats and pigeons. However, conditioning has also been studied in invertebrates, and very important data on the neural basis of conditioning has come from experiments on the sea slug, Aplysia. Most relevant experiments have used the classical conditioning procedure, although instrumental (operant) conditioning experiments have also been used, and the strength of classical conditioning is often measured through its operant effects, as in conditioned suppression (see Phenomena section above) and autoshaping.

Stimulus-substitution theory
According to Pavlov, conditioning does not involve the acquisition of any new behavior, but rather the tendency to respond in old ways to new stimuli. Thus, he theorized that the CS merely substitutes for the US in evoking the reflex response. This explanation is called the stimulus-substitution theory of conditioning. A critical problem with the stimulus-substitution theory is that the CR and UR are not always the same. Pavlov himself observed that a dog's saliva produced as a CR differed in composition from that produced as a UR. The CR is sometimes even the opposite of the UR. For example: the unconditional response to electric shock is an increase in heart rate, whereas a CS that has been paired with the electric shock elicits a decrease in heart rate. (However, it has been proposed that only when the UR does not involve the central nervous system are the CR and the UR opposites.)

Rescorla–Wagner model
The Rescorla–Wagner (R–W) model is a relatively simple yet powerful model of conditioning. The model predicts a number of important phenomena, but it also fails in important ways, thus leading to a number of modifications and alternative models. However, because much of the theoretical research on conditioning in the past 40 years has been instigated by this model or reactions to it, the R–W model deserves a brief description here.

The Rescorla-Wagner model argues that there is a limit to the amount of conditioning that can occur in the pairing of two stimuli. One determinant of this limit is the nature of the US. For example: pairing a bell with a juicy steak is more likely to produce salivation than pairing the bell with a piece of dry bread, and dry bread is likely to work better than a piece of cardboard. A key idea behind the R–W model is that a CS signals or predicts the US. One might say that before conditioning, the subject is surprised by the US. However, after conditioning, the subject is no longer surprised, because the CS predicts the coming of the US. (The model can be described mathematically and that words like predict, surprise, and expect are only used to help explain the model.) Here the workings of the model are illustrated with brief accounts of acquisition, extinction, and blocking. The model also predicts a number of other phenomena, see main article on the model.

Equation
$$\Delta V=\alpha\beta (\lambda - \Sigma V)$$

This is the Rescorla-Wagner equation. It specifies the amount of learning that will occur on a single pairing of a conditioning stimulus (CS) with an unconditioned stimulus (US). The above equation is solved repeatedly to predict the course of learning over many such trials.

In this model, the degree of learning is measured by how well the CS predicts the US, which is given by the "associative strength" of the CS. In the equation, V represents the current associative strength of the CS, and ∆V is the change in this strength that happens on a given trial. ΣV is the sum of the strengths of all stimuli present in the situation. λ is the maximum associative strength that a given US will support; its value is usually set to 1 on trials when the US is present, and 0 when the US is absent. α and β are constants related to the salience of the CS and the speed of learning for a given US. How the equation predicts various experimental results is explained in following sections. For further details, see the main article on the model.

R–W model: acquisition
The R–W model measures conditioning by assigning an "associative strength" to the CS and other local stimuli. Before a CS is conditioned it has an associative strength of zero. Pairing the CS and the US causes a gradual increase in the associative strength of the CS. This increase is determined by the nature of the US (e.g. its intensity). The amount of learning that happens during any single CS-US pairing depends on the difference between the total associative strengths of CS and other stimuli present in the situation (ΣV in the equation), and a maximum set by the US (λ in the equation). On the first pairing of the CS and US, this difference is large and the associative strength of the CS takes a big step up. As CS-US pairings accumulate, the US becomes more predictable, and the increase in associative strength on each trial becomes smaller and smaller. Finally, the difference between the associative strength of the CS (plus any that may accrue to other stimuli) and the maximum strength reaches zero. That is, the US is fully predicted, the associative strength of the CS stops growing, and conditioning is complete.

R–W model: extinction
The associative process described by the R–W model also accounts for extinction (see "procedures" above). The extinction procedure starts with a positive associative strength of the CS, which means that the CS predicts that the US will occur. On an extinction trial the US fails to occur after the CS. As a result of this "surprising" outcome, the associative strength of the CS takes a step down. Extinction is complete when the strength of the CS reaches zero; no US is predicted, and no US occurs. However, if that same CS is presented without the US but accompanied by a well-established conditioned inhibitor (CI), that is, a stimulus that predicts the absence of a US (in R-W terms, a stimulus with a negative associate strength) then R-W predicts that the CS will not undergo extinction (its V will not decrease in size).

R–W model: blocking
The most important and novel contribution of the R–W model is its assumption that the conditioning of a CS depends not just on that CS alone, and its relationship to the US, but also on all other stimuli present in the conditioning situation. In particular, the model states that the US is predicted by the sum of the associative strengths of all stimuli present in the conditioning situation. Learning is controlled by the difference between this total associative strength and the strength supported by the US. When this sum of strengths reaches a maximum set by the US, conditioning ends as just described.

The R–W explanation of the blocking phenomenon illustrates one consequence of the assumption just stated. In blocking (see "phenomena" above), CS1 is paired with a US until conditioning is complete. Then on additional conditioning trials a second stimulus (CS2) appears together with CS1, and both are followed by the US. Finally CS2 is tested and shown to produce no response because learning about CS2 was "blocked" by the initial learning about CS1. The R–W model explains this by saying that after the initial conditioning, CS1 fully predicts the US. Since there is no difference between what is predicted and what happens, no new learning happens on the additional trials with CS1+CS2, hence CS2 later yields no response.

Theoretical issues and alternatives to the Rescorla–Wagner model
One of the main reasons for the importance of the R–W model is that it is relatively simple and makes clear predictions. Tests of these predictions have led to a number of important new findings and a considerably increased understanding of conditioning. Some new information has supported the theory, but much has not, and it is generally agreed that the theory is, at best, too simple. However, no single model seems to account for all the phenomena that experiments have produced. Following are brief summaries of some related theoretical issues.

Content of learning
The R–W model reduces conditioning to the association of a CS and US, and measures this with a single number, the associative strength of the CS. A number of experimental findings indicate that more is learned than this. Among these are two phenomena described earlier in this article
 * Latent inhibition: If a subject is repeatedly exposed to the CS before conditioning starts, then conditioning takes longer. The R–W model cannot explain this because preexposure leaves the strength of the CS unchanged at zero.
 * Recovery of responding after extinction: It appears that something remains after extinction has reduced associative strength to zero because several procedures cause responding to reappear without further conditioning.

