Sotheara Leang

Eric Castelli

Dominique Vaufreydaz

Sethserey Sam

PRELIMINARY STUDY ON SSCF-DERIVED POLAR COORDINATE FOR ASR AUTHOR VERSION

Keywords: Automatic Speech Recognition, Spectral Subband Centroid Frequency, Speaker Normalization

The transition angles are defined to describe the vowelto-vowel transitions in the acoustic space of the Spectral Subband Centroids, and the findings show that they are similar among speakers and speaking rates. In this paper, we propose to investigate the usage of polar coordinates in favor of angles to describe a speech signal by characterizing its acoustic trajectory and using them in Automatic Speech Recognition. According to the experimental results evaluated on the BRAF100 dataset, the polar coordinates achieved significantly higher accuracy than the angles in the mixed and cross-gender speech recognitions, demonstrating that these representations are superior at defining the acoustic trajectory of the speech signal. Furthermore, the accuracy was significantly improved when they were utilized with their first and second-order derivatives (∆, ∆∆), especially in cross-female recognition. However, the results showed they were not much more gender-independent than the conventional Mel-frequency Cepstral Coefficients (MFCCs).

Introduction

Automatic Speech Recognition (ASR) plays a vital role in human-computer interactions, particularly in voice assistants. It allows such intelligent devices to translate a speech signal into textual information to obtain semantic comprehension before taking action. Due to the rapid growth of related disciplines, ASR systems have remarkably improved and are extensively used in several sectors, significantly improving work efficiency and reducing human demands.

Most state-of-the-art ASR systems based on Hidden Markov Model (HMM) or Deep Learning characterize the speech signal with acoustic features derived from the absolute frequency measurements such as Mel-frequency Cepstral Coefficients (MFCCs), Perceptual Linear Predictive Cepstrum (PLP) and Mel-filter Bank [START_REF] Suman K Saksamudre | A review on different approaches for speech recognition system[END_REF][START_REF] Gupta | The state of the art of feature extraction techniques in speech recognition[END_REF]. How-ever, it is well known that the frequency space of speakers varies greatly, especially if we compare that of men with that of women or children: women and children produce speech with higher frequencies than men [START_REF] Pooja V Janse | A comparative study between mfcc and dwt feature extraction technique[END_REF][START_REF] Ménard | Auditory normalization of french vowels synthesized by an articulatory model simulating growth from birth to adulthood[END_REF][START_REF] Potamianos | Robust recognition of children's speech[END_REF][START_REF] James | The role of f 0 and formant frequencies in distinguishing the voices of men and women[END_REF][START_REF] Carré | Speech: A dynamic process[END_REF]. Furthermore, if the physiological, emotional, or sociological factors (i.e., age, speaking rate, accent) are considered, the acoustic space varies significantly due to the inter-speaker and intra-speaker differences [START_REF] Benzeghiba | Automatic speech recognition and speech variability: A review[END_REF].

To take into account the large variability of speech (interspeaker variability, environmental variability, etc.), current ASR systems require a very large amount of data for their training. This is even more true in the case of systems based on Deep Learning. This is a major handicap for developing such systems for poorly endowed or endangered languages. It is, in fact, often quite challenging and very costly to develop an ASR system for minority or low-resource languages when a large dataset of the transcribed speech does not exist [START_REF] Besacier | Automatic speech recognition for under-resourced languages: A survey[END_REF][START_REF] Yu | Acoustic modeling based on deep learning for low-resource speech recognition: An overview[END_REF]. One research direction to reduce the training data size is to make the ASR systems intrinsically speaker-independent. If this is possible for the acoustic part of the systems, only one speaker would be sufficient for the learning phase. This research proposes to describe the speech signal based on "acoustic gestures", with the hypothesis that it is more robust and capable of improving the performance of ASR systems, particularly with respect to the natural variability of the speech. We suggest defining the acoustic gestures of the speech signal using polar coordinates rather than the transition angles on the acoustic space of the Spectral Subband Centroid Frequency (SSCF). We anticipate that the polar coordinates provide a better representation of the speech because they can define not only the transition direction but also the acoustic trajectory of the speech signal.

This paper is structured as follows: Section 2 includes related work. The proposed method is described in Section 3. The experiments are introduced in Section 4. Section 5 contains the results and discussions. Finally, section 6 presents the conclusion and future work.

Related Work

The Spectral Subband Centroids (SSCs) were examined and utilized as complement parameters to the cepstral features for speech recognition. They have properties similar to the formant frequencies and are robust to additive Gaussian noise [START_REF] Kuldip | Spectral subband centroid features for speech recognition[END_REF]. The SSC frequencies (SSCFs) were computed by dividing the frequency band into a number of subbands and computing the centroid of each subband using power spectrum of the speech signal (Equation 1).

