Check size dependency of the sources of the hemifield-onset evoked potential.

The checkerboard-onset evoked potential does not obtain its adult waveform until late puberty. The changing waveform results from the development of the underlying sources originating in distinct areas of the visual cortex. Since the waveform of checkerboard-onset evoked potential also varies with check size, we studied the dependency of the activity of these sources on check size. A dipole source localization procedure yielded the position and orientation of the equivalent dipoles and the constituent components of the pattern-onset evoked potential, each corresponding to one of these dipoles. For every check size used, the checkerboard-onset evoked potential could be described by a summation of the relative amplitudes of these components. Since the relative amplitude versus check size curves showed a different behavior for each source, they provided evidence of functionally distinct cortical generators. The strength of the striate source was especially sensitive to the fine structure of a pattern, whereas the extrastriate sources contributed mainly for coarse pattern elements.