Flicker adaptation of low-level cortical visual neurons contributes to temporal dilation

Neural correlates of aftereffects induced by adaptations to single and average durations

Duration perception can be heavily distorted owing to repetitive exposure to a relatively long or short sensory event, often causing a duration aftereffect. Here, we used a novel procedure to show that adaptations to both single and average durations produced the duration aftereffect. Participants completed a duration reproduction task (Experiment 1) or a duration category rating task (Experiment 2) after long-term adaptations to a stimulus of medium duration and to stimuli of averagely medium duration. We found that adaptations to both single and average durations resulted in duration aftereffects. The simultaneously recorded functional magnetic resonance imaging (fMRI) data revealed that the reduction in neural activity due to long-term adaptation to single duration was observed in the right supramarginal gyrus (SMG) of the parietal lobe, while adaptation to average duration resulted in fMRI adaptations in the left postcentral gyrus (PCG) and middle cingulate gyrus (MCG). At the individual level, the magnitude of the behavioral aftereffect was positively correlated with the magnitude of fMRI adaptation in the right SMG after adaptation to single duration, while there were no significantly positive correlations between the behavioral aftereffect and fMRI adaptations in the left PCG and MCG. These results suggest that there are different neural mechanisms for aftereffects caused by adaptations to single and average durations.

Stimulus (dis)similarity can modify the effect of a task-irrelevant sandwiching stimulus on the perceived duration of brief visual stimuli

The perceived duration of a target visual stimulus is shorter when a brief non-target visual stimulus precedes and trails the target than when it appears alone. This time compression requires spatiotemporal proximity of the target and non-target stimuli, which is one of the perceptual grouping rules. The present study examined whether and how another grouping rule, stimulus (dis)similarity, modulated this effect. In Experiment 1, time compression occurred only when the preceding and trailing stimuli (black-white checkerboard) were dissimilar from the target (unfilled round or triangle) with spatiotemporal proximity. In contrast, it was reduced when the preceding or trailing stimuli (filled rounds or triangles) were similar to the target. Experiment 2 revealed time compression with dissimilar stimuli, independent of the intensity or saliency of the target and non-target stimuli. Experiment 3 replicated the findings of Experiment 1 by manipulating the luminance similarity between target and non-target stimuli. Furthermore, time dilation occurred when the non-target stimuli were indistinguishable from the target stimuli. These results indicate that stimulus dissimilarity with spatiotemporal proximity induces time compression, whereas stimulus similarity with spatiotemporal proximity does not. These findings were discussed in relation to the neural readout model.

Adaptation reveals multi-stage coding of visual duration

In conflict with historically dominant models of time perception, recent evidence suggests that the encoding of our environment's temporal properties may not require a separate class of neurons whose raison d'être is the dedicated processing of temporal information. If true, it follows that temporal processing should be imbued with the known selectivity found within non-temporal neurons. In the current study, we tested this hypothesis for the processing of a poorly understood stimulus parameter: visual event duration. We used sensory adaptation techniques to generate duration aftereffects: bidirectional distortions of perceived duration. Presenting adapting and test durations to the same vs different eyes utilises the visual system's anatomical progression from monocular, pre-cortical neurons to their binocular, cortical counterparts. Duration aftereffects exhibited robust inter-ocular transfer alongside a small but significant contribution from monocular mechanisms. We then used novel stimuli which provided duration information that was invisible to monocular neurons. These stimuli generated robust duration aftereffects which showed partial selectivity for adapt-test changes in retinal disparity. Our findings reveal distinct duration encoding mechanisms at monocular, depth-selective and depth-invariant stages of the visual hierarchy.