SSCF m = hm lm f w m (f )P γ (f ) df hm lm w m (f )P γ (f ) df (1)
Where l m and h m are the lower and upper bounds of subband m; w m is the subband filter; P (f ) is the power spectrum at frequency bin f ; γ is a coefficient regulating dynamic range of the power spectrum.

The SSCFs were subsequently explored in Vietnamese vowel-to-vowel (VV) transitions. Six subband filters were proposed to capture more information about the speech signal by splitting the frequency band in mel-scale into the equals-size subbands. The study found that the SSCF parameters have similar shape to the formant frequencies and provide a good estimation and continuous values, even during the consonant production [START_REF] Tran | Acoustic gesture modeling. Application to a Vietnamese speech recognition system[END_REF].

The VV trajectories can be considered straight lines on the SSCF1-SSCF2 plane (which corresponds roughly to the F1-F2 plane) (see Figure 1). For each of these lines, it is then possible to define an angle with reference to the SSCF1 axis. For each pair of the transitions, V1V2 and V2V1, the sum of the two angles is equal to 180 • (see Figure 2). Starting from this observation, the angles were proposed to characterize the direction of the vocalic transition between the adjacency frames on the SSCF planes. The angles were measured by hand over 80% of the transition duration. According to the study, the angles were asserted to be speaker-independent because they were roughly the same across diverse speakers, including men and women, and similar results were obtained for the different speaking rates (see Figure 3) [START_REF] Tran | Acoustic gesture modeling. Application to a Vietnamese speech recognition system[END_REF].

Proposed Method

Previous work assumed that the spectral trajectories of vowel-to-vowel transitions are quasi-straight lines on SSCF1-SSCF2 plane. Thus it is possible to compute the angles to define the vocalic transitions [START_REF] Tran | Acoustic gesture modeling. Application to a Vietnamese speech recognition system[END_REF]. The angles are generalized to calculate across all planes of SSCF i -SSCF i+1 .

In each SSCF i -SSCF i+1 plane, the angle between a transition window of N frames are defined as in Equation 2. The SSCF angles of six vowel-to-vowel transitions on the SSCF1-SSCF2 plane produced by a Vietnamese male speaker (source [START_REF] Tran | Acoustic gesture modeling. Application to a Vietnamese speech recognition system[END_REF]).

Angle(j) = π 180 • arctan ∆SSCF i+1 ∆SSCF i (2)
Where j is a frame at index j; ∆SSCF i is the difference between SSCF on the ith axis at the end and the beginning of the transition.

The transition angles are computed directly from the arctan function, which has a discontinuity at -π and +π, resulting in the steps throughout the transition (see Figure 4). Therefore, obtaining the angle directly from the arctan function is not a continuous parameter. Our experiments showed that using only arctan angles to define the acoustic trajectory of the speech signal is probably not a very good idea because the jumps will produce a kind of "noise" in the data.

That is why we propose to characterize the spectral trajectory in the acoustic space of the SSCFs using the polar coordinates. We assert that they give a more accurate representation because they are continuous variables and can specify not only the transition direction but also the tra-2 Figure 3: The SSCF angles on the SSCF1-SSCF2 plane at fast and normal rates produced by a Vietnamese male and female (source [START_REF] Tran | Acoustic gesture modeling. Application to a Vietnamese speech recognition system[END_REF]). jectory of the spectrum. For a given frame, the two polar parameters (angle and radius) on the SSCF i-SSCF i+1 plane are defined as in Equation 3.

Angle(j) = 180 • π arctan SSCF i+1 SSCF i Radius(j) = SSCF 2 i+1 + SSCF 2 i ( 3
)
Where j represents a frame at index j; SSCF i and SSCF i+1 represent the axis of the SSCF plane, respectively.

Experiments

The study made use of the Kaldi toolkit [START_REF] Povey | The kaldi speech recognition toolkit[END_REF]. Note that the language model was trained using SRILM [START_REF]The sri language modeling toolkit[END_REF] on the transcripts of the speech corpus because we want to mea- sure the effect of the acoustic model, not the effect of outof-vocabulary (OOV) words that existed in the test sets.

The lexicon was constructed from the vocabulary of the language model using Phonetisaurus [START_REF]Phonetisaurus g2p[END_REF].

