Jonathan Curot
email: jonathan.curot@cnrs.fr

MD, PhD Franck-Emmanuel Roux

MD, PhD Jean-Christophe Sol

MD Jéremie Pariente

MD, PhD Emmanuel J Barbeau

Awake craniotomy and memory induction through electrical stimulation: Why are Penfield's findings not replicated in the modern era?

Keywords: awake craniotomy, memory, electrical brain stimulation, reminiscences, epilepsy, déjà-vu

From the 1930s through the early 1960s, Wilder Penfield collected a large number of memories induced by electrical brain stimulation (EBS) during awake craniotomy. As a result, he was a major contributor to several neuroscientific and neuropsychological concepts of long-term memory. His 1963 paper, which recorded all the cases of memories he induced in his operating room, remains a substantial point of reference in neuroscience in 2019, although some of his interpretations are now debatable. However, it is highly surprising that since Penfield's reports there has been no other surgical publication on memories induced during awake surgery. In this review, we explore this phenomenon and analyze some of the reasons that might explain it. We hypothesize that the main reasons for lack of subsequent reports are related to changes in operative procedures (i.e., use of anesthetics, time constraints, insufficient debriefings) and changes in EBS parameters, rather than to the sites that are stimulated, the pathology treated, or the tasks used. If reminiscences are still induced, they should be reported in detail to add valuable contributions to the understanding of long-term memory networks, especially memories that are difficult to reproduce in the laboratory, such as autobiographical memories.

Chronic direct electrical brain stimulation (EBS) is regularly used in the form of deep brain stimulation (DBS) for the treatment of Parkinson's disease [START_REF] Benabid | Deep brain stimulation of the subthalamic nucleus for the treatment of Parkinson's disease[END_REF] and is being actively developed for the treatment of psychiatric disorders, 2 epilepsy, [START_REF] Li | Deep brain stimulation for drug-resistant epilepsy[END_REF] and cluster headache. [START_REF] Chabardès | Endoventricular Deep Brain Stimulation of the Third Ventricle: Proof of Concept and Application to Cluster Headache[END_REF] Its use is also being explored in neuroprosthetic applications for individuals who are blind or paralyzed. [START_REF] Collinger | High-performance neuroprosthetic control by an individual with tetraplegia[END_REF] We also now live in an era when the use of EBS for the chronic modulation of memory is conceivable.

Multiple recent findings indeed support the idea that EBS could potentially be used to improve human memory performance, [START_REF] Suthana | Memory Enhancement and Deep-Brain Stimulation of the Entorhinal Area[END_REF][START_REF] Titiz | Theta-burst microstimulation in the human entorhinal area improves memory specificity[END_REF][START_REF] Fell | Memory modulation by weak synchronous deep brain stimulation: A pilot study[END_REF][START_REF] Ezzyat | Direct Brain Stimulation Modulates Encoding States and Memory Performance in Humans[END_REF] increase hippocampal neurogenesis, [START_REF] Stone | Stimulation of Entorhinal Cortex Promotes Adult Neurogenesis and Facilitates Spatial Memory[END_REF] or even slow down atrophy in Alzheimer's disease. [START_REF] Sankar | Deep brain stimulation influences brain structure in Alzheimer's disease[END_REF] During the period from the 1930s to the early 1960s, Wilder Penfield and his colleagues became the first to induce experiential phenomena such as déjà-vu, but also the recollection of memories by EBS applied to the neocortex of patients during awake craniotomy. [START_REF] Penfield | part 4. The brain's record of auditory and visual experience 1[END_REF] They deeply influenced scientists' conceptions of long-term declarative memory. [START_REF] Curot | Memory scrutinized through electrical brain stimulation: A review of 80 years of experiential phenomena[END_REF] EBS induces a wide variety of memories, covering all aspects of declarative memory, although in different proportions (see Table 1 and online database DOI: 10.6084/m9.figshare.4733026). The "tape-recorder" hypothesis advocated by Penfield suggests that memory may be highly stable as time passes and it could be his most iconic contribution to the field of memory. [START_REF] Penfield | part 4. The brain's record of auditory and visual experience 1[END_REF] Although this concept is debatable nowadays, it is still a widely held belief. [START_REF] Curot | Memory scrutinized through electrical brain stimulation: A review of 80 years of experiential phenomena[END_REF] Table 1: Examples of the main different types of reminiscences induced by EBS during awake craniotomy, SEEG and electrocorticography (subdural grids). More reminiscences are available on the web. We provide a permalink to an online database, which is a census of all reminiscences induced by EBS ever published (DOI:10.6084/m9.figshare.4733026).