Role of attention in learning
Latent inhibition might happen because a subject stops focusing on a CS that is seen frequently before it is paired with a US. In fact, changes in attention to the CS are at the heart of two prominent theories that try to cope with experimental results that give the R–W model difficulty. In one of these, proposed by Nicholas Mackintosh, the speed of conditioning depends on the amount of attention devoted to the CS, and this amount of attention depends in turn on how well the CS predicts the US. Pearce and Hall proposed a related model based on a different attentional principle Both models have been extensively tested, and neither explains all the experimental results. Consequently, various authors have attempted hybrid models that combine the two attentional processes. Pearce and Hall in 2010 integrated their attentional ideas and even suggested the possibility of incorporating the Rescorla-Wagner equation into an integrated model.

Context
As stated earlier, a key idea in conditioning is that the CS signals or predicts the US (see "zero contingency procedure" above). However, for example, the room in which conditioning takes place also "predicts" that the US may occur. Still, the room predicts with much less certainty than does the experimental CS itself, because the room is also there between experimental trials, when the US is absent. The role of such context is illustrated by the fact that the dogs in Pavlov's experiment would sometimes start salivating as they approached the experimental apparatus, before they saw or heard any CS. Such so-called "context" stimuli are always present, and their influence helps to account for some otherwise puzzling experimental findings. The associative strength of context stimuli can be entered into the Rescorla-Wagner equation, and they play an important role in the comparator and computational theories outlined below.

Comparator theory
To find out what has been learned, we must somehow measure behavior ("performance") in a test situation. However, as students know all too well, performance in a test situation is not always a good measure of what has been learned. As for conditioning, there is evidence that subjects in a blocking experiment do learn something about the "blocked" CS, but fail to show this learning because of the way that they are usually tested.

"Comparator" theories of conditioning are "performance based", that is, they stress what is going on at the time of the test. In particular, they look at all the stimuli that are present during testing and at how the associations acquired by these stimuli may interact. To oversimplify somewhat, comparator theories assume that during conditioning the subject acquires both CS-US and context-US associations. At the time of the test, these associations are compared, and a response to the CS occurs only if the CS-US association is stronger than the context-US association. After a CS and US are repeatedly paired in simple acquisition, the CS-US association is strong and the context-US association is relatively weak. This means that the CS elicits a strong CR. In "zero contingency" (see above), the conditioned response is weak or absent because the context-US association is about as strong as the CS-US association. Blocking and other more subtle phenomena can also be explained by comparator theories, though, again, they cannot explain everything.

Computational theory
An organism's need to predict future events is central to modern theories of conditioning. Most theories use associations between stimuli to take care of these predictions. For example: In the R–W model, the associative strength of a CS tells us how strongly that CS predicts a US. A different approach to prediction is suggested by models such as that proposed by Gallistel & Gibbon (2000, 2002). Here the response is not determined by associative strengths. Instead, the organism records the times of onset and offset of CSs and USs and uses these to calculate the probability that the US will follow the CS. A number of experiments have shown that humans and animals can learn to time events (see Animal cognition), and the Gallistel & Gibbon model yields very good quantitative fits to a variety of experimental data. However, recent studies have suggested that duration-based models cannot account for some empirical findings as well as associative models.

Element-based models
The Rescorla-Wagner model treats a stimulus as a single entity, and it represents the associative strength of a stimulus with one number, with no record of how that number was reached. As noted above, this makes it hard for the model to account for a number of experimental results. More flexibility is provided by assuming that a stimulus is internally represented by a collection of elements, each of which may change from one associative state to another. For example, the similarity of one stimulus to another may be represented by saying that the two stimuli share elements in common. These shared elements help to account for stimulus generalization and other phenomena that may depend upon generalization. Also, different elements within the same set may have different associations, and their activations and associations may change at different times and at different rates. This allows element-based models to handle some otherwise inexplicable results.

The SOP model
A prominent example of the element approach is the "SOP" model of Wagner. The model has been elaborated in various ways since its introduction, and it can now account in principle for a very wide variety of experimental findings. The model represents any given stimulus with a large collection of elements. The time of presentation of various stimuli, the state of their elements, and the interactions between the elements, all determine the course of associative processes and the behaviors observed during conditioning experiments.

The SOP account of simple conditioning exemplifies some essentials of the SOP model. To begin with, the model assumes that the CS and US are each represented by a large group of elements. Each of these stimulus elements can be in one of three states: Of the elements that represent a single stimulus at a given moment, some may be in state A1, some in state A2, and some in state I.
 * primary activity (A1) - Roughly speaking, the stimulus is "attended to." (References to "attention" are intended only to aid understanding and are not part of the model.)
 * secondary activity (A2) - The stimulus is "peripherally attended to."
 * inactive (I) – The stimulus is "not attended to."

When a stimulus first appears, some of its elements jump from inactivity I to primary activity A1. From the A1 state they gradually decay to A2, and finally back to I. Element activity can only change in this way; in particular, elements in A2 cannot go directly back to A1. If the elements of both the CS and the US are in the A1 state at the same time, an association is learned between the two stimuli. This means that if, at a later time, the CS is presented ahead of the US, and some CS elements enter A1, these elements will activate some US elements. However, US elements activated indirectly in this way only get boosted to the A2 state. (This can be thought of the CS arousing a memory of the US, which will not be as strong as the real thing.) With repeated CS-US trials, more and more elements are associated, and more and more US elements go to A2 when the CS comes on. This gradually leaves fewer and fewer US elements that can enter A1 when the US itself appears. In consequence, learning slows down and approaches a limit. One might say that the US is "fully predicted" or "not surprising" because almost all of its elements can only enter A2 when the CS comes on, leaving few to form new associations.

The model can explain the findings that are accounted for by the Rescorla-Wagner model and a number of additional findings as well. For example, unlike most other models, SOP takes time into account. The rise and decay of element activation enables the model to explain time-dependent effects such as the fact that conditioning is strongest when the CS comes just before the US, and that when the CS comes after the US ("backward conditioning") the result is often an inhibitory CS. Many other more subtle phenomena are explained as well.

A number of other powerful models have appeared in recent years which incorporate element representations. These often include the assumption that associations involve a network of connections between "nodes" that represent stimuli, responses, and perhaps one or more "hidden" layers of intermediate interconnections. Such models make contact with a current explosion of research on neural networks, artificial intelligence and machine learning.