Datasets

The BRAF100 corpus [START_REF] Vaufreydaz | A new methodology for speech corpora definition from internet documents[END_REF] was used for French speech recognition. It contains around 28 hours of recordings from 100 native speakers (50 are men) aged from 15 to 63. Each speaker recorded between 102 and 105 different sentences and a common extract of "La Science et l'Hypothèse" of Henry Poincarée. The specification of the corpus is given in Table 1.

The corpus was prepared into three sets, each containing the train and test sets with comparable speaker sizes. The first set is for normal speech recognition, and the ASR model was trained and evaluated on the dataset of male and female speakers. The remaining two sets are for cross-gender speech recognition. The ASR model is trained on a male dataset and evaluated on a female dataset in cross-male recognition, whereas crossfemale recognition does the opposite. The specification of datasets are specified in the Table 2.

DNN-HMM Model

The ASR systems were constructed using a hybrid DNN-HMM model. The overall architecture of the DNN is given in Figure 5. The network takes nine consecutive contextual frames as the input and feeds them into three hidden layers, each consisting of 512 neurons, and the hyperbolic tangent function (tanh) was employed as an activation function. These hidden layers are used to learn the representation of the spliced input features. The output from the hidden layers was then fetched into a fully connected (FC) layer to estimate the state of the triphone HMM using the softmax function.

Feature Configuration

In this study, the angles and the polar coordinates were evaluated for characterizing the spectral trajectory of the speech signal on the SSCF planes and compared to the classical MFCC, the most widely used acoustic feature for speech recognition.

The angles were defined over a window size of one. The SSCFs were computed from short-time frames of 25 ms length with a 10 ms overlap, and the moving average was applied across a three-frame window to reduce the influence of the noise. A pre-emphasis factor of 0.97 and a Hamming window were utilized during feature extraction.

The MFCC with 6 and 13 dimensions were used as the baseline features. They were computed using 6 and 13 subband filters on the short-time frames with the same configuration as SSCFs, and a cepstral liftering coefficient of 22 was used while extracting the features.

Results and Discussions

The results in word error rate (WER) of normal and crossgender speech recognitions are presented in Table 3. In normal recognition cases, the MFCC significantly outperformed the angle and polar coordinate. As expected, the 13 MFCC produced a greater result than the 6 MFCC because it captured more signal information (higher precision in the frequency domain). In addition, the performance was marginally improved when speed and acceleration were used (∆, ∆∆).

The angle had the greatest error rate of 94%, demonstrating that it is not a good parameter due to its discontinuity. However, the estimation of transition trajectory using polar coordinate surpassed angle by a wide margin. Further-more, the performance was significantly improved when it was combined with speed and acceleration, particularly in cross-female recognition.

In the transgender recognition cases, similar patterns were obtained if the different types of parameters were compared. Nevertheless, all parameters had significantly higher error rates in all cases. It can be seen that the recognition rates for one gender are poorer if the learning is performed with speakers of the other gender. This shows that these parameters are not really independent of gender.

In cross-male recognition, the 13 MFCC performed worse than the 6 MFCC, and the situation was quite similar when used with delta features (∆, ∆∆). However, this was not the case for cross-female recognition.

Overall, we still find that recognition rates are similar between the polar parameters and the MFCC (particularly for the 6 MFCC) compared to the angles.

However, the polar parameters characterizing the spectral trajectories do not seem much more gender-independent than the classical MFCC. This may be due to several aspects that we have not yet investigated in this first study:

1) We considered SSCF0 (roughly corresponding to the fundamental frequency F0) participating in the same way as the other SSCFi similar to the formants. However, it is well known that the fundamental frequency F0 is quite different between men and women. Moreover, calculating a trajectory angle between SSCF0 and SSCF1 (F0 and F1) is probably not judicious because it does not correspond to real physical phenomena that SSCF0 (F0) characterizes the vibration of vocal cords, whereas the other SSCF characterize the resonances of the vocal tract.

2) A second point to investigate concerns the fact that characterizing the trajectories only by angles. If this is valid in the SSCF1-SSCF2 (F1-F2) plane where the trajectory during vowel-vowel transitions is indeed a quasistraight line, it is not the case for the other SSCF i-SSCF i+1 planes for i > 2. We must consider characterizing this trajectory by another means.

3) A dynamic gesture is classically characterized by its trajectory and speed (and acceleration) of the movement on the trajectory. It is therefore important to characterize velocity and acceleration more judiciously than simply computing the first and second derivatives (∆, ∆∆).

Conclusion

In this work, we investigated the use of polar coordinates in SSCF planes to describe the speech signal based on its acoustic trajectory. The finding showed that they significantly outperformed angles in both normal and crossgender recognitions, demonstrating that the polar representation provides better information in characterizing the acoustic elements of the speech signal.

