Characterizing surround suppression in motion direction perception

Visual perception is often achieved by surround modulation mechanisms, which help us pool individual information in our visual field. This mechanism is also prominent in motion perception, namely motion discrimination. A center-surround antagonistic organization aids motion perception using these modula-tion mechanisms, which are facilitation or suppression. Decisions to which modu-lation type will take place are usually manipulated by changing size and contrast of drifting Gabor disc stimuli. Because a systematic investigation of these mech-anisms is not conducted in prior research, we have used three different types of stimulus (small disc, annular, and large disc grating) to see whether there is a fa-cilitative or suppressive pattern between duration thresholds of these stimuli. To achieve this, we performed a behavioral study, that would assess motion discrim-ination thresholds of human participants for these stimuli via adaptive staircase procedures. Then, we looked for three possible pooling regimes by comparing duration thresholds of small disc-annular-large disc grating stimulus sets: effi-cient or inefficient (weak or strong suppression) pooling. We found that duration thresholds of large disc gratings were almost greater than small disc or annular gratings almost all the time in both contrast levels, which indicated a strong sup-pression. We have also conducted simulations of divisive normalization models and observed that modulation mechanisms were more prominent in simulated duration thresholds using MT parameters from literature than V1. Although previous literature suggests that MT region produces facilitative or suppressive effects within itself, we are not able to postulate a specific brain region creating strong suppressive pooling we observed in our behavioral data. This is why this study is a critical next step for future neuroimaging studies.