CHECK SIZE DEPENDENCY OF THE SOURCES OF THE HEMIFIELD-ONSET EVOKED-POTENTIAL

The checkerboard-onset evoked potential does not obtain its adult wave form until late puberty. The changing waveform results from the develo pment of the underlying sources originating in distinct areas of the v isual cortex. Since the waveform of checkerboard-onset evoked potentia l also varies with check size, we studied the dependency of the activi ty of these sources on check size. A dipole source localization proced ure yielded the position and orientation of the equivalent dipoles and the constituent components of the pattern-onset evoked potential, eac h corresponding to one of these dipoles. For every check size used, th e checkerboard-onset evoked potential could be described by a summatio n of the relative amplitudes of these components. Since the relative a mplitude versus check size curves showed a different behavior for each source, they provided evidence of functionally distinct cortical gene rators. The strength of the striate source was especially sensitive to the fine structure of a pattern, whereas the extrastriate sources con tributed mainly for coarse pattern elements.