Even though the polar coordinates produced more significant results, there is still room for improvement. Our experiments showed that they are not significantly more gender-independent than the classical MFCC. We propose several directions for investigation in order to better account for the dynamic aspects of speech production. Examining how the cochlear system works will provide us with more useful ideas for characterizing the time aspect. Furthermore, it is desirable to investigate how to efficiently manage SSCF0 because using it directly with higher SSCFs is irrelevant. Finally, the SSCF0 must be normalized to account for spectral variation between speakers, particularly between male and female voices.

Figure 1 :

 1 Figure 1: The spectral transition of /ai/ on the SSCF1-SSCF2 plane produced by a Vietnamese female speaker

Figure 4 :

 4 Figure 4: The transition angles of /ai/ on the SSCF1-SSCF2 plane produced by a Vietnamese female speaker. The steps caused by the arctan function are highlighted in red rectangular.

Figure 5 :

 5 Figure 5: Overview architecture of the DNN

Table 1 :

 1 Specification of BRAF100

	Corpus	Hour Transcript Speaker Vocab
	BRAF100	28h	10,564	100	22,199

Table 2 :

 2 The experimental datasets constructed from BRAF100

	Experiment	Train Set Hour Transcript	Speaker	Test Set Hour Transcript	Speaker
	Normal recognition	12h	4,657	22 males, 22 females	3h	1,036	5 males, 5 females
	Cross-male recognition	12h	4,661		44 males	1.5h	519	5 females
	Cross-female recognition	13h	5,278		44 females	1.5h	517	5 males
	Three training steps were conducted to produce the DNN-
	HMM model. First, a context-independent model (Mono-
	phone) was trained on the input features using 1K Gaus-
	sians over 40 iterations. Then, context-dependent (Tri-
	phone) training was performed over 35 iterations, utiliz-
	ing 2K states and 10K Gaussians. Finally, the triphone
	DNN training was conducted across 20 epochs on the
	aligned training data from the context-dependent model.
	The initial and final learning rates of 0.01 and 0.001 were
	used during the training.

Table 3 :

 3 Experimental results on BRAF100 in WER(%)

	Experiment	Parameter	Dimension MONO TRI DNN
		6 MFCC	6	62.00	31.24 10.90
		6 MFCC, ∆, ∆∆	18	26.70	11.20 08.65
		13 MFCC	13	41.90	17.09 08.39
	Normal recognition	13 MFCC, ∆, ∆∆	39	20.83	09.10 07.14
		Angle	5	97.12	94.02 94.19
		Polar	10	71.62	42.35 16.44
		Polar, ∆, ∆∆	30	46.17	21.10 13.01
		6 MFCC	6	67.41	39.58 16.51
		6 MFCC, ∆, ∆∆	18	32.19	14.84 11.51
		13 MFCC	13	62.65	41.04 20.58
	Cross-male recognition	13 MFCC, ∆, ∆∆	39	36.39	23.12 16.74
		Angle	5	96.87	97.29 94.60
		Polar	10	79.25	56.81 28.43
		Polar, ∆, ∆∆	30	57.85	33.86 21.51
		6 MFCC	6	73.71	51.54 20.44
		6 MFCC, ∆, ∆∆	18	35.63	17.59 15.30
		13 MFCC	13	66.96	43.92 18.62
	Cross-female recognition	13 MFCC, ∆, ∆∆	39	32.55	16.49 13.45
		Angle	5	93.78	94.15 93.51
		Polar	10	85.73	74.24 41.09
		Polar, ∆, ∆∆	30	64.31	48.24 29.20

Acknowledgment

This work was carried out as part of a PhD thesis funded by the French Embassy in Cambodia and the Cambodia Academy of Digital Technology.


Dr Karina J Linnell
email: k.j.linnell@gold.ac.uk

Serge Caparos

Urbanisation, the Arousal System, and Covert and Overt Attentional Selection

Keywords: information processing, urbanization, arousal, attention, selective attention, covert attention, overt attention, eye movements, locus-coeruleus, cross-cultural, culture, socio-ecological, environment

Urban exposure impacts the information processing style of human beings • This change in style is expressed both in terms of attentional engagement (the deployment of mental processing resources, or covert attentional selection) and of visual exploration (eye movements, or overt attentional selection)

• Both these effects may be underpinned by lifestyle-driven variations in arousalrelated processing

• Arousal variations may be linked to the neuromodulatory locus coeruleusnorepinephrine (LC-NE) system in the brain that is key to regulating cognitive function

Abstract

Urbanisation is growing rapidly. We review evidence that this growth is altering the default information processing style of human beings by impacting both overt and covert processes of attentional selection (i.e., attentional selection with and without eye movements respectively), in ways consistent with reduced attentional engagement and increased exploration. Whilst the factors and systems mediating these effects are likely to be many and various, we focus on one system which may be responsible for mediating effects on both covert and overt attentional selection. Specifically, the neuromodulatory locus coeruleus-norepinephrine (LC-NE) system is key to regulating cognitive function in a behaviourally-relevant and arousal-dependent manner and therefore well suited to supporting adaptation to the profound socio-ecological changes inherent in urbanisation.

