Perceived motion of contrast-modulated gratings: Predictions of the multi-channel gradient model and the role of full-wave rectification

AbstractThe paper examines the perception of motion in contrast-modulated sine-wave grating patterns. These non-rigid motion patterns give rise to a spatially-structured motion percept in which perceived speed varies with spatial position. We measured the perceived motion of the low contrast regions of amplitude-modulated gratings as a function of the carrier frequency, the carrier speed, the shape of the modulation signal and the modulation depth. We found that for the static carriers perceived speed was greatest in the low contrast regions of the display. The speed of the low contrast regions was underestimated and perceived speed decreased as the spatial frequency of the carrier increased. When the direction of the motion of the carrier was opposite to that of the contrast modulation, the low contrast regions could appear to be stationary. The perceived speed of the contrast modulation increased with modulation depth. The brightness contrast of the carrier grating had little effect on perceived speed of contrast-modulated patterns for average contrasts of over 10%. A motion model which had full-wave rectification as an explicit pre-processing stage followed by low-pass filtering or some other selection criterion, would predict that the motion of contrast-modulated gratings should appear rigid and that the motion of the envelope should be judged correctly. The Multi-channel Gradient Model however predicts both the structured motion field experienced when viewing these second-order motion patterns and the reductions in perceived speed as a function of carrier spatial frequency and carrier speed

“Are you looking at me?” How children’s gaze judgments improve with age

Adults’ judgments of another person’s gaze reflect both sensory (e.g., perceptual) and nonsensory (e.g., decisional) processes. We examined how children’s performance on a gaze categorization task develops over time by varying uncertainty in the stimulus presented to 6- to 11- year-olds (n = 57). We found that younger children responded “direct” over a wider range of gaze deviations. We also found that increasing uncertainty led to an increase in direct responses, across all age groups. A simple model to account for these data revealed that although younger children had a noisier sensory representation of the stimulus, most developmental changes in gaze were because of a change in children’s response criteria (category boundaries). These results suggest that although the core mechanisms for gaze processing are already in place by the age of 6, their development continues across the whole of childhood. (PsycINFO Database Record (c) 2016 APA, all rights reserved

Adaptive motion analysis in machine and biological vision

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Visual representation of eye gaze is coded by a non-opponent multichannel system

To date, there is no functional account of the visual perception of gaze in humans. Previous work has demonstrated that left gaze and right gaze are represented by separate mechanisms. However, these data are consistent with either a multichannel system comprising separate channels for distinct gaze directions (e.g., left, direct, and right) or an opponent-coding system in which all gaze directions are coded by just 2 pools of cells, one coding left gaze and the other right, with direct gaze represented as a neutral point reflecting equal activation of both left and right pools. In 2 experiments, the authors used adaptation procedures to investigate which of these models provides the optimal account. Both experiments supported multichannel coding. Previous research has shown that facial identity is coded by an opponent-coding system; hence, these results also demonstrate that gaze is coded by a different representational system to facial identity

The effect of TMS on visual motion sensitivity: an increase in neural noise or a decrease in signal strength?

The underlying mechanisms of action of transcranial magnetic stimulation (TMS) are still a matter of debate. TMS may impair a subject's performance by increasing neural noise, suppressing the neural signal, or both. Here, we delivered a single pulse of TMS (spTMS) to V5/MT during a motion direction discrimination task while concurrently manipulating the level of noise in the motion stimulus. Our results indicate that spTMS essentially acts by suppressing the strength of the relevant visual signal. We suggest that TMS may induce a pattern of neural activity that complements the ongoing activation elicited by the sensory signal in a manner that partially impoverishes that signal