In the ensuing decades, induction of memories was replicated under conditions other than awake craniotomy -mostly by medial temporal lobe (MTL) intracranial EBS in patients with epilepsy [START_REF] Curot | Memory scrutinized through electrical brain stimulation: A review of 80 years of experiential phenomena[END_REF][START_REF] Gloor | Experiential Phenomena of Temporal Lobe Epilepsy[END_REF][START_REF] Vignal | The dreamy state: hallucinations of autobiographic memory evoked by temporal lobe stimulations and seizures[END_REF][START_REF] Halgren | Mental phenomena evoked by electrical stimulation of the human hippocampal formation and amygdala[END_REF] but also by fornix EBS in non-epileptic patients. [START_REF] Hamani | Memory enhancement induced by hypothalamic/fornix deep brain stimulation[END_REF][START_REF] Deeb | Fornix-Region Deep Brain Stimulation-Induced Memory Flashbacks in Alzheimer's Disease[END_REF] technique, these reminiscences became a valuable tool to explore long-term memory mechanisms. [START_REF] Curot | Memory scrutinized through electrical brain stimulation: A review of 80 years of experiential phenomena[END_REF][START_REF] Barbeau | Recollection of vivid memories after perirhinal region stimulations: synchronization in the theta range of spatially distributed brain areas[END_REF][START_REF] Jacobs | Explaining how brain stimulation can evoke memories[END_REF] They should be considered as rare exploitable "snapshots" of the brief fleeting reinstatement of a past event. [START_REF] Tulving | Ecphoric Processes in Episodic Memory [and Discussion[END_REF] By suggesting that memory retrieval relies on the widespread synchronization of nodes beyond the MTL [START_REF] Barbeau | Recollection of vivid memories after perirhinal region stimulations: synchronization in the theta range of spatially distributed brain areas[END_REF] , reminiscences induced by EBS also paved the way to the neuromodulation of these networks. In addition to many neuropsychological, electrophysiogical and functional MRI memory experiments, EBS studies contributed to identify the rhinal cortex, [START_REF] Suthana | Memory Enhancement and Deep-Brain Stimulation of the Entorhinal Area[END_REF][START_REF] Titiz | Theta-burst microstimulation in the human entorhinal area improves memory specificity[END_REF] the hippocampus, [START_REF] Suthana | Memory Enhancement and Deep-Brain Stimulation of the Entorhinal Area[END_REF][START_REF] Fell | Memory modulation by weak synchronous deep brain stimulation: A pilot study[END_REF][START_REF] Jacobs | Direct electrical stimulation of human entorhinal region and hippocampus impairs memory[END_REF] the amygdala, [START_REF] Inman | Direct electrical stimulation of the amygdala enhances declarative memory in humans[END_REF] the lateral temporal cortex, [START_REF] Jacobs | Direct electrical stimulation of human entorhinal region and hippocampus impairs memory[END_REF][START_REF] Ezzyat | Closed-loop stimulation of temporal cortex rescues functional networks and improves memory[END_REF] and the fornix, [START_REF] Laxton | A phase I trial of deep brain stimulation of memory circuits in Alzheimer's disease[END_REF] as potential targets to modulate memory using chronic EBS. For example, Andres Lozano's team unexpectedly induced personal semantic memories in an overweight patient following DBS targeting the hypothalamus. [START_REF] Hamani | Memory enhancement induced by hypothalamic/fornix deep brain stimulation[END_REF] The electrodes were in fact positioned very close to the fornix, one of the major white tracts involved in declarative memory. Subsequently, the same team led a large prospective trial to test DBS of the fornix in patients with Alzheimer's disease with the aim to limit memory impairment and even restore memory capacities. [START_REF] Laxton | A phase I trial of deep brain stimulation of memory circuits in Alzheimer's disease[END_REF] However, memories induced by EBS are very rare phenomena. Overall, 0.19% of stimulations induced reminiscences in our most recent analysis of patients undergoing stereoelectroencephalography (SEEG), [START_REF] Curot | Memory scrutinized through electrical brain stimulation: A review of 80 years of experiential phenomena[END_REF] which demonstrates that unfortunately, the optimal combination of parameters required to induce and then improve memory remains unknown.

