Walking modulates visual detection performance according to stride cycle phase.

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 for the successful coordination of perceptual and motor function. Here, we test whether these phasic demands impact upon visual perception, by assessing performance in a visual detection task during natural unencumbered walking. We finely sample visual performance over the stride cycle as participants walk 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 show 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 reveals highly prevalent oscillations in visual detection performance that cluster in two idiosyncratic frequency ranges (2 or 4 cycles per stride), with a strong phase alignment across participants.

Vestibular and active self-motion signals drive visual perception in binocular rivalry.

Multisensory integration helps the brain build reliable models of the world and resolve ambiguities. Visual interactions with sound and touch are well established but vestibular influences on vision are less well studied. Here, we test the vestibular influence on vision using horizontally opposed motions presented one to each eye so that visual perception is unstable and alternates irregularly. Passive, whole-body rotations in the yaw plane stabilized visual alternations, with perceived direction oscillating congruently with rotation (leftward motion during leftward rotation, and vice versa). This demonstrates a purely vestibular signal can resolve ambiguous visual motion and determine visual perception. Active self-rotation following the same sinusoidal profile also entrained vision to the rotation cycle - more strongly and with a lesser time lag, likely because of efference copy and predictive internal models. Both experiments show that visual ambiguity provides an effective paradigm to reveal how vestibular and motor inputs can shape visual perception.

Inhibition of return in the oculomotor decision process: Dissociating visual target discrimination from saccade readiness delays.

Saccades toward previously cued or fixated locations typically have longer latencies than those toward novel locations, a phenomenon known as inhibition of return (IOR). Despite extensive debate on its potential function, it remains unclear what the role of IOR in the oculomotor decision process is. Here, we ask whether the effect on eye movement planning is best characterized as a delay in visual target discrimination or as a reduction in readiness to execute the movement (saccade readiness). To evaluate this question, we use target-distractor tasks with clear speed-accuracy trade-offs. Simultaneously cueing both the target and distractor (or neither) we find longer latencies at the cued locations. Despite this delay in latencies, accuracy improves in line with the speed-accuracy trade-off curve (Experiment 1). This suggests that while visual target discrimination can progress unimpeded, saccade readiness is reduced. Based on this reduction in readiness we predict that the more saccades rely on visual target discrimination, the less their destination will be affected by inducing IOR. Indeed, after cueing either the target or an onset distractor (Experiment 2), short-latency, stimulus-driven, saccades are strongly biased away from the cued location, while the destinations of longer latency goal-driven saccades are affected only minimally. The fact that primarily stimulus-driven saccades are affected by inducing IOR is interesting as it can explain why the spatial bias associated with IOR is not consistently found. (PsycInfo Database Record (c) 2020 APA, all rights reserved).

Brief localised monocular deprivation in adults alters binocular rivalry predominance retinotopically and reduces spatial inhibition.

Short-term deprivation (2.5 h) of an eye has been shown to boost its relative ocular dominance in young adults. Here, we show that a much shorter deprivation period (3-6 min) produces a similar paradoxical boost that is retinotopic and reduces spatial inhibition on neighbouring, non-deprived areas. Partial deprivation was conducted in the left hemifield, central vision or in an annular region, later assessed with a binocular rivalry tracking procedure. Post-deprivation, dominance of the deprived eye increased when rivalling images were within the deprived retinotopic region, but not within neighbouring, non-deprived areas where dominance was dependent on the correspondence between the orientation content of the stimuli presented in the deprived and that of the stimuli presented in non-deprived areas. Together, these results accord with other deprivation studies showing V1 activity changes and reduced GABAergic inhibition.

Time dilation effect in an active observer and virtual environment requires apparent motion: No dilation for retinal- or world-motion alone.

It is known that moving visual stimuli are perceived to last longer than stationary stimuli with the same physical duration (Kanai, Paffen, Hogendoorn, & Verstraten, 2006), and that motor actions (Tomassini & Morrone, 2016) and eye movements (Morrone, Ross, & Burr, 2005) can alter perceived duration. In the present work, we investigated the contributions of stimulus motion and self-motion to perceived duration while observers stood or walked in a virtual reality environment. Using a visual temporal reproduction task, we independently manipulated both the participants' motion (stationary or walking) and the stimulus motion (retinal stationary, real-world stationary and negative double velocity). When the observers were standing still, drifting gratings were perceived as lasting longer than duration-matched static gratings. Interestingly, we did not see any time distortion when observers were walking, neither when the gratings were kept stationary relative to the observer's point of view (i.e., no retinal motion) nor when they were stationary in the external world (i.e., producing the same retinal velocity as the walking condition with stationary grating). Self-motion caused significant dilation in perceived duration only when the gratings were moving at double speed, opposite to the observers' walking direction. Consistent with previous work (Fornaciai, Arrighi, & Burr, 2016), this suggests that the system is able to suppress self-generated motion to enhance external motion, which would have ecological benefits, for example, for threat detection while navigating through the environment.

