Biology subjects

Alais, D.

Publications and source records attributed to Alais, D..

18 recordsLinked to original sources

bioRxiv2026

Walking Produces Oscillations in Orientation and Direction Perception at the Step Rate

Walking is one of the most stereotyped oscillatory movement able-bodied individuals produce. It is highly consistent within and across individuals, with people taking ~5000 steps a day on average at approximately 2 Hz. This consistency and its ubiquitous role in daily living make walking an ideal behaviour for investigating perception under active conditions. Across two virtual reality experiments, we tested whether stride-based oscillations, as seen in visual detection performance, also occur for visual orientation and motion direction discrimination. Participants followed a simulated screen along a linear path in virtual space while visual stimuli were presented on the screen. Group-level performance in both visual tasks oscillated at approximately two cycles per stride. Peak performance in visual orientation identification aligned with the late swing phase of each step, whereas peak performance in motion direction identification aligned with the late stance or early swing phase. These findings extend recent work showing that visual perception is modulated at the step rate during locomotion.

neuroscience↗

bioRxiv2026

Entering and Exiting Awareness are Asymmetrical: Evidence from tracking Continuous Flash Suppression

This study provides a model for obtaining cleaner threshold estimates of entry into and exit from conscious awareness measured with the popular breaking continuous flash suppression (bCFS) and newer tracking CFS (tCFS) paradigms. In both, a suppressed target increases in contrast until it breaks into awareness. tCFS also measures exit thresholds by reversing the contrast change. Both CFS paradigms require a response when the threshold is reached and thus involve a reaction-time component. During this period, target contrast continues to change and threshold estimates are inflated (the "overshoot" problem). Here, we measure reaction-time components for both increasing (entering awareness) and decreasing targets (exiting awareness). We find overshoot is asymmetrical, being greater for stimuli entering (breakthrough thresholds) than for exiting awareness (suppression thresholds). To solve "the overshoot problem", we introduce a mathematical model that quantifies reaction-time components so they can be partialled out to produce clean breakthrough and suppression thresholds. The model can also be applied to previously published CFS studies. The model works for simple target detection and two-alternative target discrimination, even in the continuous tracking paradigm of tCFS. With the timing corrections applied, changes in pupil diameter around transitions in awareness are shown to align with the corrected time points, most clearly when targets exit awareness. Even after correction, the contrast decrease required for a target to exit awareness was greater than the increase required for it to enter awareness.

neuroscience↗

bioRxiv2026

Priors for perceived speed combine supramodally for visual-tactile motion but not for audio-visual motion.

Perception combines incoming sensory evidence with priors learned from preceding input. One example is the central tendency effect in which stimulus estimates regress towards a prior based on the mean of the stimulus distribution. Whether such priors are specific to each modality or shared across modalities remains unresolved. Here, we examined this question using speed perception in the visual, auditory and tactile modalities and used central tendency as a behavioral probe. By giving each modality its own speed range, clear predictions emerge when modalities are interleaved: if priors are modality specific, each modality should maintain its own central tendency bias whereas a supramodal prior would exhibit a different mean based on the pooled set of speeds. Moreover, a supra-modal prior should exhibit precision weighting, with the more reliable cue receiving more weight. For vision and touch, speed estimates converged when interleaved: both modalities shifted but the less reliable one shifted more, consistent with a precision-weighted supramodal prior. For vision and audition, the priors remained largely distinct: audition (the less reliable modality) showed a partial shift but did not converge on the precision weighted, supramodal mean. This was not explained by the large audiovisual reliability difference: it persisted when visual precision was reduced. These results show that in the motion domain, visual and tactile signals can combine crossmodally, but auditory and visual signals do not. Supramodal processing of motion therefore depends on which senses are involved and is likely explained by the lack of an early spatial mapping in audition.

neuroscience↗

bioRxiv2025

The Role of Temporal Factors in Processing Rapid Serial Visual Presentations

Sequentially presented visual stimuli are often employed to investigate the temporal limits of our visual perception and attention. Due to the finite nature of the brains processing capacity, experimental designs attempt to increase temporal separation within series to mitigate potential neural interference between stimuli. This is achieved through shortening of stimulus durations and lengthening of interstimulus intervals. Across two experiments, we parametrically varied these parameters for a serially presented checkerboard wedge stimulus and used inverted encoding modelling to assess the fidelity of the neural representation of spatial location. We found longer interstimulus intervals and shorter stimulus durations produced the most reliable decoding. Varying interstimulus interval altered the fidelity of neural representations via inter-item interference. By contrast, the influence of stimulus duration seemed largely analytic; in a fixed duration block, shorter stimulus duration enabled more presentations, which led to increased decoding accuracy. Distinct anticipatory and reactionary neural events were observed for stimuli presented in triplet sequences, which seemed consistent with forwards and backwards visual masking. The absence of an anticipatory event was observed when backward masking was stronger while a reactionary event was present for every stimulus subjected to forward masking.

