Sequential pattern-onset, -reversal and -offset VEPs: comparison of effects of checksize.

The effects of checksize on individual components of half-field pattern-onset, -reversal and -offset VEPs were studied by presenting these three modes of stimulation sequentially in a single recording epoch so as to provide near identical recording and subject conditions. Components on the side of the scalp ipsilateral and contralateral to the stimulated half-field were measured so as to separate the macular and paramacular contributions. Ten checksizes were used (6'-110'). MANOVA showed a significant effect of checksize: small checksizes enhanced onset ipsilateral CII and contralateral P105, as well as all ipsilateral reversal components and offset N85 component, suggesting they are reflecting macular function. Onset CI, and contralateral reversal N105 and offset N115 components increased in amplitude with increasing checksize, suggesting they are predominantly of paramacular origin. The morphology of the contralateral onset P105 component was checksize dependent: small checks (< 35') produced a sharply defined positivity (macular sub-component), whereas larger checks produced a broadened/bifid waveform, suggesting the emergence of a later paramacular sub-component.