The duration aftereffect does not reflect adaptation to perceived duration.

Recent studies have provided evidence for a role of duration-tuned channels in the encoding of duration. Duration encoding in these channels is thought to reflect the time between responses to the onset and offset of an event. This notion is in apparent conflict with studies that demonstrate that the perceived duration of an event can vary independently from the time separating its perceived onset and offset. Instead, these studies suggest that duration encoding is sensitive to other temporal aspects of a sensory event. In the current study, we investigated whether duration-tuned channels encode duration based on the time between the on- and offset of an event (onset-offset duration), or if they encode a duration corresponding to the perceived duration of that event. We used a duration illusion to dissociate onset-offset duration and perceived duration and measured whether repeated exposure to illusion-inducing stimuli caused adaptation to the onset-offset duration or the perceived duration of these illusion-inducing stimuli. We report clear evidence for adaptation to the onset-offset duration of illusion-inducing stimuli. This finding supports the notion that duration-tuned mechanisms respond to the time between the onset and offset of an event, without necessarily reflecting the duration perceived, and eventually reported by the participant. Implications for the duration channel model and the mechanisms underlying duration illusions are discussed.

Attention Gates the Selective Encoding of Duration.

The abundance of temporal information in our environment calls for the effective selection and utilization of temporal information that is relevant for our behavior. Here we investigated whether visual attention gates the selective encoding of relevant duration information when multiple sources of duration information are present. We probed the encoding of duration by using a duration-adaptation paradigm. Participants adapted to two concurrently presented streams of stimuli with different durations, while detecting oddballs in one of the streams. We measured the resulting duration after-effect (DAE) and found that the DAE reflects stronger relative adaptation to attended durations, compared to unattended durations. Additionally, we demonstrate that unattended durations do not contribute to the measured DAE. These results suggest that attention plays a crucial role in the selective encoding of duration: attended durations are encoded, while encoding of unattended durations is either weak or absent.

The Lifetime of Salience Extends Beyond the Initial Saccade.

Several models of selection in search predict that saccades are biased toward conspicuous objects (also referred to as salient objects). Indeed, it has been demonstrated that initial saccades are biased toward the most conspicuous candidate. However, in a recent study, no such bias was found for the second saccade, and it was concluded that the attraction of conspicuous elements is limited to only short-latency initial saccades. This conclusion is based on only a single feature manipulation (orientation contrast) and conflicts with the prediction of influential salience models. Here, we investigate whether this result can be generalized beyond the domain of orientation. In displays containing three luminance annuli (Experiment 1), we find a considerable bias toward the most conspicuous candidate for the second saccade. In Experiment 1, the target could not be discriminated peripherally. When we made the target peripherally discriminable, the second saccade was no longer biased toward the more conspicuous candidate (Experiment 2). Thus, conspicuity plays a role in saccadic selection beyond the initial saccade. Whether second saccades are biased toward conspicuous objects appears to depend on the type of feature contrast underlying the conspicuity and the peripheral discriminability of target properties.

Velocity perception in a moving observer.

Previous research has shown that when a moving stimulus is presented to a moving observer, the perceived speed of the stimulus is affected by vestibular self-motion signals (Hogendoorn, Verstraten, MacDougall, & Alais, 2017. Vision Research 130, 22-30.). This interaction was interpreted as a weighted sum of visual and vestibular motion signals. This interpretation also predicts effects of vestibular self-motion signals on perceived speed. Here, we test this prediction in two experiments. In Experiment 1, moving observers carried out a visual speed discrimination task in order to establish points of subjective equality (PSE) between stimuli presented in the same or opposite direction of self-motion. We observed robust effects of self-motion on perceived speed, with self-motion in the same direction as visual motion resulting in increases in perceived speed and vice versa. These effects were well- described by a limited-width integration window. In Experiment 2, the same observers carried out another speed discrimination task in order to establish discrimination thresholds. According to the Weber-Fechner law, these thresholds are expected to increase or decrease along with perceived speed. However, no effect of self-motion on discrimination thresholds was observed. This pattern of results suggests a limit on speed discrimination performance early in the visual system, with visuo-vestibular integration in later downstream areas. These results are consistent with previous work on heading perception.