Urbanisation, the Arousal System, and Covert and Overt Attentional Selection

Urbanisation is growing rapidly: as of 2017, 55% of the world's population lived in an urban environment, compared to just 33% in 1960 [1]. Here we review evidence that this spread of urbanisation is altering the default information processing style of human beings by impacting both covert and overt processes of attentional selection, processes that involve respectively the orienting of mental processing resources, or the 'mind's eye', versus physical eye-movements. Whilst the factors and systems mediating these effects of urbanisation are likely to be many and various, we focus on one system which may be responsible for mediating effects on both covert and overt attentional selection, processes often explained by very different systems. This system is the neuromodulatory locus coeruleus-norepinephrine (LC-NE) system that is key to "regulating cognitive function in a behaviourally-relevant and arousal-dependent manner" (p.16) [START_REF] Devilbiss | Consequences of tuning network function by tonic and phasic locus coeruleus output and stress: Regulating detection and discrimination of peripheral stimuli[END_REF] and therefore well suited to supporting adaptation to the profound socio-ecological changes inherent in urbanisation.

In support of this account, we review evidence on the impact of urbanisation on a variety of populations from around the world including not just the "WEIRD" (white, educated, industrialised, rich, and democratic) [START_REF] Henrich | The weirdest people in the world?[END_REF] and other high-income populations that are usually reported in the cognitive literature but also populations from low-and middle-income countries that today comprise the majority of the world's population [4].

Covert Attentional Selection

Much of the growing interest in the cognitive impact of urbanisation can be traced back to the pioneering work of Kaplan [START_REF] Kaplan | The restorative benefits of nature: toward an integrative framework[END_REF] on Attention Restoration Theory (ART). According to this theory, inhabitants of urban environments are exposed to a plethora of stimuli that compete for their attention, only some of which are relevant to behavioural goals; remaining focused on relevant information exhausts directed attention processes which can however be restored by exposure to less stimulating natural environments. Compatible with this theory, Berman, Jonides and Kaplan [START_REF] Berman | The cognitive benefits of interacting with nature[END_REF] showed that when American participants either walked in nature or were shown pictures of nature both their attentional control (indexed using a flanker selective-attention task) and working memory (WM; indexed using a backwards digit-span task) improved compared to when they walked in an urban area or were shown pictures of urban environments.

The present authors and their colleagues extended this work to the traditional Himba -a non-WEIRD population from a remote part of northern Namibia -and showed that, compared to urban participants in Namibia and/or in the UK, the traditional Himba showed better attentional control: they showed (i) no significant distraction from a motion singleton previously reported always to capture attention [START_REF] De Fockert | Reduced distractibility in a remote culture[END_REF] and (ii) focused spatial attention and reduced distractor interference on a flanker selection task of low perceptual load [START_REF] Linnell | Urbanization decreases attentional engagement[END_REF] which is widely assumed not to support attentional engagement and efficient selection [START_REF] Kahneman | Attention and Effort[END_REF][START_REF] Lavie | Distracted and confused? Selective attention under load[END_REF] (although see Linnell and Caparos [START_REF] Linnell | Perceptual load and early selection: An effect of attentional engagement?[END_REF]). These findings extended previous suggestions by Jung that remote peoples are generally better at concentrating [START_REF]Modern man in search of a soul[END_REF].

On the basis of these findings, Linnell et al [START_REF] Linnell | Urbanization decreases attentional engagement[END_REF] invoked the work of Aston-Jones on the dynamic control of attentional state [START_REF] Aston-Jones | An integrative theory of locus-coeruleus-norepinephrine function: adaptive gain and optimal performance[END_REF] to argue that natural environments -on the one hand -promote the adoption of engaged attentional states that facilitate attentional control and performance of the task at hand while urban environments -on the other hand -promote the adoption of explorative states that facilitate non-selective processing of the diverse stimulation and opportunities they afford.