Neural basis of learning and memory
Pavlov proposed that conditioning involved a connection between brain centers for conditioned and unconditioned stimuli. His physiological account of conditioning has been abandoned, but classical conditioning continues to be used to study the neural structures and functions that underlie learning and memory. Forms of classical conditioning that are used for this purpose include, among others, fear conditioning, eyeblink conditioning, and the foot contraction conditioning of Hermissenda crassicornis, a sea-slug. Both fear and eyeblink conditioning involve a neutral stimulus, frequently a tone, becoming paired with an unconditioned stimulus. In the case of eyeblink conditioning, the US is an air-puff, while in fear conditioning the US is threatening or aversive such as a foot shock.

The American neuroscientist David A. McCormick performed experiments that demonstrated "...discrete regions of the cerebellum and associated brainstem areas contain neurons that alter their activity during conditioning – these regions are critical for the acquisition and performance of this simple learning task. It appears that other regions of the brain, including the hippocampus, amygdala, and prefrontal cortex, contribute to the conditioning process, especially when the demands of the task get more complex."

Fear and eyeblink conditioning involve generally non overlapping neural circuitry, but share molecular mechanisms. Fear conditioning occurs in the basolateral amygdala, which receives glutaminergic input directly from thalamic afferents, as well as indirectly from prefrontal projections. The direct projections are sufficient for delay conditioning, but in the case of trace conditioning, where the CS needs to be internally represented despite a lack of external stimulus, indirect pathways are necessary. The anterior cingulate is one candidate for intermediate trace conditioning, but the hippocampus may also play a major role. Presynaptic activation of protein kinase A and postsynaptic activation of NMDA receptors and its signal transduction pathway are necessary for conditioning related plasticity. CREB is also necessary for conditioning related plasticity, and it may induce downstream synthesis of proteins necessary for this to occur. As NMDA receptors are only activated after an increase in presynaptic calcium(thereby releasing the Mg2+ block), they are a potential coincidence detector that could mediate spike timing dependent plasticity. STDP constrains LTP to situations where the CS predicts the US, and LTD to the reverse.

Behavioral therapies
Some therapies associated with classical conditioning are aversion therapy, systematic desensitization and flooding. Aversion therapy is a type of behavior therapy designed to make patients cease an undesirable habit by associating the habit with a strong unpleasant unconditioned stimulus. For example, a medication might be used to associate the taste of alcohol with stomach upset. Systematic desensitization is a treatment for phobias in which the patient is trained to relax while being exposed to progressively more anxiety-provoking stimuli (e.g. angry words). This is an example of counterconditioning, intended to associate the feared stimuli with a response (relaxation) that is incompatible with anxiety Flooding is a form of desensitization that attempts to eliminate phobias and anxieties by repeated exposure to highly distressing stimuli until the lack of reinforcement of the anxiety response causes its extinction. "Flooding" usually involves actual exposure to the stimuli, whereas the term "implosion" refers to imagined exposure, but the two terms are sometimes used synonymously.

Conditioning therapies usually take less time than humanistic therapies.

Conditioned drug response
A stimulus that is present when a drug is administered or consumed may eventually evoke a conditioned physiological response that mimics the effect of the drug. This is sometimes the case with caffeine; habitual coffee drinkers may find that the smell of coffee gives them a feeling of alertness. In other cases, the conditioned response is a compensatory reaction that tends to offset the effects of the drug. For example, if a drug causes the body to become less sensitive to pain, the compensatory conditioned reaction may be one that makes the user more sensitive to pain. This compensatory reaction may contribute to drug tolerance. If so, a drug user may increase the amount of drug consumed in order to feel its effects, and end up taking very large amounts of the drug. In this case a dangerous overdose reaction may occur if the CS happens to be absent, so that the conditioned compensatory effect fails to occur. For example, if the drug has always been administered in the same room, the stimuli provided by that room may produce a conditioned compensatory effect; then an overdose reaction may happen if the drug is administered in a different location where the conditioned stimuli are absent.

Conditioned hunger
Signals that consistently precede food intake can become conditioned stimuli for a set of bodily responses that prepares the body for food and digestion. These reflexive responses include the secretion of digestive juices into the stomach and the secretion of certain hormones into the blood stream, and they induce a state of hunger. An example of conditioned hunger is the "appetizer effect." Any signal that consistently precedes a meal, such as a clock indicating that it is time for dinner, can cause people to feel hungrier than before the signal. The lateral hypothalamus (LH) is involved in the initiation of eating. The nigrostriatal pathway, which includes the substantia nigra, the lateral hypothalamus, and the basal ganglia have been shown to be involved in hunger motivation.

Conditioned emotional response
The influence of classical conditioning can be seen in emotional responses such as phobia, disgust, nausea, anger, and sexual arousal. A familiar example is conditioned nausea, in which the CS is the sight or smell of a particular food that in the past has resulted in an unconditioned stomach upset. Similarly, when the CS is the sight of a dog and the US is the pain of being bitten, the result may be a conditioned fear of dogs. An example of conditioned emotional response is conditioned suppression.

As an adaptive mechanism, emotional conditioning helps shield an individual from harm or prepare it for important biological events such as sexual activity. Thus, a stimulus that has occurred before sexual interaction comes to cause sexual arousal, which prepares the individual for sexual contact. For example, sexual arousal has been conditioned in human subjects by pairing a stimulus like a picture of a jar of pennies with views of an erotic film clip. Similar experiments involving blue gourami fish and domesticated quail have shown that such conditioning can increase the number of offspring. These results suggest that conditioning techniques might help to increase fertility rates in infertile individuals and endangered species.

Pavlovian-instrumental transfer
Pavlovian-instrumental transfer is a phenomenon that occurs when a conditioned stimulus (CS, also known as a "cue") that has been associated with rewarding or aversive stimuli via classical conditioning alters motivational salience and operant behavior. In a typical experiment, a rat is presented with sound-food pairings (classical conditioning). Separately, the rat learns to press a lever to get food (operant conditioning). Test sessions now show that the rat presses the lever faster in the presence of the sound than in silence, although the sound has never been associated with lever pressing.

Pavlovian-instrumental transfer is suggested to play a role in the differential outcomes effect, a procedure which enhances operant discrimination by pairing stimuli with specific outcomes.

title: Classical conditioning


text:
Brecqhou

Brecqhou (or Brechou; ) is one of the Channel Islands, located off the west coast of Sark where they are now geographically detached from each other. Brecqhou is politically part of both Sark and the Bailiwick of Guernsey. It has been established in the courts that Brecqhou is a tenement of Sark. The Ministry of Justice, the department of the United Kingdom government with responsibility for the Channel Islands, considers Brecqhou part of Sark.