Object size determines the spatial spread of visual time

A key question for temporal processing research is how the nervous system extracts event duration, despite a notable lack of neural structures dedicated to duration encoding. This is in stark contrast with the orderly arrangement of neurons tasked with spatial processing. In this study, we examine the linkage between the spatial and temporal domains. We use sensory adaptation techniques to generate after-effects where perceived duration is either compressed or expanded in the opposite direction to the adapting stimulus' duration. Our results indicate that these after-effects are broadly tuned, extending over an area approximately five times the size of the stimulus. This region is directly related to the size of the adapting stimulus-the larger the adapting stimulus the greater the spatial spread of the after-effect. We construct a simple model to test predictions based on overlapping adapted versus non-adapted neuronal populations and show that our effects cannot be explained by any single, fixed-scale neural filtering. Rather, our effects are best explained by a self-scaled mechanism underpinned by duration selective neurons that also pool spatial information across earlier stages of visual processing.

Relative Time Compression for Slow-Motion Stimuli through Rapid Recalibration

A number of psychophysical studies have shown that moving stimuli appear to last longer than static stimuli. Here, we report that the perceived duration for slow moving stimuli can be shorter than for static stimuli under specific circumstances. Observers were tested using natural movies presented at various speeds (0.0× = static, 0.25× = slow, or 1.9× = fast, relative to original speed) and indicated whether test duration was perceived as longer or shorter than comparison movies presented at their original speed. While fast movies were perceived as longer than slow and static movies (in accordance with previous studies), we found that slow movies were perceived as shorter (i.e., time compressed) compared to static images. Similar results were obtained for artificial stimuli consisting of drifting gratings. However, time compression for slow stimuli disappeared if comparison stimuli were replaced by a white static disk that removed repetitive exposures to moving stimuli. Results suggest that duration estimation is modulated by contextual effects induced by the specific diet – or distribution – of prior visual stimuli to which observers are exposed. A simple model, which includes a rapid recalibration of human time estimation via adaptation to preceding stimuli, succeeds in reproducing our experimental data.

Flickering task-irrelevant distractors induce dilation of target duration depending upon cortical distance

Flickering stimuli are perceived to be longer than stable stimuli. This so-called “flicker-induced time dilation” has been investigated in a number of studies, but the factors critical for this effect remain unclear. We explored the spatial distribution of the flicker effect and examined how the flickering task-irrelevant distractors spatially distant from the target induce time dilation. In two experiments, we demonstrated that flickering distractors dilated the perceived duration of the target stimulus even though the target stimulus itself was stable. In addition, when the distractor duration was much longer than the target duration, a flickering distractor located ipsilateral to the target caused greater time dilation than did a contralateral distractor. Thus the amount of dilation depended on the distance between the cortical areas responsible for the stimulus locations. These findings are consistent with the recent study reporting that modulation of neural oscillators encoding the interval duration could explain flicker-induced time dilation.

Multiple channels of visual time perception

The proposal that the processing of visual time might rely on a network of distributed mechanisms that are vision-specific and timescale-specific stands in contrast to the classical view of time perception as the product of a single supramodal clock. Evidence showing that some of these mechanisms have a sensory component that can be locally adapted is at odds with another traditional assumption, namely that time is completely divorced from space. Recent evidence suggests that multiple timing mechanisms exist across and within sensory modalities and that they operate in various neural regions. The current review summarizes this evidence and frames it into the broader scope of models for time perception in the visual domain.

Visual duration aftereffect is position invariant

Adaptation to relatively long or short sensory events leads to a negative aftereffect, such that the durations of the subsequent events within a certain range appear to be contracted or expanded. The distortion in perceived duration is presumed to arise from the adaptation of duration detectors. Here, we focus on the positional sensitivity of those visual duration detectors by exploring whether the duration aftereffect may be constrained by the visual location of stimuli. We adopted two different paradigms, one that tests for transfer across visual hemifields, and the other that tests for simultaneous selectivity between visual hemifields. By employing these experimental designs, we show that the duration aftereffect strongly transfers across visual hemifields and is not contingent on them. The lack of position specificity suggests that duration detectors in the visual system may operate at a relatively later stage of sensory processing.

