Biology subjects

Davidson, M. J.

Publications and source records attributed to Davidson, M. J..

12 recordsLinked to original sources

bioRxiv2024

Exposure to low doses of Batrachochytrium dendrobatidis reveals variation in resistance in the Critically Endangered southern corroboree frog

Chytridiomycosis poses a significant extinction threat to many amphibians, including the critically endangered southern corroboree frog (Pseudophryne corroboree). Captive breeding programs have become essential to maintain populations while effective long-term conservation strategies are developed. Understanding the variation in susceptibility to chytridiomycosis within this species is essential in exploring the potential for selective breeding to enhance disease resistance. In this study, we conducted a large-scale Batrachochytrium dendrobatidis (Bd) exposure experiment involving 972 juvenile P. corroboree selected to ensure a broad genetic representation of the species. Three replicate experiments were conducted under uniform conditions, to assess individual susceptibility and compare results across replicate experiments. Significant variation was observed within and between experiments, with individual survival rates ranging from 44-74% across experiments, influenced notably by the zoo in which frogs were bred. Remarkably, 21-47% of exposed frogs remained Bd-negative, suggesting potential innate resistance. Infection intensity correlated positively with body condition, in one experiment, while age and size showed inconsistent effects on survival and infection rates across experiments, but younger and smaller frogs were more susceptible to infection and had lower survival. Among frogs that became infected, none cleared infection, with most progressing to terminal stages within an average of 69 days (ranging from 33 to 97 days). However, a few individuals maintained stable infection loads without displaying clinical signs of chytridiomycosis. This observed phenotypic variation in P. corroboree responses to Bd highlights the potential for selective breeding to improve survival outcomes in this species. The dataset generated from this study will be instrumental in guiding breeding strategies that strengthen conservation efforts for this critically endangered species.

zoology↗

bioRxiv2024

An automatic domain-general error signal is shared across tasks and predicts confidence in different sensory modalities

Understanding the ability to self-evaluate decisions is an active area of research. This research has primarily focused on the neural correlates of self-evaluation during visual-tasks, and whether pre- or post-decisional neural correlates capture subjective confidence in that decision. This focus has been useful, yet also precludes an investigation of key every-day features of metacognitive self-evaluation: that decisions are rapid, must be evaluated without explicit feedback, and unfold in a multisensory world. These considerations lead us to hypothesise that an automatic domain-general metacognitive signal may be shared between sensory modalities, which we tested in the present study with multivariate decoding of electroencephalographic (EEG) data. Participants (N=21, 12 female) first performed a visual task with no request for self-evaluations of performance, prior to an auditory task that included rating decision confidence on each trial. A multivariate classifier trained to predict errors in the speeded visual-task generalised to predict errors in the subsequent non-speeded auditory discrimination. This generalisation was unique to classifiers trained on the visual response-locked data, and further predicted subjective confidence on the subsequent auditory task. This evidence of overlapping neural activity across the two tasks provides evidence for automatic encoding of confidence independent of any explicit request for metacognitive reports, and a shared basis for metacognitive evaluations across sensory modalities.

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↗

bioRxiv2023

The speed and phase of locomotion dictate saccade probability and simultaneous low-frequency power spectra

Everyday we make thousands of saccades and take thousands of steps as we explore our environment. Despite their common co-occurrence in a typical active state, we know little about the coordination between eye-movements and walking behaviour and related changes in cortical activity. Technical limitations have been a major impediment which we overcome here by leveraging the advantages of an immersive wireless virtual reality (VR) environment with three-dimensional position tracking, together with simultaneous recording of eye-movements and mobile electroencephalography (EEG). Using this approach with participants engaged in unencumbered walking along a clear, level path, we find that the likelihood of eye-movements at both slow and natural walking speeds entrains to the rhythm of footfall, peaking shortly after the heel-strike of each step. Simultaneous EEG recordings reveal a concomitant modulation entrained to heel-strike, with increases and decreases in oscillatory power for a broad range of frequencies. The peak of these effects occurred in the theta and alpha range for both walking speeds. Together, our data show that the step-rate of locomotion influences other behaviours such as eye movements and produces related modulations of simultaneous EEG following the same rhythmic pattern. These results reveal gait as an important factor to be considered when interpreting saccadic and time-frequency EEG data in active observers.

