Interactions between contrast and spatial displacement in visual motion processing

SummaryThe scaling of behavioral performance and neuronal responses with visual luminance contrast is one of the most basic and well-accepted aspects of visual processing [1,2]. This relationship between contrast and visual perception can be experienced informally (for example, by driving in fog), and numerous studies have confirmed that many visual skills are impaired under low-contrast conditions. A number of studies, however, have found a more complex interaction between contrast and visual processing. For single neurons, the spatiotemporal structure of visual receptive fields is different under conditions of low and high contrast [3–9], and psychophysical experiments have revealed counterintuitive cases where low-contrast conditions change the content [10] and even improve the accuracy [11] of visual perception. We have evaluated human subjects' ability to estimate the direction of moving dot fields presented at high or low contrast and compared these results to neural responses recorded from cortical area MT of alert macaque monkeys. The results reported here show that motion stimuli involving small spatial displacements yield better visual performance at lower contrast, and that this improvement is mirrored in the activity of MT neurons. These data link responses in area MT with behavioral performance and demonstrate that brighter is not always better for motion processing