Linnell, Caparos, and Davidoff [START_REF] Linnell | Urbanisation increases left-bias in line bisection: An expression of elevated levels of intrinsic alertness?[END_REF] drew still further on the work of Aston-Jones and colleagues by suggesting that this effect of urbanisation is mediated by the neuromodulatory LC-NE arousal system [START_REF] Nieuwenhuis | Decision making, the P3, and the locus coeruleus norepinephrine system[END_REF]. This system has been shown by Aston-Jones and colleagues to be key to modulating attentional state and cognitive function in keeping with behavioural demands and closely linked arousal-related processing [START_REF] Aston-Jones | An integrative theory of locus-coeruleus-norepinephrine function: adaptive gain and optimal performance[END_REF][START_REF] Aston-Jones | Role of locus-coeruleus in attention and behavioural flexibility[END_REF]. Specifically, the LC ensures that middling amounts of arousal are associated with attentional engagement, while elevated arousal is associated with exploration, by increasing its tonic activity with increasing arousal.

Linnell et al [START_REF] Linnell | Urbanization decreases attentional engagement[END_REF][START_REF] Linnell | Urbanisation increases left-bias in line bisection: An expression of elevated levels of intrinsic alertness?[END_REF] suggested that urbanisation provokes a shift in the LC-NE arousal system towards increased exploration and decreased engagement. This suggestion is in keeping with the association between urbanisation and stress [START_REF] Lederbogen | City living and urban upbringing affect neural social stress processing in humans[END_REF][START_REF] Peen | The current status of urban-rural differences in psychiatric disorders[END_REF] (for a review of the links between stress and LC activation, see Aston-Jones, Gonzalez, & Doran [START_REF] Aston-Jones | Role of the locus-coeruleus-norepinephrine system in arousal and circadian regulation of the sleep-wake cycle[END_REF]). It is also in keeping with Broadbent's [START_REF] Broadbent | Environment and performance[END_REF] suggestion that urban populations express higher levels of arousal and converging evidence for such differences between the traditional Himba and urban UK controls [START_REF] Linnell | Urbanisation increases left-bias in line bisection: An expression of elevated levels of intrinsic alertness?[END_REF]. Such population differences open up the possibility that a widely reported (leftwards) spatial bias (pseudo-neglect [START_REF] Jewell | Pseudoneglect: a review and meta-analysis of performance factors in line bisection tasks[END_REF][START_REF] Manly | Rightward shift in spatial awareness with declining alertness[END_REF]) -that has long been assumed to represent normal human functioning -may be the product of hyper-arousal in urban populations [START_REF] Linnell | Urbanisation increases left-bias in line bisection: An expression of elevated levels of intrinsic alertness?[END_REF].

Urbanisation has also been shown to impact the anterior cingulate cortex and amygdala [START_REF] Lederbogen | City living and urban upbringing affect neural social stress processing in humans[END_REF] which may, we suggest here, result in more complex, regulatory effects on the LC-NE arousal system. Once this possibility is taken into account our LC-NE arousal account may be able to subsume the directed-attention/WM capacity account of urbanisation [START_REF] Berman | The cognitive benefits of interacting with nature[END_REF]: recent work by Unsworth and Robison [START_REF] Unsworth | A locus coeruleus-norepinephrine account of individual differences in working memory capacity and attention control[END_REF][START_REF] Unsworth | The importance of arousal for variation in working memory capacity and attention control: A latent variable pupillometry study[END_REF][START_REF] Robison | Pupillometry tracks fluctuations in working memory performance[END_REF] suggests that the well-documented relation between attentional control and WM is partially underpinned by individual variation in the efficiency with which LC-NE functioning is regulated, so that if urbanisation impacts the efficiency with which the LC-NE system is regulated it will also affect attentional control and WM capacity as documented by Berman and colleagues.

Beyond covert selection, our LC-NE arousal account can also encompass effects of urbanisation on overt selection. We provide converging evidence for this in the next section.

It is worth noting at this juncture that effects on covert and overt selection are not generally studied in a common explanatory framework (although see Alotaibi, Underwood & Smith [START_REF] Alotaibi | Cultural differences in attention: Eye movement evidence from a comparative visual search task[END_REF]).

Overt Attentional Selection

We argue here for the first time that work by Aston-Jones and colleagues on the LC-NE arousal system is compatible with arousal-related processing mediating the impact of urbanisation not just on the balance between exploratory and task-engaged styles in covert selection (as discussed in the previous section) but also in overt selection or eye-movement looking patterns.

