Perceived global flow direction reveals local vector weighting by luminance

Abstract

Global flow occurs when random dots, each selecting their direction of motion randomly each frame from a distribution of directions spanning up to 180°, appear to move as a whole in the mean direction of the components. This percept arises because the visual system integrates the many independent local motion signals over space and time. Through a series of direction discrimination experiments with random-dot cinematograms (RDCs), we show that varying the luminance of dots over a suprathreshold range profoundly affects perceived direction; the brightest dots appear to be weighted more and dimmer dots weighted less when determining perceived global direction. This effect is not observable if all dots in the display have the same luminance but only when the display contains dots with different luminance values. The results are consistent with energy models of motion detectors whose responses are contrast dependent. A Monte Carlo simulation of global direction discrimination employing a 12-mechanism line-element model that weighted the local motion vectors by the normalized squared contrast of the component dots (a proxy for contrast energy) captured well the features of the experimental data.

Contrast Dependence of Smooth Pursuit Eye Movements following a Saccade to Superimposed Targets

“…The present experiments extend these findings, by characterizing smooth pursuit using spatially overlapping stimuli that varied in luminance contrast, and by examining the temporal dynamics of these changes on post-saccadic smooth pursuit eye movements. These results are also consistent with a recent study showing that global motion processing of a random-dot field is computed by weighting the motion energy of each individual dot by the relative contrast of that dot [14] . In the earlier study, a random-dot cinematogram was presented, and observers perceived global motion flow as biased in favor of the dots with the greatest luminance.…”

“…Despite these differences, both studies are consistent in finding preferential processing of the most salient motion stimuli. Taken together, these studies suggest a common mechanism, operating at different stages of motion processing: the integration of individual elements into global motion [14] as well as motion transparency, as found in the present study.…”

Humans Optimize Decision-Making by Delaying Decision Onset

“…For all trials, the distractor dots had higher contrast relative to background than the target dots. This ensured that the distractor motion was more salient than the target motion [19], [20], making selective attention necessary for the successful performance of the task. The task was presented to the subjects both in the context of the cyclic deadline paradigm and as a free reaction time task.…”

Effects of retinal eccentricity and acuity on global-motion processing

“…The version of the model examined in the present study was adapted from models previously presented by Bennett et al, (2007), Watamaniuk, Sekuler, & McKee (2011), Watamaniuk, Sekuler, & Williams (1989), and Williams, Tweeten & Sekuler (1991). An important characteristic of this model is that it uses one or more mechanisms tuned to a particular aspect of a visual stimulus such as spatial frequency or the direction of local motion.…”

“…The activation of a channel increases exponentially as a function of the correspondence between the pooled motion direction and the optimal motion direction of the channel. Summation of the activation level of all channels can be used to estimate of the direction of global motion (Bennett et al, 2007; Watamaniuk et al, 2011). …”