The aftereffect of perceived duration is contingent on auditory frequency but not visual orientation

Recent sensory history plays a critical role in duration perception. It has been established that after adapting to a particular duration, the test durations within a certain range appear to be distorted. To explore whether the aftereffect of perceived duration can be constrained by sensory modality and stimulus feature within a modality, the current study applied the technique of simultaneous sensory adaptation, by which observers were able to simultaneously adapt to two durations defined by two different stimuli. Using both simple visual and auditory stimuli, we found that the aftereffect of perceived duration is modality specific and contingent on auditory frequency but not visual orientation of the stimulus. These results demonstrate that there are independent timers responsible for the aftereffects of perceived duration in each sensory modality. Furthermore, the timer for the auditory modality may be located at a relatively earlier stage of sensory processing than the timer for the visual modality.

Audition dominates vision in duration perception irrespective of salience, attention, and temporal discriminability

Whereas the visual modality tends to dominate over the auditory modality in bimodal spatial perception, the auditory modality tends to dominate over the visual modality in bimodal temporal perception. Recent results suggest that the visual modality dominates bimodal spatial perception because spatial discriminability is typically greater for the visual than for the auditory modality; accordingly, visual dominance is eliminated or reversed when visual-spatial discriminability is reduced by degrading visual stimuli to be equivalent or inferior to auditory spatial discriminability. Thus, for spatial perception, the modality that provides greater discriminability dominates. Here, we ask whether auditory dominance in duration perception is similarly explained by factors that influence the relative quality of auditory and visual signals. In contrast to the spatial results, the auditory modality dominated over the visual modality in bimodal duration perception even when the auditory signal was clearly weaker, when the auditory signal was ignored (i.e., the visual signal was selectively attended), and when the temporal discriminability was equivalent for the auditory and visual signals. Thus, unlike spatial perception, where the modality carrying more discriminable signals dominates, duration perception seems to be mandatorily linked to auditory processing under most circumstances.

The effect of simultaneous flickering light stimulation on global form and motion perception thresholds

The question regarding the exact function of the primary visual cortex (V1) in vision has been around ever since the description of residual vision after damage to this cortical area by Riddoch in 1917. In 2002, Schoenfeld and colleagues proposed that V1 can be saturated by flashes of light, by which the function of V1-bypassing visual pathways can be "unmasked". The Schoenfeld group found that light flashes applied on stimulus onset led to the elevation of brightness increment detection thresholds, but left motion detection thresholds unaltered. Although the proposed method (i.e. the use of light flashes to induce refractoriness in V1) could be a simple, cheap and elegant way of exploring V1 functions, no study has followed up on this. Therefore it is not known if it works at all with other types of stimuli. For that reason, we decided to revisit the idea in a modified form. Global form and motion perception thresholds were assessed with static Glass pattern stimuli and random dot kinematograms, with and without 12Hz flickering light stimulation. Global motion thresholds were almost unaltered by flickering stimulation, while a significant threshold elevation was caused in the global form perception task. The strongest conclusion allowed by our data is that simultaneous flickering photostimulation elevates global form perception thresholds but not global motion perception thresholds. This is in some way related to the refractoriness generated in an unsatisfactorily defined part of V1. We suggest that this does not necessarily reflect the activity of V1-bypassing pathways, and propose that the application of light flashes is a method that deserves more attention in the exploration of the V1-dependent and independent elements of visual consciousness in human subjects.

Action enhances auditory but not visual temporal sensitivity

People naturally dance to music, and research has shown that rhythmic auditory stimuli facilitate production of precisely timed body movements. If motor mechanisms are closely linked to auditory temporal processing, just as auditory temporal processing facilitates movement production, producing action might reciprocally enhance auditory temporal sensitivity. We tested this novel hypothesis with a standard temporal-bisection paradigm, in which the slope of the temporal-bisection function provides a measure of temporal sensitivity. The bisection slope for auditory time perception was steeper when participants initiated each auditory stimulus sequence via a keypress than when they passively heard each sequence, demonstrating that initiating action enhances auditory temporal sensitivity. This enhancement is specific to the auditory modality, because voluntarily initiating each sequence did not enhance visual temporal sensitivity. A control experiment ruled out the possibility that tactile sensation associated with a keypress increased auditory temporal sensitivity. Taken together, these results demonstrate a unique reciprocal relationship between auditory time perception and motor mechanisms. As auditory perception facilitates precisely timed movements, generating action enhances auditory temporal sensitivity.