According to Aston-Jones, Rajkowski, and Cohen [START_REF] Aston-Jones | Role of locus-coeruleus in attention and behavioural flexibility[END_REF], there is a close relationship between LC activity and eye movement looking patterns: "Decreased steady foveation [i.e., a decreased tendency to keep the eyes still at the onset of each trial], and increased 'scanning' eye movements, were associated with epochs of elevated LC activity [and], conversely, consistent visual fixation occurred during periods of intermediate LC discharge." [START_REF] Rajkowski | Locus coeruleus activity in monkey: Phasic and tonic changes are associated with altered vigilance[END_REF] (p.1312). Others too have suggested that more exploratory looking patterns accompany higher levels of arousal/LC activity [START_REF] De Barbaro | Micro-analysis of infant looking in a naturalistic social setting: insights from biologically based models of attention[END_REF][START_REF] Pajkossy | Tonic noradrenergic activity modulates explorative behavior and attentional set shifting: evidence from pupillometry and gaze pattern analysis[END_REF]. If in addition urbanisation promotes hyperarousal, as suggested in the previous section, it is then a small step to suggest that it will make looking patterns -in terms of fixation durations and/or saccade lengths -more exploratory and, indeed, recent work from 2018 is compatible with this suggestion. Specifically, Köster, Itakura, Yovsi, and Kärtner [START_REF] Köster | Visual attention in 5-year-olds from three different cultures[END_REF] compared looking patterns between 5year-olds from rural populations in Cameroon (Africa) and from urban populations in Germany and Japan, during free viewing of pictures of figural objects. They showed that the rural Cameroonians looked significantly longer at figural objects than their urban counterparts, whose eye movements were more wide-ranging, encompassing more of the time the background or context against which the figural object was presented.

Particularly noteworthy was how much larger the differences in looking patterns were between the rural and urban populations, as compared to the differences between the two urban populations, the Germans and Japanese. The cross-cultural literature has explained differences in looking patterns between Western and East-Asian populations [START_REF] Chua | Cultural variation in eye movements during scene perception[END_REF][START_REF] Masuda | When does the culturally dominant mode of attention appear or disappear? Comparing patterns of eye movement during the visual flicker task between European Canadians and Japanese[END_REF] using a social-organisation account [START_REF] Nisbett | Culture and systems of thought: Holistic versus analytic cognition[END_REF][START_REF] Nisbett | Culture and point of view[END_REF]. According to this account, individualist versus collectivist cultures engender information processing styles that are respectively analytical versus holistic, promoting the selection of respectively figural versus background information (although see Rayner and colleagues [START_REF] Rayner | Eye movements during information processing tasks: Individual differences and cultural effects[END_REF]).

Yet, despite the social organisation of rural Cameroonians being more collectivist than that of either of the urban samples studied, the rural group was actually the one that showed the most focused looking-patterns. Thus, while social organisation is undoubtedly an important driver of looking patterns, the work of Köster et al [START_REF] Köster | Visual attention in 5-year-olds from three different cultures[END_REF] suggests that urbanisation can be a more important one. What we propose here is that this impact of urbanisation on overt selection may be at least partially mediated by the LC-NE arousal system, such that urbanisation promotes more exploratory looking behaviours.

The rural-urban differences in exploratory looking behaviours that are predicted by our LC-NE arousal-based account may explain rural-urban differences in susceptibility to the Ebbinghaus illusion: specifically, reduced susceptibility to this contextual size illusion has been reported in rural Cameroonians compared to both German and Japanese urban groups [START_REF] Köster | Visual attention in 5-year-olds from three different cultures[END_REF] and this finding replicates previous findings of reduced susceptibility in the traditional Himba compared to urban groups from the UK and Japan [START_REF] Bremner | The ontogeny of visual context effects: Tracing effects of culture and the urban environment on the development of the Ebbinghaus illusion in British and Namibian observers[END_REF][START_REF] Caparos | Exposure to an urban environment alters the local bias of a remote culture[END_REF].

Reduced susceptibility to illusions such as the Ebbinghaus -which depend for their existence on the processing of irrelevant contextual information -is expected to go hand-inhand with looking patterns that are focused on figural objects [START_REF] Köster | Visual attention in 5-year-olds from three different cultures[END_REF]. An arousal-based account, along the lines that the rural Cameroonians and remote Namibians are not hyperaroused and make fewer (exploratory) eye movements, can potentially explain this finding given the link that exists between eye movements and illusion size: Mruczek and colleagues [START_REF] Mruczek | Dynamic illusory size contrast: a relative-size illusion modulated by stimulus motion and eye movements[END_REF][START_REF] Mruczek | The Dynamic Ebbinghaus: motion dynamics greatly enhance the classic contextual size illusion[END_REF] have argued that eye movements create informational uncertainty about retinal-size and distance cues to size and thus increase the weighting of relational cues from irrelevant contextual information and, in so doing, augment the size illusion that is the Ebbinghaus.

