Extrastriate loci for the perception of stereo depth and illusory contours: Evidence from fMRI

Purpose. We have previously demonstrated localized regions in human cortex associated with stereo depth and illusory contours. In this study we aimed to use a high-resolution MRI method for direct visualization of the Stria of Gennari ² to determine the relationship of these areas to primary visual cortex. Methods. Images were acquired on a GE Signa 1.5T scanner equipped for echoplanar imaging and a resolution of 1.5x1.5mm². Multiple axial slices were acquired during baseline, visual stimulation, and post stimulation periods. Signal intensities were compared during baseline and stimulation periods to identify "active" voxels. Stimulation conditions included: 1) red-green random spot anaglyphs that produced a stereo-defined square; 2) a random spot pattern with no stereo, 3) four pacman at the corners of a Kanizsa square (illusory contours) and 4) four non-aligned pacmen. Results. The activity elicited by stereo-contours and illusory contours was observed predominately in right occipital cortex. These areas were outside the areas of visual cortex in which the Stria of Gennari was visualized. Conclusion. Extra-striate cortical areas in human brain are active during the perception of stereo depth and illusory contours.