Electrical parameters and electrode types vary across studies. [START_REF] Lee | Electrical engram: How deep brain stimulation affects memory[END_REF] The best electrical parameters have rarely been compared in humans. [START_REF] Fell | Memory modulation by weak synchronous deep brain stimulation: A pilot study[END_REF] Prospective trials that contrast the efficiency of a panel of EBS frequencies and intensities are needed.

Even the optimal brain structures that should be the target of EBS to improve memory are to a great extent unknown. In fact, stimulating the same structure can lead to contradictory results [START_REF] Suthana | Memory Enhancement and Deep-Brain Stimulation of the Entorhinal Area[END_REF][START_REF] Jacobs | Direct electrical stimulation of human entorhinal region and hippocampus impairs memory[END_REF] and, when effective, to only small improvement in cognitive scores. [START_REF] Fell | Memory modulation by weak synchronous deep brain stimulation: A pilot study[END_REF][START_REF] Koubeissi | Low-frequency electrical stimulation of a fiber tract in temporal lobe epilepsy[END_REF] Nonetheless, we previously demonstrated that the rhinal cortex was the main gateway to memory following EBS. [START_REF] Curot | Memory scrutinized through electrical brain stimulation: A review of 80 years of experiential phenomena[END_REF] In contrast, EBS of some other structures presumably involved in memory, such as the posterior cingulate cortex, [START_REF] Curot | Memory scrutinized through electrical brain stimulation: A review of 80 years of experiential phenomena[END_REF][START_REF] Foster | Direct cortical stimulation of human posteromedial cortex[END_REF] never induce memory retrieval or can even impair memory recall during free recall memory tasks. [START_REF] Natu | Stimulation of the posterior cingulate cortex impairs episodic memory encoding[END_REF] The choice of EBS locations would be all the more strategic since the EBS location influences the nature (e.g., episodic or semantic) of the memories that are recalled. [START_REF] Curot | Memory scrutinized through electrical brain stimulation: A review of 80 years of experiential phenomena[END_REF] Such an influence of the EBS location suggests that EBS can activate memory networks in a predictable manner.

In this context, studies reporting memories induced by EBS are a valuable basis for clinical trials aimed at modulating human memory networks to improve encoding or retrieval performance. [START_REF] Lee | Electrical engram: How deep brain stimulation affects memory[END_REF] Therefore, it would appear that awake craniotomy, according to Penfield's seminal studies, should be able to make a specific contribution to this endeavor. By adding data to the pool of extra-operative EBS, awake craniotomy could help to identify the optimal location(s) and electrical parameters for neuromodulation, while empiricism is currently the rule.

