Binocular disparity is one of the most important cues for depth perception in humans. Previously, some of the authors performed electroencephalogram (EEG) measurement in subjects viewing random dot stereograms (RDSs) with 3 binocular disparities: no, small or large disparity, and estimated the brain sites where the visual information was processed for human stereopsis, using equivalent current dipole source localization (ECDL) method. The results showed that: 1) the postcentral gyrus (PstCG) is involved in visual processing of stereopsis; and 2) all-channel average EEGs converge and the convergence time for larger RDS disparity is longer than that for smaller one.
With reference with the above findings, the present study divided the cerebral visual processing for stereopsis into the processing before and after when PstCG is estimated, and compared the brain sites and their temporal transition in observing RDSs with different disparities. Subjects wore crystal shutter glasses for observing RDSs and an electrode cap for EEG measurement. By difference waves between small and no disparities and between large and no disparities, temporal features for cerebral processing of the RDSs with disparities were extracted.
Application of the ECDL method to the average data for small and large disparities revealed that the visual processing before the PstCG localization consists of two pathways: one is from V1 to V4 and then to the TE field; and the other from V1 to the MT field and then to the PstCG. This result did not depend on the amount of disparity. After the PstCG localization, ECDs were localized to the superior colliculus (SC) and frontal visual field (FEF), both of which are involved in ocular movements. At the interval between the FEF localization and the EEG convergence, ECDs were located at the inferior frontal gyrus (IFG) and middle frontal gyrus (MFG). For RDSs with large disparities, the IFG and MFG ECDs were estimated earlier than those for small disparities, while for large disparities the convergence time and the time when ECDs were localized to the IFG just before the convergence time were later than those for small disparities.
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