Effect of spatial context on perceived walking direction.

Contextual modulation occurs for many aspects of high-level vision but is relatively unexplored for the perception of walking direction. In a recent study, we observed an effect of the temporal context on perceived walking direction. Here, we examined the spatial contextual modulation by measuring the perceived direction of a target point-light walker in the presence of two flanker walkers, one on each side. Experiment 1 followed a within-subjects design. Participants (n = 30) completed a spatial context task by judging the walking direction of the target in 13 different conditions: a walker alone in the center or with two flanking walkers either intact or scrambled at a flanker deviation of ±15°, ±30°, or ±45°. For comparison, participants completed an adaptation task where they reported the walking direction of a target after adaptation to ±30° walking direction. We found the expected repulsive effects in the adaptation task but attractive effects in the spatial context task. In Experiment 2 (n = 40), we measured the tuning of spatial contextual modulation across a wide range of flanker deviation magnitudes ranging from 15° to 165° in 15° intervals. Our results showed significant attractive effects across a wide range of flanker walking directions with the peak effect at around 30°. The assimilative versus repulsive effects of spatial contextual modulation and temporal adaptation suggest dissociable neural mechanisms, but they may operate on the same population of sensory channels coding for walking direction, as evidenced by similarity in the peak tuning across the walking direction of the inducers.

Colour information biases facial age estimation and reduces inter-observer variability.

Age estimation is a perceptual task that people perform automatically and effortlessly on a daily basis. Colour has been identified as one of the facial cues that contributes to age perception. To investigate further the role of colour in age perception, we manipulated the chromatic content of facial images holistically. In Experiment 1, images were shown in colour or grey scale; in Experiment 2, images were shown with red-green contrast increased or decreased; in Experiment 3, images were shown with modified yellow-blue contrast. We examined whether the presence of chromatic information biases the perception of age and/or affects inter-observer variability in age judgements, and whether specific chromatic information affects the perception of age. We found that the same face tended to be judged as younger with increased red-green contrast compared to decreased red-green contrast, suggesting that red-green contrast directly affects age perception. Inter-observer variability in age ratings was significantly lower when participants were asked to rate colour compared with grey scale versions of images. This finding indicates that colour carries information useful cues for age estimation.

Adaptation to walking direction in biological motion.

The direction that we see another person walking provides us with an important cue to their intentions, but little is known about how the brain encodes walking direction across a neuronal population. The current study used an adaptation technique to investigate the sensory coding of perceived walking direction. We measured perceived walking direction of point-light stimuli before and after adaptation, and found that adaptation to a specific walking direction resulted in repulsive perceptual aftereffects. The magnitude of these aftereffects was tuned to the walking direction of the adaptor relative to the test, with local repulsion of perceived walking direction for test stimuli oriented on either side of the adapted walking direction. The specific tuning profiles that we observed are well explained by a population-coding model, in which perceived walking direction is coded in terms of the relative activity across a bank of sensory channels with peak tuning distributed across the full 360° range of walking directions. Further experiments showed specificity in how horizontal (azimuth) walking direction is coded when moving away from the observer compared to when moving toward the observer. Moreover, there was clear specificity in these perceptual aftereffects for walking direction compared to a nonbiological form of 3D motion (a rotating sphere). These results indicate the existence of neural mechanisms in the human visual system tuned to specific walking directions, provide insight into the number of sensory channels and how their responses are combined to encode walking direction, and demonstrate the specificity of adaptation to biological motion. (PsycInfo Database Record (c) 2023 APA, all rights reserved).

The long and short of it? A novel geometric illusion.

Here we present what we believe to be a novel geometric illusion where identical lines are perceived as being of differing lengths. Participants were asked to report which of the two parallel rows of horizontal lines contained the longer individual lines (two lines on one row and 15 on the other). Using an adaptive staircase we adjusted the length of the lines on the row containing two to estimate the point of subjective equality (PSE). At the PSE, the two lines were consistently shorter than the row containing the fixed length of 15 lines demonstrating a disparity in perceived length such that lines of identical length are perceived as longer in a row of two than in a row of 15. The illusion magnitude was unaffected by which row was presented above the other. Additionally, the effect persisted when using one as opposed to two test lines, and when the line stimuli on both rows were presented with alternating luminance polarity the illusion magnitude decreased, but was not abolished. The data indicate a robust geometric illusion that may be modulated by perceptual grouping processes.

PyTrack: An end-to-end analysis toolkit for eye tracking.

Eye tracking is a widely used tool for behavioral research in the field of psychology. With technological advancement, we now have specialized eye-tracking devices that offer high sampling rates, up to 2000 Hz, and allow for measuring eye movements with high accuracy. They also offer high spatial resolution, which enables the recording of very small movements, like drifts and microsaccades. Features and parameters of interest that characterize eye movements need to be algorithmically extracted from raw data as most eye trackers identify only basic parameters, such as blinks, fixations, and saccades. Eye-tracking experiments may investigate eye movement behavior in different groups of participants and in varying stimuli conditions. Hence, the analysis stage of such experiments typically involves two phases, (i) extraction of parameters of interest and (ii) statistical analysis between different participants or stimuli conditions using these parameters. Furthermore, the datasets collected in these experiments are usually very large in size, owing to the high temporal resolution of the eye trackers, and hence would benefit from an automated analysis toolkit. In this work, we present PyTrack, an end-to-end open-source solution for the analysis and visualization of eye-tracking data. It can be used to extract parameters of interest, generate and visualize a variety of gaze plots from raw eye-tracking data, and conduct statistical analysis between stimuli conditions and subject groups.

Splitting time: Sound-induced illusory visual temporal fission and fusion.

Auditory stimuli have been shown to alter visual temporal perception. For example, illusory temporal order is perceived when an auditory tone cues one side of space prior to the onset of simultaneously presented visual stimuli. Competing accounts attempt to explain such effects. The spatial gradient account of attention suggests speeded processing of visual stimuli in the cued space, whereas the impletion account suggests a Gestalt-like process where an attempt is made to arrive at a "realistic" representation of an event given ambiguous conditions. Temporal ventriloquism-where visual temporal order judgment performance is enhanced when a spatially uninformative tone is presented prior to, and after, visual stimuli onset-argues that the temporal relationship of the auditory stimuli to visual stimuli, as well as the number of auditory stimuli equaling the visual stimuli, drives the mechanisms underlying these and related effects. Results from a series of experiments highlight putative inconsistencies in both the spatial gradient account of attention and the classical temporal ventriloquism account. We present novel behavioral effects-illusory temporal order via spatially uninformative tones, and illusory simultaneity via a single tone prior to visual stimuli onset-that can be accounted for by an expanded version of the impletion account. (PsycINFO Database Record (c) 2020 APA, all rights reserved).