Asymmetry in visual evoked potentials to gratings registered in the two hemispheres of the human brain.

The study aimed at testing, by a visual evoked potential method, the hypothesis of the hemispheric specialization in processing of high and low-spatial frequencies. Twenty four right-handed subjects (12 males and 12 females) were presented with square-wave vertical gratings of various spatial frequencies (0.67, 0.86, 1.20, 2.00, 2.40, 3.00, 3.30, 6.00 and 7.50 c/deg). Gratings were presented in nine separate blocks each containing 64 exposures. Time of exposure was 30 ms and the interstimulus interval varied from 2 to 3.5 s. VEPs were recorded with electrodes located at O1 and O2 and referred to Cz according to the 10/20 system. Amplitudes and latencies of two VEPs components (N130-150 and P200-240) were analyzed. The results showed larger amplitudes of VEPs registered in the right hemisphere of both males and females. This difference, however, was apparent in the earlier component of VEPs in females and in the later component in males. The observed hemispheric asymmetry did not depend on the spatial frequency of grating. Females demonstrated longer latencies than males for both N and P components. Our data suggest that the right hemisphere predominates in processing grating stimuli, but the dynamics of this process differ in the two sexes. The results do not support Sergent's hypothesis which postulate the right hemisphere specialization for low spatial frequencies and the left hemisphere specialization for high spatial frequencies.