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Center for Mind and Brain

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Davis, CA 95618

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Flexible Retinotopy

Although the visual cortex is organized retinotopically (adjacent points on your retina are mapped onto adjacent regions in the visual cortex), we show how motion modulates this map.



Examples of stimuli from the experiments



For the display on the left, are the top circles closer than the bottom circles? While it may look like they are, this is actually not the case: both the top and bottom circles are completely aligned. The display on the right is exactly the same except the direction of motion has been reversed.



Drifting stimuli behind a static aperture produce a stronger BOLD response at their trailing edge (origin of motion).



2007

Spatially asymmetric response to moving patterns in the visual cortex: re-examining the local sign hypothesis.

One of the most fundamental functions of the visual system is to code the positions of objects. Most studies, especially those using fMRI, widely assume that the location of the peak retinotopic activity generated in the visual cortex by an object is the position assigned to that object-this is a simplified version of the local sign hypothesis. Here, we employed a novel technique to compare the pattern of responses to moving and stationary objects and found that the local sign hypothesis is false. By spatially correlating populations of voxel responses to different moving and stationary stimuli in different positions, we recovered the modulation transfer function for moving patterns. The results show that the pattern of responses to a moving object is best correlated with the response to a static object that is located behind the moving one. The pattern of responses across the visual cortex was able to distinguish object positions separated by about 0.25 deg visual angle, equivalent to approximately 0.25 mm cortical distance. We also found that the position assigned to a pattern is not simply dictated by the peak activity-the shape of the luminance envelope and the resulting shape of the population response, including the shape and skew in the response at the edges of the pattern, influences where the visual cortex assigns the object's position. Therefore, visually coded position is not conveyed by the peak but by the overall profile of activity.

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2003

Flexible retinotopy: motion-dependent position coding in the visual cortex.

Although the visual cortex is organized retinotopically, it is not clear whether the cortical representation of position necessarily reflects perceived position. Using functional magnetic resonance imaging (fMRI), we show that the retinotopic representation of a stationary object in the cortex was systematically shifted when visual motion was present in the scene. Whereas the object could appear shifted in the direction of the visual motion, the representation of the object in the visual cortex was always shifted in the opposite direction. The results show that the representation of position in the primary visual cortex, as revealed by fMRI, can be dissociated from perceived location.

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