neuroscience↗

bioRxiv2025

A Minimal Physiological Model of Perceptual Suppression and Breakthrough in Visual Rivalry

Visual rivalry paradigms provide a powerful tool for probing the mechanisms of visual awareness and perceptual suppression. While the dynamics and determinants of perceptual switches in visual rivalry have been extensively studied and modeled, recent advances in experimental design - particularly those that quantify the depth and variability of perceptual suppression - have outpaced the development of computational models. Here we extend an existing dynamical model of binocular rivalry to encompass two novel experimental paradigms: a threshold detection variant of binocular rivalry, and tracking continuous flash suppression. Together, these tasks provide complementary measures of the dynamics and magnitude of perceptual suppression. Through numerical simulation we demonstrate that a single mechanism, competitive (hysteretic) inhibition between slowly adapting monocular populations, is sufficient to account for the suppression depth findings across both paradigms. This unified model offers a foundation for the development of a quantitative theory of perceptual suppression in visual rivalry.

neuroscience↗

bioRxiv2025

Tactile and Visual Spatial Frequency Perception Follows Optimal Integration but Is Not Affected by Spatial Proximity

Spatial frequency is a fundamental feature in both visual and somatosensory perception, yet how these modalities integrate spatial frequency information remains unclear. This study investigates whether visuotactile spatial frequency perception follows the principles of Maximum Likelihood Estimation (MLE) and whether spatial proximity influences multisensory integration, using virtual reality (VR) and high-precision 3D-printed tactile stimuli. Experiment 1 found that the visuotactile integration of spatial frequency cues follows the MLE rule. However, Experiment 2 revealed that this integration is not affected by spatial proximity. These findings provide additional insights into the feature dependency of multisensory integration between vision and touch and highlight the potential for independent processing before integration, offering new perspectives on the mechanisms of spatial frequency processing in both visual and tactile modalities.

neuroscience↗

bioRxiv2025

Characteristics of Tactile Orientation Perception: Oblique Effect, Active vs Passive Exploration, and Serial Dependence

Orientation perception is one of the most fundamental aspects of somatosensory perception, similar to its visual counterpart. In the current study, we examined several features of tactile orientation perception and their interactions, aiming to better understand the underlying mechanisms. We found that active exploration exhibits better orientation acuity compared to passive exploration. We also observed an exploration-independent and paradigm-independent robust anisotropy (i.e., tactile oblique effect) in tactile orientation acuity where the proximal-distal axis demonstrates superior orientation acuity compared to oblique and medial-lateral orientations. Finally, we demonstrated that, similar to its visual counterpart, tactile orientation perception showed a systematic attractive serial dependence effect, in which current orientation perception is biased towards the previous trial. However, this attractive effect is driven by post-perceptual processing/response rather than the stimulus per se.

neuroscience↗

bioRxiv2024

Perception of audio-visual synchrony is modulated by walking speed and step cycle phase

Investigating sensory processes in active human observers is critical for a holistic understanding of perception. Recent research has demonstrated that locomotion can alter visual detection performance in a rhythmic manner, illustrating how a very frequent and natural behaviour can influence sensory performance. Here we extend this line of work to incorporate variations in walking speed, and test whether multi-sensory processing is impacted by the speed and phase of locomotion. Participants made audio-visual synchrony judgements while walking at two speeds over a range of stimulus onset asynchronies (SOAs). We find that sensitivity to multi-sensory synchrony decreases at slow walking speeds and is accompanied by an increase in reaction times, compared to when walking at a natural pace. A further analysis of the shortest SOAs was conducted to test whether subjective synchrony modulated over the step cycle. This revealed that synchrony judgements were quadratically modulated with perceived synchrony being higher in the swing phase of each step and lower when both feet were grounded during stance phase. Together, these results extend an earlier report that walking dynamically modulates visual sensitivity by contributing two new findings: first, that walking speed modulates perceived synchrony of audio-visual stimuli, and second, that modulations within the step-cycle extend to multisensory synchrony judgements which peak in the swing phase of each step.