Paradoxically however, reports of memories induced by EBS during awake craniotomy have completely stopped since Penfield. In an extensive review of the literature on EBS, we found no report of electrically induced memories during awake craniotomy after Penfield. [START_REF] Curot | Memory scrutinized through electrical brain stimulation: A review of 80 years of experiential phenomena[END_REF] We did find a description of a patient who reported dream-like experiences after EBS of the posterior cingulate cortex, but these experiences were not clearly related to memory. [START_REF] Herbet | Disrupting posterior cingulate connectivity disconnects consciousness from the external environment[END_REF] It is particularly intriguing to note that Penfield's results have not been replicated, given that fundamentally, the goals and indications of awake craniotomy have not changed. Operating on awake patients is essential to delineate tumor resections, arteriovenous malformations, or seizure onset-zones in patients with drug-refractory epilepsy. [START_REF] Gopinath | Seizure outcome following primary motor cortex-sparing resective surgery for perirolandic focal cortical dysplasia[END_REF][START_REF] Kim | Awake craniotomy for brain tumors near eloquent cortex: Correlation of intraoperative cortical mapping with neurological outcomes in 309 consecutive patients[END_REF] Awake craniotomy is also performed for functional mapping of motor and/or sensory functions, 32 language, 33 visual functions, [START_REF] Wolfson | The role of awake craniotomy in reducing intraoperative visual field deficits during tumor surgery[END_REF] and verbal or visuospatial memory. [START_REF] Ojemann | Verbal memory deficits after left temporal lobectomy for epilepsy. Mechanism and intraoperative prediction[END_REF][START_REF] Holmes | Neuronal activity in human right lateral temporal cortex related to visuospatial memory and perception[END_REF] Several possibilities can be considered to explain why no memory induced by EBS during awake craniotomy has been reported these past five decades.

1-

Changes in the indications for intra-operative mapping? One could argue that nowadays most indications for awake craniotomy do not include epilepsy, as in Penfield's reports, but rather tumors. This could explain the lack of induced reminiscences, as some authors have argued that prior "sensitization" by seizure activity is necessary for the induction of memories by EBS (see Theodore Rasmussen's comment in the Discussion at the end of the 1966 paper by Chapman et al. [START_REF] Chapman | Memory changes induced by stimulation of hippocampus or amygdala in epilepsy patients with implanted electrodes[END_REF] ). However, reminiscences following EBS sometimes occur in patients undergoing tumor resection (even in Penfield's series, e.g., Patient J.T. [START_REF] Penfield | part 4. The brain's record of auditory and visual experience 1[END_REF] ). Recent reports of reminiscences induced by EBS (in settings other than awake craniotomy) are not limited to the epileptic population [START_REF] Curot | Memory scrutinized through electrical brain stimulation: A review of 80 years of experiential phenomena[END_REF] or to pathology with a direct impact on memory functions (e.g., obesity [START_REF] Hamani | Memory enhancement induced by hypothalamic/fornix deep brain stimulation[END_REF] ). Glial tumors are also often associated with epilepsy and a significant proportion of patients who benefit from awake craniotomy for glioma are probably epileptic, although this is not always mentioned in available studies.

2-An influence of the task and of baseline state of the brain at the moment of EBS?

One possibility could be that reminiscences are induced only when patients perform specific tasks and/or when EBS is applied during specific brain states. However, Penfield's patients performed no memory task during EBS sessions. In contrast, Ojemann's team, which used memory tasks (such as the presentation of words or visual items), to our knowledge never reported any recollection of memories induced by EBS. [START_REF] Ojemann | Verbal memory deficits after left temporal lobectomy for epilepsy. Mechanism and intraoperative prediction[END_REF] More complex tasks involving higher demands on memory networks might be more efficient to trigger conscious retrieval, [START_REF] Jacobs | Explaining how brain stimulation can evoke memories[END_REF] for example answering questions related to specific periods of life.