Name
The name Brecqhou derives from the Old Norse brekka (slope or escarpment; compare Bricquebec) and holmr (island or islet; see -hou). It was also formerly known as "Merchant's Island" (L'Isle aux Marchands). The spellings Brechou, Brehou, Brehoe appear on old maps.

Geography
A mere islet, Brecqhou has a surface area of just 74 acres. The island is separated from Sark by an extremely narrow sound (Le Goulliot Passage) which can be perilous for rowers. It is traversed frequently by yachts during each summer and by fishing boats year round and even forms a part of the route taken by occasional powerboating events in the islands.

Feudal relationship with Sark
In Sark, the word tenant is used, and often pronounced as in French, in the sense of feudal landholder rather than the common English meaning of lessee. The landholdings of Sark are held by 40 tenants representing the parcels of the 40 families who colonised Sark. As explained on the Sark government website: "There is no true freehold, all land being held on perpetual lease (fief) from the Seigneur, and the 40 properties (Tenements) into which the Island is divided (as well as a few other holdings in perpetual fief) can only pass by strict rules of inheritance or by sale." The relevance of the seigneurial privileges and duties that distinguish feudal from civil landowning has decreased as most of the duties relate to agriculture and defence.

Since 1929, the island has been connected to the title of the tenement La Moinerie de Haut, one of the 40 tenements whose owner had to keep a gun for the defence of the fief and, until forfeit in 2008, had a seat in the Chief Pleas. Originally, La Moinerie de Haut, named after the medieval monastery whose site is close to it, was a parcel of land in northwest Sark that was, at that time, owned by the Seigneur himself. When Sibyl Hathaway sold the island of Brecqhou to Angelo Clarke in 1929, she transferred that seat in the Chief Pleas to unrepresented Brecqhou. This was scant loss for her, as she owned more than one tenement and every member of the Chief Pleas was entitled to only one vote.

From 1993, the tenement of Brecqhou was owned by the Barclay brothers, the co-owners of The Daily Telegraph newspaper and former co-owners of The Scotsman. The brothers bought the island for £2.3 million in September 1993. Under the Reform (Sark) Law 1951, the tenant was David Barclay. After assuming ownership, the brothers had intermittent legal disputes with the government of Sark, and expressed a desire to make Brecqhou politically independent. They drove cars on the island and had a helicopter, both of which are banned under Sark law.

 * Customary and express written rule of Sark over Brecqhou may be evidenced in the retention of all seigneurial rights per the sale instrument from the Dame (female Seigneur) of Sark, Hathaway, to Clarke in 1929.
 * According to the Barclays this retention was invalid, as Brecqhou may not have been part of the fief of Sark. They cited facts such as that the letters patent establishing the fief do not mention the smaller island. While the Seigneur in long succession came to own Brecqhou (not before 1681), they cited practices and acts to suggest Brecqhou may not have been merged into the fief of Sark. Therefore the claim runs that the Seigneur could not legally hold the privileges valid elsewhere such as might survive sale, whatever its terms.

This conflict caused a lawsuit (1996–2000) and the founding in 2006 of a Brecqhou liaison sub-committee of Sark's Chief Pleas.

Sir David Barclay died in January 2021, leaving his brother as the sole tenant of Brecqhou.

Tenants

 * 1929–1932: Angelo Clarke
 * 1932–1944: Thomas Arthur Clarke (1871–1944; left the island 20 June 1940)
 * 1949–1966: John Thomson Donaldson
 * 1966–1987: Leonard Joseph Matchan
 * 1987–1992: Susan Groves (not recognised by Seigneur of Sark)
 * 1993–2021: David and Frederick Barclay
 * 2021–: Sir Frederick Barclay

Flag and stamps
The former tenant, Leonard Joseph Matchan, had devised a personal flag (identical to the Sark flag, with the exception that the Matchan coat of arms was emblazoned on the bottom right). Although frequently considered the island flag, this was only a personal flag, and is no longer in use.

The flag has since been adapted to equate with the current tenant, Sir Frederick Barclay. The current flag of Brecqhou remains in the same format as the previous flag, but replaces the Matchan coat of arms with the twins' coats of arms in the bottom right.

Leonard Joseph Matchan issued postage stamps (labelled "Brechou") on 30 September 1969, but they were suppressed the following day by the Guernsey Post Office when they took over responsibility from Royal Mail for the issue of stamps. Matchan occupied Brecqhou until his death on 6 October 1987. The current tenants issued stamps annually between 1999 and 2013.

Public visits
In 2012, it was reported that the island is open to the public, by prior arrangement.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: Brecqhou


text:
User:Sirius~enwiki

I'll write something less meaningless (or more meaningful) here as soon as I can think of it.

In the meantime, the only article I've started (I'm horribly new here) is Elf Power, which is at present pretty much a stub, however, I hope to extend it within about a week.

Please don't email me. (You can't, anyway, as the address I've supplied is no longer valid. Still, please, don't email me.)

title: User:Sirius~enwiki


text:
Biofuel

Biofuel is a fuel that is produced over a short time span from biomass, rather than by the very slow natural processes involved in the formation of fossil fuels such as oil. Biofuel can be produced from plants or from agricultural, domestic or industrial biowaste. Biofuels are mostly used for transportation, but can also be used for heating and electricity. Biofuels (and bioenergy in general) are regarded as a renewable energy source. The use of biofuel has been subject to criticism regarding the "food vs fuel" debate, varied assessments of their sustainability, and possible deforestation and biodiversity loss as a result of biofuel production.

In general, biofuels emit fewer greenhouse gas emissions when burned in an engine and are generally considered carbon-neutral fuels as the carbon emitted has been captured from the atmosphere by the crops used in production. However, life-cycle assessments of biofuels have shown large emissions associated with the potential land-use change required to produce additional biofuel feedstocks. The outcomes of lifecycle assessments (LCAs) for biofuels are highly situational and dependent on many factors including the type of feedstock, production routes, data variations, and methodological choices. This could be added to emphasize the complexity and variability in assessing the environmental impacts of biofuels. Estimates about the climate impact from biofuels vary widely based on the methodology and exact situation examined. Therefore, the climate change mitigation potential of biofuel varies considerably: in some scenarios emission levels are comparable to fossil fuels, and in other scenarios the biofuel emissions result in negative emissions.