neuroscience↗

bioRxiv2024

Walking entrains unique oscillations for central and peripheral visual detection

It is important to investigate perception in the context of natural behaviour in order to reach a holistic account of how sensory processes are coordinated with actions. In particular, the effect of walking upon perceptual and cognitive functions has recently been investigated in the context of how common voluntary actions may dynamically impact upon visual detection. This work has revealed that walking can enhance peripheral visual processing, and that during walking, performance on a visual detection task oscillates through good and bad periods within the phases of the stride-cycle. Here, we extend this work by examining whether oscillations in visual detection performance are uniform across the visual field while walking. Participants monitored parafoveal ([~]3.7 d.v.a) and peripheral ([~]7 d.v.a) locations left/right of fixation for the onset of targets while walking at a natural pace in wireless virtual reality. For targets at all locations accuracy, reaction times and response likelihood oscillated within each individuals stride-cycle, at primarily 2 or 4 cycles per stride. Importantly, oscillations in accuracy and reaction time shared the same frequency at both locations but were decreased in amplitude and phase-lagged in the periphery, revealing an interaction between visual field locations and oscillations in performance. Together, these results demonstrate that oscillations in visual performance entrained by the stride-cycle occur with unique amplitudes and phases across the visual field.

neuroscience↗

bioRxiv2024

Positive serial dependence in ratings of food images for appeal and calories

Food is fundamental to survival and our brains are highly attuned to rapidly process food stimuli. Neural signals show foods can be discriminated as edible or inedible as early as 85 ms after stimulus onset1, distinguished as processed or unprocessed beginning at 130 ms2 and as high or low density from 165 ms3. Recent evidence revealed specialised processing of food stimuli in the ventral visual pathway4-6, an area that underlies perception of faces and other important objects. For many visual objects, present perception can be biased towards recent perceptual history (known as serial dependence7,8). We examined serial dependence for food in two large samples (n>300) that rated sequences of food images for either appeal or calories. Calorie ratings by males and females agreed closely but appeal ratings were higher in males. High calorie ratings were associated with high appeal, especially in males. Serial analyses testing if current trial ratings were influenced by the previous one showed both appeal and calorie ratings exhibited clear positive dependences (i.e., a high preceding rating increased current trial ratings). The serial effect for appeal was roughly twice that for calories and males showed a greater serial effect than females for both ratings. Serial amplitude was larger in those who reported a longer elapsed time since they last ate and was larger in the BMI>25 group compared to BMI<25. These findings square with recently found food selectively in visual temporal cortex, reveal a new mechanism influencing food decision-making and suggest a new sensory-level component that could complement cognitive strategies in diet intervention.

neuroscience↗

bioRxiv2023

Tactile Adaptation to Orientation Produces a Robust Tilt Aftereffect and Exhibits Crossmodal Transfer When Tested in Vision

Orientation processing is one of the most fundamental functions in both visual and somatosensory perception. Converging findings suggest that orientation processing in both modalities is closely linked: somatosensory neurons share a similar orientation organisation as visual neurons, and the visual cortex has been found to be heavily involved in tactile orientation perception. The tilt aftereffect (TAE) is a demonstration of orientation adaptation and is used widely in behavioural experiments to investigate orientation mechanisms in vision. By testing the classic TAE paradigm in both tactile and crossmodal orientation tasks between vision and touch we were able to show that tactile perception of orientation shows a very robust TAE, similar to its visual counterpart. We further show that orientation adaptation in touch transfers to produce a TAE when tested in vision, but not vice versa. We also observed a similar asymmetricity in the intramodal serial dependence effect within the test sequence. These findings provide concrete evidence that vision and touch engage a similar orientation processing mechanism, but the asymmetry in the crossmodal transfer of TAE and serial dependence provides more insights into the underlying mechanism of this link.