3-

Insufficient patient interviewing? Patients may not report what they experience if they are not specifically asked. Operating room conditions clearly do not favor the spontaneous verbalization of memories if the patient is not specifically questioned. Although the majority of patients do not experience major discomfort during current surgical procedures, which are well tolerated in the majority of cases, [START_REF] Whittle | Patient perceptions of "awake" brain tumour surgery[END_REF] it is nonetheless possible that Penfield was more committed to questioning patients because of his personal interest in these phenomena. The modern focus on efficiency could also be a factor. Penfield might have taken more time to talk to his patients during the surgical procedure, and in current practice operating time might be considered too limited (or too costly) to allow for the kind of interview that might elicit descriptions of EBSinduced reminiscences.

4-A role of afterdischarges (ADs)?

The influence of ADs (transient EEG changes after EBS, corresponding to bursts of epileptiform activity) is a historical debate as some authors have posited that they are necessary for reminiscences to occur (inhibition of MTL resulting in the "liberation" of neocortical structures), [START_REF] Halgren | Mental phenomena evoked by electrical stimulation of the human hippocampal formation and amygdala[END_REF] while others have suggested that they may have an inhibitory effect that prevents the occurrence of reminiscence after EBS. [START_REF] Vignal | The dreamy state: hallucinations of autobiographic memory evoked by temporal lobe stimulations and seizures[END_REF] Notwithstanding, it is difficult to interpret the effect of activation or inhibition of such a propagated and selfperpetuating epileptiform discharge. Moreover, most awake craniotomy procedures do not include concomitant electrocorticography to verify the occurrence of ADs. Early studies that reported EBS-induced reminiscences used stimulation parameters intense enough to induce an AD. [START_REF] Penfield | part 4. The brain's record of auditory and visual experience 1[END_REF] Penfield used to increase intensity to get ADs during awake craniotomy. However, besides the necessity to limit the risk of seizures during awake craniotomy, [START_REF] Nossek | Intraoperative seizures during awake craniotomy: Incidence and consequences: Analysis of 477 patients[END_REF] in previous work, we demonstrated that nearly one-half of reminiscences were devoid of any AD visible on EEG. [START_REF] Curot | Memory scrutinized through electrical brain stimulation: A review of 80 years of experiential phenomena[END_REF] Therefore, ADs do not appear to be necessary for reminiscences.

5-

Subtle changes in the locations of EBS?

One might wonder if the reason is not a progressive transition over the past decades towards a more focused mapping of areas highly 10 eloquent for language. A combination of the improvement in preoperative functional MRI mapping and a better knowledge of neuropsychological outcomes after surgery would explain why less time is spent to stimulate areas in which resection does not usually lead to permanent language impairment (such as anterior and ventral temporal areas). However, Penfield obtained memories mostly by lateral temporal EBS. Furthermore, lateral temporal EBS during electrocorticography also induces reminiscences. [START_REF] Moriarity | Human "memories" can be evoked by stimulation of the lateral temporal cortex after ipsilateral medial temporal lobe resection[END_REF] The targets of EBS during contemporary awake craniotomy are still primarily the same locations (Fig. 1). Therefore, the key is not in the location alone.

Fig. 1: Comparison of EBS locations that induced experiential responses (including memories) in Penfield's era (red circles) and locations stimulated during awake craniotomy in the modern era (yellow areas). The red circles show the main locations at which Penfield and his collaborators found that stimulation induced experiential responses during AC. The images represent results obtained in 40 patients (out of a pool of 1,132). This figure was

created on the basis of Penfield's results. [START_REF] Penfield | part 4. The brain's record of auditory and visual experience 1[END_REF] The areas in yellow present the data from the Toulouse awake craniotomy database. However, no reminiscence was reported by any patient in the Toulouse cases. More details about the exact locations among a total of 2,574 stimulations that were performed for functional mapping during awake craniotomy in 90 patients are available in the 2015 study by Roux et al. [START_REF] Holmes | Neuronal activity in human right lateral temporal cortex related to visuospatial memory and perception[END_REF]

6-

Differences in electrical parameters?