Representing dynamic stimulus information during occlusion.

Human observers maintain a representation of the visual features of objects when they become occluded. This representation facilitates the interpretation of occluded events and allows us to quickly identify objects upon reappearing. Here we investigated whether visual features that change over time are also represented during occlusion. To answer this question we used an illusion from the time perception domain in which the perceived duration of an event increases as its temporal frequency content increases. In the first experiment we demonstrate temporal frequency induced modulation of duration both when the object remains visible as well as when it becomes temporarily occluded. Additionally, we demonstrate that time dilation for temporarily occluded objects cannot be explained by modulations of duration as a result of pre- and post-occlusion presentation of the object. In a second experiment, we corroborate this finding by demonstrating that modulation of the perceived duration of occluded events depends on the expected temporal frequency content of the object during occlusion. Together these results demonstrate that the dynamic properties of an object are represented during occlusion. We conclude that the representations of occluded objects contain a wide range of features derived from the period when the object was still visible, including information about both the static and dynamic properties of the object.

Who is the Usual Suspect? Evidence of a Selection Bias Toward Faces That Make Direct Eye Contact in a Lineup Task.

The speed and ease with which we recognize the faces of our friends and family members belies the difficulty we have recognizing less familiar individuals. Nonetheless, overconfidence in our ability to recognize faces has carried over into various aspects of our legal system; for instance, eyewitness identification serves a critical role in criminal proceedings. For this reason, understanding the perceptual and psychological processes that underlie false identification is of the utmost importance. Gaze direction is a salient social signal and direct eye contact, in particular, is thought to capture attention. Here, we tested the hypothesis that differences in gaze direction may influence difficult decisions in a lineup context. In a series of experiments, we show that when a group of faces differed in their gaze direction, the faces that were making eye contact with the participants were more likely to be misidentified. Interestingly, this bias disappeared when the faces are presented with their eyes closed. These findings open a critical conversation between social neuroscience and forensic psychology, and imply that direct eye contact may (wrongly) increase the perceived familiarity of a face.

An investigation of the spatial selectivity of the duration after-effect.

Adaptation to the duration of a visual stimulus causes the perceived duration of a subsequently presented stimulus with a slightly different duration to be skewed away from the adapted duration. This pattern of repulsion following adaptation is similar to that observed for other visual properties, such as orientation, and is considered evidence for the involvement of duration-selective mechanisms in duration encoding. Here, we investigated whether the encoding of duration - by duration-selective mechanisms - occurs early on in the visual processing hierarchy. To this end, we investigated the spatial specificity of the duration after-effect in two experiments. We measured the duration after-effect at adapter-test distances ranging between 0 and 15° of visual angle and for within- and between-hemifield presentations. We replicated the duration after-effect: the test stimulus was perceived to have a longer duration following adaptation to a shorter duration, and a shorter duration following adaptation to a longer duration. Importantly, this duration after-effect occurred at all measured distances, with no evidence for a decrease in the magnitude of the after-effect at larger distances or across hemifields. This shows that adaptation to duration does not result from adaptation occurring early on in the visual processing hierarchy. Instead, it seems likely that duration information is a high-level stimulus property that is encoded later on in the visual processing hierarchy.

Vestibular signals of self-motion modulate global motion perception.

Certain visual stimuli can have two possible interpretations. These perceptual interpretations may alternate stochastically, a phenomenon known as bistability. Some classes of bistable stimuli, including binocular rivalry, are sensitive to bias from input through other modalities, such as sound and touch. Here, we address the question whether bistable visual motion stimuli, known as plaids, are affected by vestibular input that is caused by self-motion. In Experiment 1, we show that a vestibular self-motion signal biases the interpretation of the bistable plaid, increasing or decreasing the likelihood of the plaid being perceived as globally coherent or transparently sliding depending on the relationship between self-motion and global visual motion directions. In Experiment 2, we find that when the vestibular direction is orthogonal to the visual direction, the vestibular self-motion signal also biases the direction of one-dimensional motion. This interaction suggests that the effect in Experiment 1 is due to the self-motion vector adding to the visual motion vectors. Together, this demonstrates that the perception of visual motion direction can be systematically affected by concurrent but uninformative and task-irrelevant vestibular input caused by self-motion.

Grouping of optic flow stimuli during binocular rivalry is driven by monocular information.