Global demand for biofuels is predicted to increase by 56% over 2022-2027. By 2027 worldwide biofuel production is expected to supply 5.4% of the world's fuels for transport including 1% of aviation fuel. Demand for aviation biofuel is forecast to increase.

The two most common types of biofuel are bioethanol and biodiesel. Brazil is the largest producer of bioethanol, while the EU is the largest producer of biodiesel. The energy content in the global production of bioethanol and biodiesel is 2.2 and 1.8 EJ per year, respectively.

Bioethanol is an alcohol made by fermentation, mostly from carbohydrates produced in sugar or starch crops such as maize, sugarcane, or sweet sorghum. Cellulosic biomass, derived from non-food sources, such as trees and grasses, is also being developed as a feedstock for ethanol production. Ethanol can be used as a fuel for vehicles in its pure form (E100), but it is usually used as a gasoline additive to increase octane ratings and improve vehicle emissions.

Biodiesel is produced from oils or fats using transesterification. It can be used as a fuel for vehicles in its pure form (B100), but it is usually used as a diesel additive to reduce levels of particulates, carbon monoxide, and hydrocarbons from diesel-powered vehicles.

Terminology
The term biofuel is used in different ways. One definition is "Biofuels are biobased products, in solid, liquid, or gaseous forms. They are produced from crops or natural products, such as wood, or agricultural residues, such as molasses and bagasse."

Other publications reserve the term biofuel for liquid or gaseous fuels, used for transportation.

The IPCC Sixth Assessment Report defines biofuel as "A fuel, generally in liquid form, produced from biomass. Biofuels include bioethanol from sugarcane, sugar beet or maize, and biodiesel from canola or soybeans.". It goes on to define biomass in this context as "organic material excluding the material that is fossilised or embedded in geological formations". This means that coal or other fossil fuels is not a form of biomass in this context.

Conventional biofuels (first generation)
First-generation biofuels (also denoted as "conventional biofuels") are made from food crops grown on arable land. The crop's sugar, starch, or oil content is converted into biodiesel or ethanol, using transesterification, or yeast fermentation.

Advanced biofuels
To avoid a "food versus fuel" dilemma, second-generation biofuels and third-generation biofuels (also called advanced biofuels or sustainable biofuels or drop-in biofuels) are made from feedstocks which do not directly compete with food or feed crop such as waste products and energy crops. A wide range of waste feedstocks such as those derived from agriculture and forestry activities like rice straw, rice husk, wood chips, and sawdust can be used to produce advanced biofuels through biochemical and thermochemical processes.

The feedstock used to make the fuels either grow on arable land but are byproducts of the main crop, or they are grown on marginal land. Second-generation feedstocks also include straw, bagasse, perennial grasses, jatropha, waste vegetable oil, municipal solid waste and so forth.

Ethanol
Biologically produced alcohols, most commonly ethanol, and less commonly propanol and butanol, are produced by the action of microorganisms and enzymes through the fermentation of sugars or starches (easiest to produce) or cellulose (more difficult to produce).The IEA estimates that ethanol production used 20% of sugar supplies and 13% of corn supplies in 2021.

Ethanol fuel is the most common biofuel worldwide, particularly in Brazil. Alcohol fuels are produced by fermentation of sugars derived from wheat, corn, sugar beets, sugar cane, molasses and any sugar or starch from which alcoholic beverages such as whiskey, can be made (such as potato and fruit waste, etc.). Production methods used are enzyme digestion (to release sugars from stored starches), fermentation of the sugars, distillation and drying. The distillation process requires significant energy input to generate heat. Heat is sometimes generated with unsustainable natural gas fossil fuel, but cellulosic biomass such as bagasse is the most common fuel in Brazil, while pellets, wood chips and also waste heat are more common in Europe. Corn-to-ethanol and other food stocks has led to the development of cellulosic ethanol.

Other bioalcohols
Methanol is currently produced from natural gas, a non-renewable fossil fuel. In the future it is hoped to be produced from biomass as biomethanol. This is technically feasible, but the production is currently being postponed for concerns that the economic viability is still pending. The methanol economy is an alternative to the hydrogen economy to be contrasted with today's hydrogen production from natural gas.

Butanol is formed by ABE fermentation (acetone, butanol, ethanol) and experimental modifications of the process show potentially high net energy gains with biobutanol as the only liquid product. Biobutanol is often claimed to provide a direct replacement for gasoline, because it will produce more energy than ethanol and allegedly can be burned "straight" in existing gasoline engines (without modification to the engine or car), is less corrosive and less water-soluble than ethanol, and could be distributed via existing infrastructures. Escherichia coli strains have also been successfully engineered to produce butanol by modifying their amino acid metabolism. One drawback to butanol production in E. coli remains the high cost of nutrient rich media, however, recent work has demonstrated E. coli can produce butanol with minimal nutritional supplementation. Biobutanol is sometimes called biogasoline, which is incorrect as it is chemically different, being an alcohol and not a hydrocarbon like gasoline.

Biodiesel

Biodiesel is the most common biofuel in Europe. It is produced from oils or fats using transesterification and is a liquid similar in composition to fossil/mineral diesel. Chemically, it consists mostly of fatty acid methyl (or ethyl) esters (FAMEs). Feedstocks for biodiesel include animal fats, vegetable oils, soy, rapeseed, jatropha, mahua, mustard, flax, sunflower, palm oil, hemp, field pennycress, Pongamia pinnata and algae. Pure biodiesel (B100, also known as "neat" biodiesel) currently reduces emissions with up to 60% compared to diesel Second generation B100. , researchers at Australia's CSIRO have been studying safflower oil as an engine lubricant, and researchers at Montana State University's Advanced Fuels Center in the US have been studying the oil's performance in a large diesel engine, with results described as a "breakthrough".

Biodiesel can be used in any diesel engine and modified equipment when mixed with mineral diesel. It can also be used in its pure form (B100) in diesel engines, but some maintenance and performance problems may occur during wintertime utilization, since the fuel becomes somewhat more viscous at lower temperatures, depending on the feedstock used.

Electronically controlled 'common rail' and 'Unit Injector' type systems from the late 1990s onwards can only use biodiesel blended with conventional diesel fuel. These engines have finely metered and atomized multiple-stage injection systems that are very sensitive to the viscosity of the fuel. Many current-generation diesel engines are designed to run on B100 without altering the engine itself, although this depends on the fuel rail design. Since biodiesel is an effective solvent and cleans residues deposited by mineral diesel, engine filters may need to be replaced more often, as the biofuel dissolves old deposits in the fuel tank and pipes. It also effectively cleans the engine combustion chamber of carbon deposits, helping to maintain efficiency.