There have been significant changes in the electrical parameters used for EBS (pulse type, pulse width, frequency of stimulation, intensity and duration) since Penfield's work. Variations in these parameters may influence the amount of current delivered by EBS, its extension, and its direction. [START_REF] Nathan | Determination of current density distributions generated by electrical stimulation of the human cerebral cortex[END_REF] Penfield used different types of EBS, bipolar and monopolar (a monopolar silver ball electrode for surface exploration and a multiple-contact needle electrode for depth stimulation), and he applied the stimulation for longer periods of time than nowadays. [START_REF] Penfield | part 4. The brain's record of auditory and visual experience 1[END_REF] The amount of current delivered is difficult to quantify, but it is likely that it was higher than the amount that would be delivered with intracranial electrodes or even by EBS during awake craniotomy in the present era. However, parameters limiting current spread (bipolar EBS, low intensity) and high-frequency EBS are in fact more likely to induce memories during SEEG [START_REF] Curot | Memory scrutinized through electrical brain stimulation: A review of 80 years of experiential phenomena[END_REF][START_REF] Lee | Electrical engram: How deep brain stimulation affects memory[END_REF] (Fig. 2). One hypothesis is that a specific combination of electrical parameters for a specific location may be required to induce the recall of memories. The fact that the amount of current required to induce a clinical sign or an AD depends on the lobe stimulated [START_REF] Pouratian | Variability of intraoperative electrocortical stimulation mapping parameters across and within individuals[END_REF] and varies from point to point 43 supports this hypothesis.

Electrical parameters, especially intensity, should be individualized at each location. [START_REF] Roux | Variability of intraoperative electrostimulation parameters in conscious individuals: language cortex[END_REF] Overall, it is possible that modern parameters for EBS during awake craniotomy are inadequate to induce memory recall following neocortical EBS. increased by 1 mA (final amplitude 1.5-12 mA) and 3-4 seconds duration. [START_REF] Roux | Electrostimulation mapping of comprehension of auditory and visual words[END_REF]

7-

Changes in operating procedures? Penfield only used local anesthesia for the beginning of the surgical procedure and sedation only after testing. [START_REF] Bulsara | Improvements in brain tumor surgery: the modern history of awake craniotomies[END_REF] Nowadays, the most frequently reported anesthetic techniques are the asleep-awake-asleep technique and monitored anesthesia care. [START_REF] Stevanovic | Anaesthesia management for awake craniotomy: Systematic review and meta-analysis[END_REF] Sedation with short-acting drugs such as propofol (used since the 1990s) and remifentanil or midazolam, [START_REF] Stevanovic | Anaesthesia management for awake craniotomy: Systematic review and meta-analysis[END_REF] allow the anesthesiologist to closely control the patient's level of analgesia and sedation, whereas patients undergoing postoperative SEEG and electrocorticography are free of anesthetic agents during the immediate postoperative period and during postoperative monitoring. Propofol and midazolam are known to have amnestic effects. [START_REF] Veselis | Propofol and midazolam inhibit conscious memory processes very soon after encoding: An event-related potential study of familiarity and recollection in volunteers[END_REF] Although they are short-acting drugs, remnant effects cannot be prevented during the awake period. Since the advent of propofol, it has been reported that 8% to 37% of patients have no recollection of being awake and tested despite being awake and cooperative during the awake craniotomy procedure. [START_REF] Whittle | Patient perceptions of "awake" brain tumour surgery[END_REF] Therefore, we cannot exclude the possibility that conscious sedation with propofol may transiently alter the activation of memory-related structures (such as the hippocampus) and prevent the recall of memories after EBS.

8-

The need for intra-procedure intracranial EEG recordings? Anesthetic agents could also influence brain state, including the frequency and the phase of oscillations, at the moment stimulation is applied. This parameter is difficult to control and assess, but certainly important.

