| I have addressed the physiological mechanisms for, and perceptual consequences of, integrating visual motion. Where possible, I have tried to determine the rules by which the visual system decides whether to integrate or not. My first set of experiments was motivated by the following observations. Humans and primates can see motions at small and large scales. Also, neurons in area MT have large receptive fields, which are known to play a role in the perception of visual motion. I conducted a series of electrophysiological experiments to determine whether MT neurons compute global motion, which was defined in terms of widely separated apparent motion. I used stimuli in which there could be opposing local and global motion. I found that MT neurons are unaffected by global motion, that their responses are entirely determined by local motion. My control experiments suggest that they do not compute global motion even in the absence of local motion. My second set of experiments concerned how the visual system decides whether to integrate or segment motions. I presented drifting square-wave plaids and asked subjects to indicate whether they appeared to move coherently, as a single object, or transparently, as two objects moving in different directions. I found that a plaid's component and pattern speed affected how it was perceived. Plaids were more transparent at faster pattern speeds and were coherent otherwise. I developed a Bayesian model that can explain these results. Key components of the model are based on preferences of the system to see slow and singular motion. My final set of questions was motivated by the idea that adaptation causes repulsion by reducing the gain of mechanisms that encode properties of a stimulus. In psychophysical experiments, I measured the pattern of biases in perceived direction that result from adapting to coherent and transparent drifting square-wave plaids. My results suggest that adapting to plaids causes repulsion away from their pattern directions, even when they are not perceived. |