neuroscience↗

bioRxiv2023

A common mechanism processes auditory and visual motion

We report behavioural findings implying common motion processing for auditory and visual motion. We presented brief translational motion stimuli drifting leftwards or rightwards in the visual or auditory modality at various speeds. Observers made a speed discrimination on each trial, comparing current speed against mean speed (i.e., method of single stimuli). Data were compiled into psychometric functions and means and slopes compared. Slopes between auditory and visual motion were identical, consistent with a common noise source, although mean speed for audition was veridical while visual speeds were significantly underestimated. An inter-trial analysis revealed clear motion priming in both audition and vision (i.e., faster perceived speed after a fast preceding speed, and vice versa - a positive serial dependence). Plotting priming as a function of preceding speed revealed the same slope for each modality. We also tested whether motion priming was modality specific. Whether vision preceded audition, or audition preceded vision, a positive serial bias (i.e., priming) was always observed. We conclude a common process underlies auditory and visual motion, and that this explains the closely matched data in vision and audition, as well as the crossmodal data showing equivalent motion priming regardless of the preceding trials modality.

animal behavior and cognition↗

bioRxiv2023

tCFS: A new 'CFS tracking' paradigm reveals uniform suppression depth regardless of target complexity or salience

When the eyes view separate and incompatible images, the brain suppresses one image and promotes the other into visual awareness. Periods of interocular suppression can be prolonged during continuous flash suppression (CFS) - when one eye views a static target while the other views a complex dynamic stimulus. Measuring the time needed for a suppressed image to break CFS (bCFS) has been widely used to investigate unconscious processing, and the results have generated controversy regarding the scope of visual processing without awareness. Here, we address this controversy with a new CFS tracking paradigm (tCFS) in which the suppressed monocular target steadily increases in contrast until breaking into awareness (as in bCFS) after which it decreases until it again disappears (reCFS), with this cycle continuing for many reversals. Unlike bCFS, tCFS provides a measure of suppression depth by quantifying the difference between breakthrough and suppression thresholds. tCFS confirms that: (i) breakthrough thresholds indeed differ across target types (e.g., faces vs gratings, as bCFS has shown) - but (ii) suppression depth does not vary across target types. Once the breakthrough contrast is reached for a given stimulus, all stimuli require a strikingly uniform reduction in contrast to reach the corresponding suppression threshold. This uniform suppression depth points to a single mechanism of CFS suppression, one that likely occurs early in visual processing because suppression depth was not modulated by target salience or complexity. More fundamentally, it shows that variations in bCFS thresholds alone are insufficient for inferring whether the barrier to achieving awareness exerted by interocular suppression is weaker for some categories of visual stimuli compared to others. Significance statementResearch on unconscious vision has proliferated recently, often employing the continuous flash suppression (CFS) method in which rapidly changing shapes viewed by one eye suppress the other eyes image from awareness. If that suppressed image is progressively strengthened in contrast it will eventually break suppression and emerge into visible awareness. Low breakthrough thresholds are claimed to indicate unconscious processing during suppression. We introduce a method that quantifies breakthrough thresholds and suppression thresholds, thus providing a lower bound missing from previous CFS research. Comparing various image types, including those claimed to undergo unconscious processing, all images show equal suppression when both thresholds are measured. We thus find no evidence of differential unconscious processing and conclude reliance on breakthrough thresholds is misleading without considering suppression thresholds and leads to spurious claims about unconscious processing.

neuroscience↗

bioRxiv2023

Peripersonal tracking accuracy is limited by the speed and phase of locomotion

Recent evidence suggests that perceptual and cognitive functions are codetermined by rhythmic bodily states. Prior investigations have focused on the cardiac and respiratory rhythms, both of which are also known to synchronise with locomotion - arguably our most common and natural of voluntary behaviours. Unlike the cardiorespiratory rhythms, walking is entirely under voluntary control, enabling a test of how natural and voluntary rhythmic action may affect sensory function. Here, we show that the speed and phase of human locomotion constrains sensorimotor performance. We used a continuous visuo-motor tracking task in a wireless, body-tracking virtual environment, and found that the accuracy and reaction time of continuous reaching movements were decreased at slower walking speeds, and rhythmically modulated according to the phases of the step-cycle. Decreased accuracy when walking at slow speeds suggests an advantage for interlimb coordination at normal walking speeds, in contrast to previous research on dual-task walking and reach-to-grasp movements. Phasic modulations of reach precision within the step-cycle also suggest that the upper limbs are affected by the ballistic demands of motor-preparation during natural locomotion. Together these results show that the natural phases of human locomotion impose constraints on sensory function and demonstrate the value of examining dynamic and natural behaviour in contrast to the traditional and static methods of psychological science.

neuroscience↗

bioRxiv2020

Propagation and update of auditory perceptual priors through alpha and theta rhythms

