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Zhongchao Tan, Haidong Lu; The critical speed for motion streak processing in early visual cortex. Journal of Vision 2011;11(11):772. doi: 10.1167/11.11.772.
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A fast-moving object is perceived as a streak because of the temporal integrating of the visual system. The explanation of motion streaks in terms of linear filtering process provides support for the observation of cortical response maps under extreme motion velocities. The cortical domains with orientation preference perpendicular to the motion direction are preferentially activated when the dots are moving at low velocities, while those with orientation preference parallel to the motion direction are preferentially activated when the dots are moving at high velocities. However, it is still unclear how stimulus velocity may influence the cortical population activities and to what extent the ensemble spatiotemporal preferences determine this reversal process. Here we used intrinsic optical imaging method to systematically measure the cortical orientation response in cat areas 17 and 18 to drifting dots at different velocities. For both areas, beyond a critical velocity, the cortical activity reversed to patterns with orthogonal orientation preference. We find that the critical reversal velocity is roughly three times higher for area 18 relative to area 17. Our results demonstrated an interesting phenomenon: for an intermediate range of velocities, the evoked cortical activity patterns are reversed between areas 17 and 18. The difference in the reversal speed can arise from the linear feed-forward processing and the spatiotemporal filtering properties of neurons in both visual areas. We suggest that such a co-operation of areas 17 and 18 can explain the psychophysical observation that the perceived critical speed increase with the size of the moving dots. Our perception is therefore a pooling of distributed activities across multiple cortical areas.
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