Spectral composition and the dynamics of brain oscillations in local field potentials just prior to stimulation and at the location of stimulation influence the effect of EBS. [START_REF] Lesser | Short-term variations in response distribution to cortical stimulation[END_REF] Dynamics in the theta band may especially influence memory induction, since theta synchrony between MTL and neocortex mediates recollection. [START_REF] Barbeau | Recollection of vivid memories after perirhinal region stimulations: synchronization in the theta range of spatially distributed brain areas[END_REF][START_REF] Fuentemilla | Theta oscillations orchestrate medial temporal lobe and neocortex in remembering autobiographical memories[END_REF] However, mimicking theta oscillatory patterns by the use of theta burst stimulation in the lateral neocortex [START_REF] Kim | Networkbased brain stimulation selectively impairs spatial retrieval[END_REF] and in the entorhinal cortex 7 led to contradictory results, respectively a reduction and an improvement in memory performances.

These uncertainties reinforce the need for more knowledge concerning EBS effects and changes in baseline brain state. Such studies would require EEG recordings in the operation room, which could be a constraint.

9-

Lack of interest or a publication bias? A simple explanation could be that memories are still induced by EBS during awake craniotomy but are not reported in publications because of a lack of interest in experiential phenomena. It is possible that in this era neurosurgeons are only focused on issues of language and not of memory. But this argument does not seem valid, as there is still an interest in spontaneous phenomena caused by EBS during awake craniotomy.

Different kinds of unexpected psychic phenomena such as out of body experiences, 51 a strong desire to move a limb, [START_REF] Desmurget | Movement intention after parietal cortex stimulation in humans[END_REF] and fear 53 are still reported. One possibility is a publication bias related to difficulties publishing single cases or the tendency to avoid publishing results that have already been reported.

Conclusions

Although historically they are major contributors to the understanding of long-term memory mechanisms, reminiscences induced by EBS have not been reported in the awake craniotomy literature of the past five decades. We are not able to give the exact reasons for this phenomenon. However, following our brief analysis above, we can hypothesize that the main reasons are more likely to be related to changes in operative procedures (use of anesthetic drugs, time constraints, and insufficient interviews) and in EBS parameters rather than to the current locations of EBS, pathology, or tasks.

If reminiscences are still induced during awake craniotomy, they should be reported in detail in order to add to the understanding of long-term memory networks, especially memories that are difficult to reproduce in a laboratory, such as autobiographical memories. [START_REF] Curot | Memory scrutinized through electrical brain stimulation: A review of 80 years of experiential phenomena[END_REF][START_REF] Barbeau | Recollection of vivid memories after perirhinal region stimulations: synchronization in the theta range of spatially distributed brain areas[END_REF][START_REF] Jacobs | Explaining how brain stimulation can evoke memories[END_REF] Such reminiscences are a complementary tool to rare experiments using naturalistic memory scenarios based on video-clips, movies or wearable cameras. [START_REF] Baldassano | Discovering Event Structure in Continuous Narrative Perception and Memory[END_REF][START_REF] Chen | Shared memories reveal shared structure in neural activity across individuals[END_REF][START_REF] Bird | Consolidation of Complex Events via Reinstatement in Posterior Cingulate Cortex[END_REF] Most available memory paradigms (cued words, targeted questions, presentation of visual items, spatial navigation tasks) are indeed insufficient to transcribe the rich experience and the ecological aspects of our autobiographical memories.

Therefore, here are our recommendations:

(1) Neurosurgeons should not hesitate to publish any case-report on experiential phenomena induced during awake craniotomy. A public repository would help to decrease publication biases. Each case provides further clues to determine the best EBS parameters for activating memory networks.

(2) A neurologist or psychologist in the operating room could help the patients to verbalize their subjective experience using appropriate and specific questions, especially when the temporal lobes are explored. Extensive reports should also be favored as they are the key to understanding the type of memory that was activated and the type of parameter that was used. [START_REF] Curot | Memory scrutinized through electrical brain stimulation: A review of 80 years of experiential phenomena[END_REF] (3) Awake mapping should be done as rigorously as possible within the limits of the operating constraints. In this context, setting a baseline state of patients' propensity to report subjective feelings during an operation would be helpful. This could be achieved through sham EBS applied randomly.