During binocular rivalry, perception alternates between two dissimilar images, presented dichoptically. Although binocular rivalry is thought to result from competition at a local level, neighboring image parts with similar features tend to be perceived together for longer durations than image parts with dissimilar features. This simultaneous dominance of two image parts is called grouping during rivalry. Previous studies have shown that this grouping depends on a shared eye-of-origin to a much larger extent than on image content, irrespective of the complexity of a static image. In the current study, we examine whether grouping of dynamic optic flow patterns is also primarily driven by monocular (eye-of-origin) information. In addition, we examine whether image parameters, such as optic flow direction, and partial versus full visibility of the optic flow pattern, affect grouping durations during rivalry. The results show that grouping of optic flow is, as is known for static images, primarily affected by its eye-of-origin. Furthermore, global motion can affect grouping durations, but only under specific conditions. Namely, only when the two full optic flow patterns were presented locally. These results suggest that grouping during rivalry is primarily driven by monocular information even for motion stimuli thought to rely on higher-level motion areas.

Faces in Context: Does Face Perception Depend on the Orientation of the Visual Scene?

The mechanisms held responsible for familiar face recognition are thought to be orientation dependent; inverted faces are more difficult to recognize than their upright counterparts. Although this effect of inversion has been investigated extensively, researchers have typically sliced faces from photographs and presented them in isolation. As such, it is not known whether the perceived orientation of a face is inherited from the visual scene in which it appears. Here, we address this question by measuring performance in a simultaneous same-different task while manipulating both the orientation of the faces and the scene. We found that the face inversion effect survived scene inversion. Nonetheless, an improvement in performance when the scene was upside down suggests that sensitivity to identity increased when the faces were more easily segmented from the scene. Thus, while these data identify congruency with the visual environment as a contributing factor in recognition performance, they imply different mechanisms operate on upright and inverted faces.

Revisiting the global effect and inhibition of return.

Saccades toward previously cued locations have longer latencies than saccades toward other locations, a phenomenon known as inhibition of return (IOR). Watanabe (Exp Brain Res 138:330-342. doi: 10.1007/s002210100709 , 2001) combined IOR with the global effect (where saccade landing points fall in between neighboring objects) to investigate whether IOR can also have a spatial component. When one of two neighboring targets was cued, there was a clear bias away from the cued location. In a condition where both targets were cued, it appeared that the global effect magnitude was similar to the condition without any cues. However, as the latencies in the double cue condition were shorter compared to the no cue condition, it is still an open question whether these results are representative for IOR. Considering the double cue condition can provide valuable insight into the interaction of the mechanisms underlying the two phenomena, here, we revisit this condition in an adapted paradigm. Our paradigm does result in longer latencies for the cued locations, and we find that the magnitude of the global effect is reduced significantly. Unexpectedly, this holds even when only including saccades with the same latencies for both conditions. Thus, the increased latencies associated with IOR cannot directly explain the reduction in global effect. The global effect reduction can likely best be seen as either a result of short-term depression of exogenous visual signals or a result of IOR established at the center of gravity of cues.

Intracranial Recordings of Occipital Cortex Responses to Illusory Visual Events.

UNLABELLED: Ambiguous visual stimuli elicit different perceptual interpretations over time, creating the illusion that a constant stimulus is changing. We investigate whether such spontaneous changes in visual perception involve occipital brain regions specialized for processing visual information, despite the absence of concomitant changes in stimulation. Spontaneous perceptual changes observed while viewing a binocular rivalry stimulus or an ambiguous structure-from-motion stimulus were compared with stimulus-induced perceptual changes that occurred in response to an actual stimulus change. Intracranial recordings from human occipital cortex revealed that spontaneous and stimulus-induced perceptual changes were both associated with an early transient increase in high-frequency power that was more spatially confined than a later transient decrease in low-frequency power. We suggest that the observed high-frequency and low-frequency modulations relate to initiation and maintenance of a percept, respectively. Our results are compatible with the idea that spontaneous changes in perception originate from competitive interactions within visual neural networks. SIGNIFICANCE STATEMENT: Ambiguous visual stimuli elicit different perceptual interpretations over time, creating the illusion that a constant stimulus is changing. The literature on the neural correlates of conscious visual perception remains inconclusive regarding the extent to which such spontaneous changes in perception involve sensory brain regions. In an attempt to bridge the gap between existing animal and human studies, we recorded from intracranial electrodes placed on the human occipital lobe. We compared two different kinds of ambiguous stimuli, binocular rivalry and the phenomenon of ambiguous structure-from-motion, enabling generalization of our findings across different stimuli. Our results indicate that spontaneous and stimulus-induced changes in perception (i.e., "illusory" and "real" changes in the stimulus, respectively) may involve sensory regions to a similar extent.