hMT+ Activity Predicts the Effect Of Spatial Attention On Surround Suppression

Surround suppression refers to the decrease in behavioral sensitivity and neural response to a central stimulus due to the presence of surrounding stimuli. Several aspects of surround suppression in human motion perception have been studied in detail, including its atypicality in some clinical populations. However, how the extent of spatial attention affects the strength of surround suppression has not been systematically studied before. To address this question, we presented human participants with center and surround drifting gratings and sought to find whether attending only to the center ( narrow attention) versus both to the center and surround ( wide attention) modulates the suppression strength in motion processing. Using psychophysics and functional magnetic resonance imaging (fMRI), we measured motion direction discrimination thresholds and cortical activity in the primary visual cortex (V1) and middle temporal complex (hMT+). We found increased perceptual thresholds and, thus, stronger surround suppression under the wide attention condition. We also found that the pattern of hMT+ activity was consistent with the behavioral results. Furthermore, a mathematical model that combines spatial attention and divisive normalization was able to explain the pattern in the behavioral and fMRI results. These findings provide a deeper understanding of how attention affects center-surround interactions and suggest possible neural mechanisms with relevance to both basic and clinical vision science. Significance StatementOur sensitivity to the drift direction of a grating decreases when it is surrounded by an iso-oriented high-contrast surround. Here, we report the first systematic neuroimaging study on how the extent of spatial attention modulates the strength of this surround suppression. We found that the strength of surround suppression increases as the extent of spatial attention increases, and, pointing to its critical role, hMT+ activity predicts these results. Furthermore, a mathematical model combining spatial interactions and attention could explain the results. In addition to their relevance to basic vision science, these findings have clinical implications because they suggest that the atypical surround suppression observed in some populations could be related to abnormal spatial attention mechanisms.