Similarly, it is possible that an arousal-based account invoking differences in looking patterns can also explain the pattern of findings reported in a study of simultaneous lightness contrast (SLC) comparing traditional Himba participants with urban Himba and UK populations [START_REF] Linnell | Urban experience alters lightness perception[END_REF]. Specifically, this study involved comparative lightness judgements of pairs of grey target discs arrayed side by side, one on a white ground and the other on a black. While all groups showed SLC by over-estimating the lightness of the disc on the black ground, traditional Himba showed much increased SLC compared to the urban groups. This finding is initially puzzling, especially in light of the generally more accurate judgements of the traditional Himba [START_REF] Bremner | The ontogeny of visual context effects: Tracing effects of culture and the urban environment on the development of the Ebbinghaus illusion in British and Namibian observers[END_REF][START_REF] Caparos | Exposure to an urban environment alters the local bias of a remote culture[END_REF]; it could however be explained if the traditional Himba move their eyes less and consequently sample the black-white boundary between target grounds less [START_REF] Linnell | Urban experience alters lightness perception[END_REF], given that removal of this boundary increases SLC [START_REF] Gilchrist | Perceptual organization in lightness[END_REF][START_REF]Eye movements and vision[END_REF]. This suggestion is compatible with the close coupling that has been demonstrated between lightness judgements and the specific information selected by eye movements [START_REF] Toscani | Selection of visual information for lightness judgements by eye movements[END_REF].

Conclusions and Future Directions

This brief review has provided evidence that through our creation of urban environments we are changing not just our environment but also our default information processing style and what we take from our environment. We suggest that the same neuromodulatory system, namely the LC-NE arousal system, may mediate effects of urbanisation on both covert and overt processes of information sampling by making them less task-focused and more exploratory, changes that are presumably adaptive to the more open-ended nature of urban living. This is absolutely not, however, to suggest that the same parts of the LC-NE system mediate these effects of urbanisation, over the same timescales. The studies reviewed here suggest that the effects of urbanisation on covert selection can be quite dramatic, with urbanised Himba -who received a remote upbringing like that of the traditional Himba and only moved to a small town in early adulthood -showing covert attention that is indistinguishable from that of urbanised controls raised in and living in a city [START_REF] Linnell | Urbanization decreases attentional engagement[END_REF]. In contrast, the effects of urbanisation on overt selection in the studies reported here would seem to be less dramatic, with the urbanised Himba falling between the traditional Himba and UK controls in terms of their levels of susceptibility to the Ebbinghaus illusion [START_REF] Bremner | The ontogeny of visual context effects: Tracing effects of culture and the urban environment on the development of the Ebbinghaus illusion in British and Namibian observers[END_REF] and levels of SLC [START_REF] Linnell | Urban experience alters lightness perception[END_REF].

Given that the effects of urbanisation on covert and overt selection appear to follow different timescales, it is likely that they are underpinned by different effects, perhaps occurring at different stages of development. Indeed, it is tempting to draw superficial parallels with the work of Meyer-Lindenberg and colleagues [START_REF] Lederbogen | City living and urban upbringing affect neural social stress processing in humans[END_REF] showing that -at least under conditions of social stress -effects of urban living are underpinned by processing differences in the amygdala while effects of urban upbringing are underpinned by processing differences in the anterior cingulate cortex. The LC-NE arousal system is however highly interconnected with diverse brain regions and future work will be needed to elucidate the precise ways in which it may mediate the impact of urbanisation on covert and overt selection. Future work will also be needed to understand what urbanisation actually means and what types of socio-ecological conditions result in the types of 'rural-urban' processing differences documented here. A recent study suggests that high population density may not be a sufficient predictor of at least some of the cognitive effects of urbanisation [START_REF] Caparos | The local perceptual bias of a non-remote and educated population[END_REF] and it may prove to be not even a necessary predictor in some of today's virtually networked populations. Other socio-ecological factors, such as the adoption of the fast-paced way of life that is associated with WEIRD and other high-income populations, may prove to be more critical predictors of the radical changes in processing style that accompany urbanisation.

Acknowledgements

The authors would like to acknowledge funding from the ESRC and BA in the UK and from the SSHRC in Canada that supported the work with African populations. We thank our local guides/research assistants, without whom the cross-cultural research we report here could not have taken place, and we are immensely grateful to the participants in our studies, who welcomed us and our research with openness, benevolence and generosity.