Cortical activity related to cue-invariant shape perception in humans

We used magnetoencephalography to search spatio-temporally for cortical act ivity 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 fig ures were defined by three Visual cues: difference of flicker, texture or l uminance between the foreground and the background in the random dot patter n. 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 m agnetic component was identified in the responses. The properties of the fi rst magnetic component differed for stimulus condition. The peak latency of the first magnetic component was different for the cues (270 ms for flicke r, 360 ms for texture and 250 ms for luminance), but not for the figures. I n contrast, the peak amplitude of the first magnetic component was differen t 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 po sterior part of the inferior temporal cortex, probably in the fusiform gyru s in four subjects, and in the lateral part of the occipital cortex which w as outside of the primary visual cortex (visual area 1) in one subject. The signal source location was different inter-individually, but almost the sa me within each subject. Reaction time was 471 ms for flicker, 569 ms for te xture 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 ma gnetic component was more clearly recorded from the right hemisphere than f rom 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 conver gence suggests that there is a restricted locus that processes the visual s hape regardless of the difference of the visual cue. The correspondence bet ween the peak latency and the reaction time suggests that the activity of t he area is responsible for the perception of visual shape. The inter-hemisp heric difference suggests a dominance of the right hemisphere in visual sha pe processing. (C) 2000 IBRO. Published by Elsevier Science Ltd. All rights reserved.