Cortical activity related to cue-invariant shape perception in humans

We used magnetoencephalography to search spatio-temporally for cortical activity related to the perception of shape defined by various visual cues in humans. The visual stimuli were three kinds of two-dimensional figures: two had fixed shapes (Diamond and Cross), the other did not (Noise). These figures were defined by three visual cues: difference of flicker, texture or luminance between the foreground and the background in the random dot pattern. Using this stimulus, we recorded the magnetic responses from the temporo-occipital regions of nine healthy subjects. Additionally, we measured the reaction time for the subjects to detect the figure by button-pressing. A magnetic component was identified in the responses. The properties of the first magnetic component differed for stimulus condition. The peak latency of the first magnetic component was different for the cues (270 ms for flicker, 360 ms for texture and 250 ms for luminance), but not for the figures. In contrast, the peak amplitude of the first magnetic component was different for the figures (96-144 fT for Diamond or Cross and 52-80 fT for Noise), but not for the cues. The signal source of the first magnetic component was estimated to lie on the ventral side of the extrastriate cortex: In the posterior part of the inferior temporal cortex, probably in the fusiform gyrus in four subjects, and in the lateral part of the occipital cortex which was outside of the primary visual cortex (visual area 1) in one subject. The signal source location was different inter-individually, but almost the same within each subject. Reaction time was 471 ms for flicker, 569 ms for texture and 426 ms for luminance, but the interval between the reaction time and the peak latency was constant (about 200 ms) for each cue. The first magnetic component was more clearly recorded from the right hemisphere than from the left.We found that the shape defined by the different visual cues activates the same localized site in the lateral extrastriate cortex. This spatial convergence suggests that there is a restricted locus that processes the visual shape regardless of the difference of the visual cue. The correspondence between the peak latency and the reaction time suggests that the activity of the area is responsible for the perception of visual shape. The inter-hemispheric difference suggests a dominance of the right hemisphere in visual shape processing.

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