On the processing of spatial frequencies as revealed by evoked-potential sourer modeling

Objectives and methods: Visually evoked potentials (VEPs) are known to be s ensitive to spatial frequency, especially in the time range between 50 and 100 ms post-stimulus. In two experiments we localized the cortical activity elicited by stimuli of varying spatial frequency in scalp-recorded brain p otentials, using multi-electrode recordings and dipole-source analysis. Results: Low spatial frequencies (<1 c/d) activated relatively lateral occi pital areas, the orientation of the neural ensembles involved being predomi nantly perpendicular to the scalp surface. In contrast, high spatial freque ncies (>4 c/d) induced activation of more medial occipital areas with the p redominant orientation of the sources being much more parallel to the scalp surface. Furthermore, at about 100 ms latency the lateral-occipital respon se to low spatial frequencies was stronger in the right hemisphere; no such asymmetry was found for the responses to the high spatial frequencies. The se findings were consistent across varying recording conditions, individual subjects, subject populations, stimulus characteristics (grating orientati on, grating vs. checkerboard), and task conditions (active vs. passive). Conclusion: The results indicate that there are differences in sensitivity to specific spatial frequencies between primary and secondary visual areas, as well as between the right and the left hemispheres. (C) 2000 Elsevier S cience Ireland Ltd. All rights reserved.