Biodiesel is an oxygenated fuel, meaning it contains a reduced amount of carbon and higher hydrogen and oxygen content than fossil diesel. This improves the combustion of biodiesel and reduces the particulate emissions from unburnt carbon. However, using pure biodiesel may increase NOx-emissions Biodiesel is also safe to handle and transport because it is non-toxic and biodegradable, and has a high flash point of about 300 °F (148 °C) compared to petroleum diesel fuel, which has a flash point of 125 °F (52 °C).

In many European countries, a 5% biodiesel blend is widely used and is available at thousands of gas stations. In France, biodiesel is incorporated at a rate of 8% in the fuel used by all French diesel vehicles. Avril Group produces under the brand Diester, a fifth of 11 million tons of biodiesel consumed annually by the European Union. It is the leading European producer of biodiesel.

Green diesel
Green diesel can be produced combination of biochemical and thermochemical processes. Conventional green diesel is produced through hydroprocessing biological oil feedstocks, such as vegetable oils and animal fats. Recently, it is produced using series of thermochemical processes such as pyrolysis and hydroprocessing. In the thermochemical route, syngas produced from gasification, bio-oil produced from pyrolysis or biocrude produced from hydrothermal liquefaction is upgraded to green diesel using hydroprocessing. Hydroprocessing is the process of using hydrogen to reform a molecular structure. For example, hydrocracking which is a widely used hydroprocessing technique in refineries is used at elevated temperatures and pressure in the presence of a catalyst to break down larger molecules, such as those found in vegetable oils, into shorter hydrocarbon chains used in diesel engines. Green diesel may also be called renewable diesel, drop-in biodiesel, hydrotreated vegetable oil (HVO fuel) or hydrogen-derived renewable diesel. Unlike biodiesel, green diesel has exactly the same chemical properties as petroleum-based diesel. It does not require new engines, pipelines or infrastructure to distribute and use, but has not been produced at a cost that is competitive with petroleum. Gasoline versions are also being developed. Green diesel is being developed in Louisiana and Singapore by ConocoPhillips, Neste Oil, Valero, Dynamic Fuels, and Honeywell UOP as well as Preem in Gothenburg, Sweden, creating what is known as Evolution Diesel.

Straight vegetable oil

Straight unmodified edible vegetable oil is generally not used as fuel, but lower-quality oil has been used for this purpose. Used vegetable oil is increasingly being processed into biodiesel, or (more rarely) cleaned of water and particulates and then used as a fuel. The IEA estimates that biodiesel production used 17% of global vegetable oil supplies in 2021.

Oils and fats reacted with 10 pounds of a short-chain alcohol (usually methanol) in the presence of a catalyst (usually sodium hydroxide [NaOH] can be hydrogenated to give a diesel substitute. The resulting product is a straight-chain hydrocarbon with a high cetane number, low in aromatics and sulfur and does not contain oxygen. Hydrogenated oils can be blended with diesel in all proportions. They have several advantages over biodiesel, including good performance at low temperatures, no storage stability problems and no susceptibility to microbial attack.

Biogasoline
Biogasoline can be produced biologically and themochemically. Using biologicalical methods, a study led by Professor Lee Sang-yup at the Korea Advanced Institute of Science and Technology (KAIST) and published in the international science journal Nature used modified E. coli fed with glucose found in plants or other non-food crops to produce biogasoline with the produced enzymes. The enzymes converted the sugar into fatty acids and then turned these into hydrocarbons that were chemically and structurally identical to those found in commercial gasoline fuel. The thermochemical approach of producing biogasoline are similar to those used to produced biodiesel. Biogasoline may also be called drop-in gasoline or renewable gasoline.

Bioethers
Bioethers (also referred to as fuel ethers or oxygenated fuels) are cost-effective compounds that act as octane rating enhancers. "Bioethers are produced by the reaction of reactive iso-olefins, such as iso-butylene, with bioethanol." Bioethers are created from wheat or sugar beets, and also be produced from the waste glycerol that results from the production of biodiesel. They also enhance engine performance, while significantly reducing engine wear and toxic exhaust emissions. By greatly reducing the amount of ground-level ozone emissions, they contribute to improved air quality.

In transportation fuel there are six ether additives: dimethyl ether (DME), diethyl ether (DEE), methyl tert-butyl ether (MTBE), ethyl tert-butyl ether (ETBE), tert-amyl methyl ether (TAME), and tert-amyl ethyl ether (TAEE).

The European Fuel Oxygenates Association identifies MTBE and ETBE as the most commonly used ethers in fuel to replace lead. Ethers were introduced in Europe in the 1970s to replace the highly toxic compound. Although Europeans still use bioether additives, the U.S. Energy Policy Act of 2005 lifted a requirement for reformulated gasoline to include an oxygenate, leading to less MTBE being added to fuel. Although bioethers are likely to replace ethers produced from petroleum in the UK, it is highly unlikely they will become a fuel in and of itself due to the low energy density.

Biogas and biomethane

Biogas is a mixture composed primarily of methane and carbon dioxide produced by the process of anaerobic digestion of organic material by micro-organisms. Other trace components of this mixture includes water vapor, hydrogen sulfide, siloxanes, hydrocarbons, ammonia, oxygen, carbon monoxide, and nitrogen. It can be produced either from biodegradable waste materials or by the use of energy crops fed into anaerobic digesters to supplement gas yields. The solid byproduct, digestate, can be used as a biofuel or a fertilizer. When and other impurities are removed from biogas, it is called biomethane. The can also be combined with hydrogen in methanation to form more methane.

Biogas can be recovered from mechanical biological treatment waste processing systems. Landfill gas, a less clean form of biogas, is produced in landfills through naturally occurring anaerobic digestion. If it escapes into the atmosphere, it acts as a greenhouse gas.

In Sweden "waste-to-energy" power plants capture methane biogas from garbage and use it power transport systems. Farmers can produce biogas from cattle manure via anaerobic digesters.

Syngas
Syngas, a mixture of carbon monoxide, hydrogen and various hydrocarbons, is produced by partial combustion of biomass (combustion with an amount of oxygen that is not sufficient to convert the biomass completely to carbon dioxide and water). Before partial combustion the biomass is dried and sometimes pyrolysed. Syngas is more efficient than direct combustion of the original biofuel; more of the energy contained in the fuel is extracted.