To maintain a continuous and coherent percept over time, the brain makes use of past sensory information to anticipate forthcoming stimuli. We recently showed that auditory experience in the immediate past is propagated through ear-specific reverberations, manifested as rhythmic fluctuations of decision bias at alpha frequency. Here, we apply the same time-resolved behavioural method to investigate how perceptual performance changes over time under conditions of high stimulus expectation, and to examine the effect of unexpected events on behaviour. As in our previous study, participants were required to discriminate the ear-of-origin of a brief monaural pure tone embedded in uncorrelated dichotic white noise. We manipulated stimulus expectation by increasing the target probability in one ear to 80%. Consistent with our earlier findings, performance did not remain constant across trials, but varied rhythmically with delay from noise onset. Specifically, decision bias showed a similar oscillation at ~9 Hz that depended on ear congruency between successive targets. This suggests rhythmic communication of auditory perceptual history occurs early and is not readily influenced by top-down expectations. In addition, we report a novel observation specific to infrequent, unexpected stimuli that gave rise to oscillations in accuracy at ~7.6 Hz one trial after the target occurred in the non-anticipated ear. This new behavioural oscillation may reflect a mechanism for updating the sensory representation once a prediction error has been detected.

neuroscience↗

bioRxiv2019

Individual difference in serial dependence results from opposite influences of perceptual choices and motor responses

Natural image statistics exhibit temporal regularities of slow changes and short-term correlations and visual perception, too, is biased towards recently seen stimuli, i.e., a positive serial dependence. Some studies report strong individual differences in serial dependence in perceptual decision making: some observers show positive serial effects, others repulsive effects, and some show no bias. To understand these contrasting results, this study separates the influences of physical stimuli per se, perceptual choices and motor responses on serial dependence in perceptual decision making. In two experiments, human observers reported which orientation (45{degrees} or -45{degrees}, at threshold contrast) they perceived. Experiment 1, used a consistent mapping between stimulus and response buttons while in Experiment 2, observers did two tasks: one with a consistent stimulus-response mapping, the other with a random stimulus-response mapping (perceptual choice and motor response unrelated). Results show that the stimulus percept (not the physical stimulus per se) affected subsequent perceptual choices in an attractive way, and that motor responses produced a repulsive serial effect. When the choice-response mapping was consistent (inseparable choice and response, typical of most experiments), individual differences in the overall serial effect was observed: some were positive, some repulsive, and some were bias-free. These individual differences likely reflect different relative weightings in individuals of a positive choice bias and a repulsive motor bias.

neuroscience↗

bioRxiv2019

Auditory perceptual history is communicated through alpha oscillations

Sensory expectations from the accumulation of information over time exert strong predictive biases on forthcoming perceptual decisions. These anticipatory mechanisms help to maintain a coherent percept in a noisy environment. Here we present novel behavioural evidence that past sensory experience biases perceptual decisions rhythmically through alpha oscillations. Participants identified the ear of origin of a brief sinusoidal tone masked by dichotic white noise, and response bias oscillated over time at [~]9 Hz. Importantly, the oscillations occurred only for trials preceded by a target to the same ear and lasted for at least two trials. These findings suggest that each stimulus elicits an oscillating memory trace, specific to the ear of origin, which subsequently biases perceptual decisions. This trace is phase-reset by the noise onset of the next trial, and remains within the circuitry of the ear in which it was elicited, modulating the sensory representations in that ear.

neuroscience↗

bioRxiv2018

Attention periodically samples competing stimuli during binocular rivalry

The attentional sampling hypothesis suggests that attention rhythmically enhances sensory processing when attending to a single (~8 Hz), or multiple (~4 Hz) objects. Here we investigated using binocular rivalry whether attention samples sensory representations that are not part of the conscious percept, during competition for perceptual dominance. When crossmodally cued toward a conscious image, subsequent changes in consciousness occurred at ~8 Hz, consistent with rates of undivided attentional sampling. However, when attention was cued toward the suppressed image, changes in consciousness slowed to ~3.5 Hz, indicating the division of attention away from the conscious visual image. In the electroencephalogram, we found that at 3.5 and 8 Hz, the strength of inter-trial phase coherence over fronto-temporal and parieto-occipital regions correlated with behavioral measures of changes in perception. When cues were not task-relevant, these effects disappeared, confirming that perceptual changes were dependent upon the allocation of attention, and that attention can flexibly sample away from a conscious image in a task-dependent manner.

neuroscience↗