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Hyun-Ah Lee, Jae-Myung Jang, Sang-Hun Lee; Direction tuning curves of motion adaptation in the visual cortex revealed by an event-related fMRI study. Journal of Vision 2005;5(8):667. doi: 10.1167/5.8.667.
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BACKGROUND Although a sampling unit of fMRI contains subpopulations of cells with diverse response selectivities, the fMRI adaptation paradigm (Grill-Spector et al., 1999) provides a means to reveal selectivities of a subpopulation of cells by measuring the extent to which cortical responses are reduced by adaptation to a specific stimulus feature. We exploited this paradigm further by conducting fMRI experiments in conjunction with a linear population model (Maurer et al., 2004) to estimate direction tuning curves of individual cells in motion-sensitive areas. METHOD A stimulus display consisted of distributed patches of dots moving in a specific direction. Two types of events-stimulus and blank-were randomly mixed in each scan. A stimulus event was composed of four epochs in the following order: a 1.5-s adapting epoch, a 0.9-s blank epoch, a 0.9-s test epoch, and a 3.9-s blank epoch. Both in the adapting and the test epochs subjects viewed moving dots and performed the speed discrimination task for attention control. Blank events, identical to stimulus events except that there was no test epoch, were included to isolate responses to the test epoch only. The direction difference between adapting and test dots was varied to assess the direction selectivity of adaptation. RESULT The amount of adaptation was maximized at 0° direction difference, and gradually dropped as the direction difference increased, resulting in a tuning curve with a Gaussian half-width of 60°∼140°. The tuning half-width was narrower in MT (60°) than in V1 (140°) while the amplitude was greater in MT (.61) than in V1 (.37). With a linear population model we derived the tuning curves of ‘representative’ responses of individual cells, which compared moderately well to those from electrophysiological studies (Kohn & Movshon, 2004). CONCLUSION An fMRI adaptation experiment combined with a linear population model can depict the detailed nature of direction selectivity of motion-sensitive cells.
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