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Abstract:
A functional hallmark of cortical neurons sensitive to visual motion is the dependency of their firing rate on motion strength. Specifically, single unit activity in macaque areas MT and MST is known to monotonically increase with motion strength such as motion coherence defined by the percentage of dot elements of a random dot kinematogram (RDK) moving in the same direction. Despite the strong similarities of the visual cortex of human and nonhuman primates, human area MT+, located in the posterior part of the inferior temporal sulcus (pITS) and probably comprising both areas MT and MST, has not consistently been found to respond stronger to coherent as compared to incoherent motion. A first goal of the present functional MRI study was to assess the responses to coherent and incoherent visual motion for both areas, separately. To this end, we exploited the fact that area MST lays immediately anterior to area MT and that only the first receives signals from both visual hemifields. A second goal was to test the influence of stimulus size. Blood oxygenation level depend (BOLD) responses were obtained from
eleven human subjects who observed a RDK (stationary dots, incoherent motion, or coherent motion) in the right visual hemifield during stationary fixation. For a fixed eccentricity the stimulus extensions were varied in such a way to cover an area either equaling, exceeding, or falling below the mean MT receptive field size of macaque area MT [1]. Unlike the posterior part of left-hemisphere pITS, the anterior part and its right-hemisphere homolog showed significantly stronger responses to coherent as compared to incoherent motion. These differences were only present for stimuli larger than the estimated MT receptive field size. Based on these findings, we suggest that functional MRI may reveal stronger responses to coherent visual motion
in human area MST provided that the stimulus allows for sufficient summation within the receptive fields. In contrast fMRI may fail to reveal the same dependency for area MT.