Syngas may be burned directly in internal combustion engines, turbines or high-temperature fuel cells. The wood gas generator, a wood-fueled gasification reactor, can be connected to an internal combustion engine.

Syngas can be used to produce methanol, dimethyl ether and hydrogen, or converted via the Fischer–Tropsch process to produce a diesel substitute, or a mixture of alcohols that can be blended into gasoline. Gasification normally relies on temperatures greater than 700 °C.

Lower-temperature gasification is desirable when co-producing biochar, but results in syngas polluted with tar.

Solid
The term "biofuels" is also used for solid fuels that are made from biomass, even though this is less common.

Algae-based biofuels
Algae can be produced in ponds or tanks on land, and out at sea. Algal fuels have high yields, a high ignition point, can be grown with minimal impact on fresh water resources, can be produced using saline water and wastewater, and are biodegradable and relatively harmless to the environment if spilled. However, production requires large amounts of energy and fertilizer, the produced fuel degrades faster than other biofuels, and it does not flow well in cold temperatures.

By 2017, due to economic considerations, most efforts to produce fuel from algae have been abandoned or changed to other applications.

Third and fourth-generation biofuels also include biofuels that are produced by bioengineered organisms i.e. algae and cyanobacteria. Algae and cyanobacteria will use water, carbon dioxide, and solar energy to produce biofuels. This method of biofuel production is still at the research level. The biofuels that are secreted by the bioengineered organisms are expected to have higher photon-to-fuel conversion efficiency, compared to older generations of biofuels. One of the advantages of this class of biofuels is that the cultivation of the organisms that produce the biofuels does not require the use of arable land. The disadvantages include the cost of cultivating the biofuel-producing organisms being very high.

Electrofuels and solar fuels
Electrofuels and solar fuels may or may not be biofuels, depending on whether they contain biological elements. Electrofuels are made by storing electrical energy in the chemical bonds of liquids and gases. The primary targets are butanol, biodiesel, and hydrogen, but include other alcohols and carbon-containing gases such as methane and butane. A solar fuel is a synthetic chemical fuel produced from solar energy. Light is converted to chemical energy, typically by reducing protons to hydrogen, or carbon dioxide to organic compounds.

Bio-digesters
A bio-digester is a mechanized toilet that uses decomposition and sedimentation to turn human waste into a renewable fuel called biogas. Biogas can be made from substances like agricultural waste and sewage. The bio-digester uses a process called anaerobic digestion to produce biogas. Anaerobic digestion uses a chemical process to break down organic matter with the use of microorganisms in the absence of oxygen to produce biogas. The processes involved in anaerobic respiration are hydrolysis, acidogenesis, acetogenesis, and methanogenesis.

Extent of production and use
Global biofuel production was 81 Mtoe in 2017 which represented an annual increase of about 3% compared to 2010. In 2017 the US was the largest biofuel producer in the world producing 37 Mtoe, followed by Brazil and South America at 23 Mtoe and Europe (mainly Germany) at 12 Mtoe.

An assessment from 2017 found that: "Biofuels will never be a major transport fuel as there is just not enough land in the world to grow plants to make biofuel for all vehicles. It can however, be part of an energy mix to take us into a future of renewable energy."

In 2021 worldwide biofuel production provided 4.3% of the world's fuels for transport, including a very small amount of aviation biofuel. By 2027 worldwide biofuel production is expected to supply 5.4% of the world's fuels for transport including 1% of aviation fuel.

The US, Europe, Brazil and Indonesia are driving the majority of biofuel consumption growth. This demand for biodiesel, renewable diesel and biojet fuel is projected to increase by 44% (21 billion litres) over 2022-2027.

Environmental impacts
Estimates about the climate impact from biofuels vary widely based on the methodology and exact situation examined.

In general, biofuels emit fewer greenhouse gas emissions when burned in an engine and are generally considered carbon-neutral fuels as the carbon they emit has been captured from the atmosphere by the crops used in biofuel production. They can have greenhouse gas emissions ranging from as low as -127.1 gCO2eq per MJ when carbon capture is incorporated into their production to those exceeding 95 gCO2eq per MJ when land-use change is significant. Several factors are responsible for the variation in emission numbers of biofuel such as feedstock and its origin, fuel production technique, system boundary definitions, and energy sources. However, many government policies such as those by European Union and the UK require that biofuels have at least 65% greenhouse gas emissions savings (or 70% if it is renewable fuels of non-biological origins) relative to fossil fuels.

Life-cycle assessments of first generation biofuels have shown large emissions associated with the potential land-use change required to produce additional biofuel feedstocks. If no land-use change is involved, first-generation biofuels can—on average—have lower emissions than fossil fuels. However, biofuel production can compete with foodcrop production. Up to 40% of corn produced in the United States is used to make ethanol and worldwide 10% of all grain is turned into biofuel. A 50% reduction in grain used for biofuels in the US and Europe would replace all of Ukraine's grain exports. Several studies have shown that reductions in emissions from biofuels are achieved at the expense of other impacts, such as acidification, eutrophication, water footprint and biodiversity loss.

The use of second generation biofuels is thought to increase environmental sustainability, since the non-food part of plants is being used to produce second-generation biofuels, instead of being disposed. But the use of second-generation biofuels increases the competition for lignocellulosic biomass, increasing the cost of these biofuels.

Third generation biofuels, produced from Algae, in theory shouldn't have as negative an impact on the environment than first or second generation biofuels, due to lower changes in land use and not requiring pesticide use for production. When looking at the data however, it has been shown that the environmental cost to produce the infrastructure and energy required for third generation biofuel production, is higher than the benefits provided from the biofuels use.

The European Commission has officially approved a measure to phase out palm oil-based biofuels by 2030. Unsustainable palm oil agriculture has caused significant environmental and social problems, including deforestation and pollution.