(4) Prospective trials comparing various EBS parameters in lateral versus MTL regions, coupled with intraoperative electrocorticography to track EEG oscillatory changes, should be designed.

We hope this review article will help neurosurgeons to share their experience.

Fig. 2 :

 2 Fig. 2: Electrical parameters for all types of EBS that induce memories: results from a review

types Examples in Penfield's reports 12 Examples obtained in reports after Penfield

  Regardless of the EBS

	Memory Episodic memory	Patient R.W.	Chapman et al. (1967). 37
	(vivid unique events,	(Right superior temporal gyrus EBS,	(Left amygdala EBS, SEEG).
	detailed emotional	awake craniotomy).	"He reported he was re-experiencing an epoch from his
	content, highly	"My mother is telling my brother he has got	own past. […] It concerned his entering a hospital in a
	contextualized in time	his coat on backwards. I can just hear	small city where he had lived seven or eight years
	and space)	them." When asked if he remembered it, he	previously. […] He stated that he could see the people
		said, "Oh yes, just before I came here."	and the places vividly and recall their names. The
		When asked if he thought these things were	episode concerned entering the hospital and greeting a
		like dreams, he said, "No." "	nurse. […] The visual aspect dominated his spontaneous
			recall but other features such as sounds smells and
			emotions were said to be present. On re-stimulation on
			the sites the same experience was induced. […]The
	Personal semantics	Patient C.M.
	(autobiographical	(Left middle temporal gyrus EBS, awake
	knowledge, without	craniotomy).
	any sense of	"She said, "I am having another one. I can't
	recollection or low	remember it. It seems like I was in my
	contextualization)	house." When asked where, she said "In the
		hallway, going towards my bedroom." "

interviewer asked for details and the patient replied that he would be able to furnish details if the stimulus was repeated again. When the stimulus began again, he directed his attention to the details, sought and was able to describe them. […] The patient said that the hospital and his contacts with doctor and nurse were extremely important events in his life, but that the specific epoch re-experienced did not stand out as particularly meaningful. […] He had completely forgotten the specific episode."

Curot et al. (2017). 13 Patient CGT2 (Right perirhinal cortex EBS, SEEG).

	"As if I had lived something. I don't know why but it
	brought me back to work. […] As if it was something I
	had lived earlier […] the feeling of having already lived
	that at work. I saw myself in the hall."
	Semantic memory
	(general knowledge
	about the world,
	culturally-shared)

Patient S. Be. (Right superior temporal gyrus EBS).

	"He observed, while the electrode was
	being held in place, "Again, 'Oh Marie, Oh
	Marie'." He explained that he had heard this
	before. "It is a theme song," he said, "on a
	radio program. The program is called the
	'Life of Luigi' ". The patient then discussed
	the identity of the song with Dr. Sears and
	he ended by singing the well-known refrain,
	"Oh Marie, Oh Marie." All in the operating
	room recognized the song."

Morris et al. (1984). 57 (Left superior temporal gyrus EBS, subdural grids).

	"At electrode C7, the patient recognized a familiar song
	he had not heard in many years. The singer was male
	with a British accent, and the verse went "Girl for
	making the morning brighter, girl for making the
	nighttime nicer". The patient could not remember the
	name of the song."
	Familiarity (item
	recognition, devoid of a
	sense of recollection
	and devoid of context)

Patient A. Bra. (Right superior temporal gyrus EBS, awake craniotomy).

	"Yes, I have heard it before. A man's voice
	talking."

Bartolomei et al. (2004). 58 Patient P3 (Left perirhinal cortex EBS, SEEG).

	"Patient P3 described the prow of a steamer on the sea
	associated with a transient feeling of familiarity."