Abstract
A number of studies have revealed that the visual system can rapidly compute average orientation in stochastic stimuli. The recent studies have also analyzed how humans estimate the average orientation in dynamic stimuli across time. The present study investigated visual mechanisms underlying the discrimination of average orientation over space and time. Visual stimulus was a dynamic texture, in which 4 or 32 frames of texture patterns were serially presented with a framerate of 33 ms. Each frame of texture was composed of 70 Gabor elements (2.3 c/deg) that were randomly distributed within a circular window of 10.7 deg diameter. The orientation of elements in each texture frame was varied according to a Gaussian distribution with a standard deviation (spatial SD) of 0, 4, 8, or 16 deg, and the spatial mean orientation of each texture frame was varied according to a particular mean and a standard deviation (temporal SD) of 0, 4, or 8 deg. Five observers viewed the dynamic texture at fovea, and indicated whether its spatiotemporal average of orientation was tilted clockwise or counter-clockwise. The spatiotemporal mean of dynamic texture was controlled by a staircase procedure, and the discrimination thresholds was estimated. It was found that discrimination threshold increased as a function of spatial SD when temporal SD is small. When temporal SD is large, however, threshold was nearly constant over a wide range of spatial SD, or even decreased as spatial SD increased in some observers. This is clearly inconsistent with the assumption that threshold is determined by spatiotemporal SD of orientation in the dynamic texture. The results suggest distinct mechanisms involved in computation of spatial average and temporal average of orientation.
Acknowledgement: Supported by the Commissioned Research of National Institute of Information and Communications Technology, and by JSPS KAK-ENHI JP15H05916, JP15H03461 and JP16J07259.