neuroscience↗

bioRxiv2023

Walking entrains unique oscillations in performance on a visual detection task

Walking is among our most frequent and natural of voluntary behaviours, yet the consequences of locomotion upon perceptual and cognitive function remain largely unknown. Recent work has highlighted that although walking feels smooth and continuous, critical phases exist within each step-cycle for the successful coordination of perceptual and motor function. Here, we tested whether these phasic demands impact upon visual perception, by assessing performance in a visual detection task during natural unencumbered walking. We finely sampled visual performance over the stride cycle as participants walked along a smooth linear path at a comfortable speed in a wireless virtual reality environment. At the group-level, accuracy, reaction times, and response likelihood showed strong oscillations, modulating at approximately 2 cycles-per-stride ([~]2 Hz) with a marked phase of optimal performance aligned with the swing phase of each step. At the participant level, Bayesian inference of population prevalence revealed highly prevalent oscillations that clustered in two idiosyncratic frequency ranges (2 or 4 cycles per stride), with a strong phase alignment across participants.

neuroscience↗

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↗

bioRxiv2021

Alpha power and stimulus-evoked activity dissociate metacognitive reports of attention, visibility and confidence in a visual detection task

Variability in the detection and discrimination of weak visual stimuli has been linked to oscillatory neural activity. In particular, the amplitude of activity in the alpha-band (8-12 Hz) has been shown to impact upon the objective likelihood of stimulus detection, as well as measures of subjective visibility, attention, and decision confidence. We aimed to clarify how preparatory alpha influences performance and phenomenology, by recording simultaneous subjective measures of attention and confidence (Experiment 1), or attention and visibility (Experiment 2) on a trial-by-trial basis in a visual detection task. Across both experiments, alpha amplitude was negatively and linearly correlated with the intensity of subjective attention. In contrast to this linear relationship, we observed a quadratic relationship between the strength of alpha oscillations and subjective ratings of confidence and visibility. We find that this same quadratic relationship links alpha amplitude to the strength of stimulus evoked responses. Visibility and confidence judgements corresponded to the strength of evoked responses, but confidence, uniquely, incorporated information about attentional state. As such, our findings reveal distinct psychological and neural correlates of metacognitive judgements of attentional state, stimulus visibility, and decision confidence.

neuroscience↗

bioRxiv2020

A neural representation of invisibility: when stimulus-specific neural activity negatively correlates with conscious experience

Although visual awareness of an object typically increases neural responses, we identify a neural response that increases prior to perceptual disappearances, and that scales with the amount of invisibility reported during perceptual filling-in. These findings challenge long-held assumptions regarding the neural correlates of consciousness and entrained visually evoked potentials, by showing that the strength of stimulus-specific neural activity can encode the conscious absence of a stimulus. Significance StatementThe focus of attention and the contents of consciousness frequently overlap. Yet what happens if this common correlation is broken? To test this, we asked human participants to attend and report on the invisibility of four visual objects which seemed to disappear, yet actually remained on screen. We found that neural activity increased, rather than decreased, when targets became invisible. This coincided with measures of attention that also increased when stimuli disappeared. Together, our data support recent suggestions that attention and conscious perception are distinct and separable. In our experiment, neural measures more strongly follow attention.

neuroscience↗

bioRxiv2018

Frequency-tagging visual background information enables multi-target perceptual filling-in to be distinguished from phenomenally matched replay

Perceptual filling-in (PFI) occurs when a physically-present visual target disappears from conscious perception, with its location filled in by the surrounding visual background. Compared to other visual illusions, these perceptual changes are crisp and simple, and can occur for multiple spatially-separated targets simultaneously. Contrasting neural activity during the presence or absence of PFI may complement other multistable phenomena to reveal the neural correlates of consciousness (NCC). We presented four peripheral targets over a background dynamically updating at 20 Hz. While participants reported on target disappearances/reappearances via button press/release, we tracked neural activity entrained by the background during PFI using steady-state visually evoked potentials (SSVEPs) recorded in the electroencephalogram. We found background SSVEPs closely correlated with subjective report, and increased with an increasing amount of PFI. Unexpectedly, we found that as the number of filled-in targets increased, the duration of target disappearances also increased, suggesting facilitatory interactions exist between targets in separate visual quadrants. We also found distinct spatiotemporal correlates for the background SSVEP harmonics. Prior to genuine PFI, the response at the second harmonic (40 Hz) increased before the first (20 Hz), which we tentatively link to an attentional effect. There was no difference between harmonics for physically removed stimuli. These results demonstrate that PFI can be used to study multi-object perceptual suppression when frequency-tagging the background of a visual display, and because there are distinct neural correlates for endogenously and exogenously induced changes in consciousness, that it is ideally suited to study the NCC.\n\nHighlightsO_LIPerceptual filling-in (PFI) has distinct advantages for investigating the neural correlates of consciousness.\nC_LIO_LIParticipants can accurately report graded changes in consciousness using four simultaneous buttons.\nC_LIO_LIFrequency-tagging of visual background information tracks changes in visual perception.\nC_LIO_LISpatiotemporal EEG responses differentiate PFI from phenomenally matched physical disappearances.\nC_LI

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↗