The production of biofuels can be very energy intensive, which if generated from non-renewable sources can heavily mitigate the benefits gained through biofuel use. A solution proposed to solve this issue is to supply biofuel production facilities with excess nuclear energy, which can supplement the energy provided by fossil fuels. This can provide a carbon inexpensive solution, to help reduce the environmental impacts of biofuel production.

title: Biofuel


text:
Talk:Diabetic ketoacidosis

Certified Diabetes Educator
Since when do people in need of medical assistance get told to call a certified diabetes educator instead of a physician, practitioner, or emergency response system? Makes me wonder if the person who wrote that section is a CDE. — Preceding unsigned comment added by 165.214.4.22 (talk) 19:57, 28 December 2012 (UTC)

 * Can't remember if I wrote that, but I would not object if a person with DM1 who is feeling unwell gets in touch with someone who can advise when to to got hospital. In the UK, diabetes specialist nurses are often the first port of call for patients. JFW | T@lk  00:22, 30 December 2012 (UTC)

DKA at presentation in kids
10.1136/bmj.d4092 is a systematic review as to why some kids get diagnosed and treated before DKA sets in. Might require citation. JFW | T@lk  21:52, 19 February 2013 (UTC)

Pathophysiology
The following was added:

Normally, the fatty acids are broken down into acetyl-Coa, which then combines with oxaloacetate in the Krebs Cycle. However, there is a paucity of oxaloacetate in the diabetic liver, as the oxaloacetate is consumed in gluconeogenesis. Therefore, two molecules of Acetyl-Coa combine to form ketone bodies.

This probably reflects the pathophysiology, but there is no source and I'm not sure if it is necessary for the flow of the text. Opinions welcome. JFW | T@lk  19:53, 1 July 2013 (UTC)

Doubt in Cause Section
The populations that are said to be more prone seems to be in US/American sense. I think that information was relevant to US demographics. Some one please look into this and clarify as I'm not an expert on this. Thanks Srikanth Aviator (talk) 15:22, 4 January 2014 (UTC)
 * I'm assuming that you are referring to type 1 diabetics when you say "The populations that are said to be more prone." Is my assumption correct? The reason that people with type 1 diabetes are more prone to DKA is because they have an absolute insulin deficiency, whereas those with type 2 do not. The insulin that type 2 diabetics are able to produce is usually sufficient to prevent them from going into DKA. In other words, it has nothing to do with nationality. It has to do with the different characteristics of the two diseases. --T1dme (talk) 20:09, 4 January 2014 (UTC)

 * Most of the studies and reports on ketosis-prone diabetes mellitus type 2 have been conducted in the USA. They show strong preponderance in African-Americans. Africans outside the USA seem to be affected also, see 10.1210/er.2007-0026.
 * What exactly is your question? JFW | T@lk  22:33, 4 January 2014 (UTC)

"Diabetic ketoacidosis may occur in those previously known to have diabetes mellitus type 2 or in those who on further investigations turn out to have features of type 2 diabetes (e.g. obesity, strong family history); this is more common in African, African-American and Hispanic people. Their condition is then labeled "ketosis-prone type 2 diabetes"." As the study was made in USA it will be more appropriate to say "this is more common in African, African-American and Hispanic people in USA" instead of "this is more common in African, African-American and Hispanic people". Right? Srikanth Aviator (talk) 16:16, 10 January 2014 (UTC)

 * Not sure. If you look at the Endocrine Reviews article I linked above, you will see that a fair number of reports came out of Africa. We'd be adding an untruth by specifying that it only affects people in the USA. JFW | T@lk  17:33, 12 January 2014 (UTC)

@JFW: Thanks for the Clarification Srikanth Aviator (talk) 18:41, 13 January 2014 (UTC)

Respiratory failure
... can occur as we already discuss. This review discusses it in more detail: 10.4239/wjd.v6.i8.1009 JFW | T@lk  06:58, 30 December 2015 (UTC)

DKA with normal glc
We should discuss Just had a case. Doc James (talk · contribs · email) 20:10, 31 January 2016 (UTC)

 * It's in ADA2009. I have added a pointer. JFW | T@lk  15:37, 4 February 2016 (UTC)

 * The link with SGLT2 inhibitors is discussed in this review 10.1210/jc.2015-1884 to which I currently have no access. JFW | T@lk  15:40, 4 February 2016 (UTC)
 * K will look. Doc James  (talk · contribs · email) 15:49, 4 February 2016 (UTC)

 * Added. JFW | T@lk

More sources
BMJ 10.1136/bmj.h5660 - revew in adults NICE DM1 in children and 10.1136/bmj.i139 BMJ summary

More to add, thus. JFW | T@lk  16:56, 10 February 2016 (UTC)

 * In fact, NEJM review should be added 10.1056/NEJMra1207788 JFW |  T@lk  12:15, 11 February 2016 (UTC)
 * Updated the lead based on the BMJ paper. Doc James  (talk · contribs · email) 05:35, 27 October 2016 (UTC)

Evidence is weak
10.3389/fendo.2017.00106 discusses the evidence for the current recommendations, and the fact that they are based on the tiniest of studies. To what extent should this be cited? JFW | T@lk  15:50, 16 August 2017 (UTC)

 * Another recent review: 10.1016/j.metabol.2015.12.007 JFW | T@lk  16:38, 16 August 2017 (UTC)

 * This editorial reviews weaknesses in the 2009 ADA position statement. More for reference than citation currently. 10.1016/S2213-8587(17)30093-1 JFW | T@lk  19:18, 15 August 2019 (UTC)

 * BMJ has done a "state of the art" 10.1136/bmj.l1114. Need to read, perhaps replacing some older sources.
 * This one is from Med Clin N Am but is free on PMC 10.1016/j.mcna.2016.12.011 JFW | T@lk  19:37, 15 August 2019 (UTC)

2021 update
I have just updated the references to the June 2021 update of the JBDS guideline. The ADA seems content to stick with the 12-year old guideline that we still reference. JBDS also addresses DKA in dialysis patients - I have not added this.

On scanning recent reviews, the following stand out:
 * 10.1016/j.ajem.2021.02.015 - review of euglycaemic DKA
 * 10.1016/j.numecd.2020.06.006 - Italian national guidelines for all age groups
 * 10.1038/s41572-020-0165-1 - broad review in Nat Rev Dis Primers
 * 10.1542/pir.2018-0231 - review in paediatrics
 * 10.1016/S2213-8587(20)30042-5 - prevention strategies
 * 10.1136/bmj.l1114 - BMJ state of the art review
 * 10.1080/17446651.2019.1561270 - approach to possible KPD in DKA
 * 10.1111/dme.13519 - role of deprivation (important systematic review)

Just in case I find myself with nothing to do for a few hours! JFW | T@lk  09:21, 10 August 2021 (UTC)

 * Also 10.1038/nrendo.2016.15 JFW | T@lk  20:18, 2 October 2021 (UTC)

JBDS update
10.1111/dme.14788 JFW | T@lk  22:56, 16 March 2022 (UTC)

Consensus statement
10.1007/s00125-024-06183-8 Online first. JFW | T@lk  11:04, 4 July 2024 (UTC)

title: Talk:Diabetic ketoacidosis