Visual motion processing in the human cortex: Attention, adaptation, and functional subdivision.

摘要

This dissertation examines how information about visual motion is represented in the human cortex. Three complementary approaches are utilized to examine the computations performed by motion-responsive regions in the human brain, all of which employ functional magnetic resonance imaging (fMRI) to noninvasively measure cortical activity. First, the effects of attention on cortical activity are quantified to assess the importance of controlling the behavioral state of subjects in neuroimaging experiments. Second, motion adaptation is used to characterize the degree of direction-selectivity in multiple brain regions, to test a simple model of the representation of the direction of visual motion, and to isolate neuronal subpopulations responsive to the direction of motion independent of the particular stimulus pattern (“pattern cells”). Third, the retinotopic organization of the human MT-complex (MT+), a particularly-motion responsive region, is assessed in order to subdivide it into regions that may be homologous to areas MT and MST found in the macaque superior temporal sulcus. Together, these studies provide evidence that: (1) attention to motion can exert strong and specific modulations of cortical activity (an effect that has confounded the interpretation of several previous neuroimaging experiments); (2) many human visual areas contain significant proportions of direction-selective neurons; (3) the direction of perceived motion is represented in the ratio of responses of neuronal subpopulations with different preferred directions; (4) human MT+ plays a special role in motion perception, exhibiting particularly high direction-selectivity and selectivity for the direction of motion independent of stimulus pattern; and (5) human MT+ can be functionally subdivided on the basis of retinotopic organization and receptive field size.