A study of the effects of contrast change on pattern VEPS, and the transition between onset, reversal and offset modes of stimulation.

We studied the relationship between the visual evoked potential (VEP) components by tracing the transition from onset/offset mode of stimulation to the reversal mode by a series of contrast change steps. VEPs on the ipsilateral and contralateral side of the scalp with respect to the left half-field checkerboard stimulus (checksizes: 12′, 50′ and 80′) were recorded in 15 subjects. Eight contrast steps for each checksize were recorded. Each test step consisted of the alternation of a constant high contrast checkerboard (A), with a second checkerboard (B) in which the contrast was changed. Checkerboard B was initially of identical spatial phase to A, but contrast was reduced systematically until B was a uniform grey field (i.e. onset/offset). In subsequent steps checkerboard B was of opposite spatial phase and contrast was increased until the final step when B was of equal high contrast (i.e. full reversal). All ipsilateral and contralateral onset components, ipsilateral offset components, and the reversal P100 component significantly enlarged with increasing contrast. The extent of amplitude change with contrast was greatest for offset, followed by onset and then reversal. A clear association could be discerned between all offset and reversal components. Onset CI and CII appeared to be related to the reversal P100 and N145, respectively. When small 12′ checks were used, onset Co appeared to have common features with the reversal N80. The contralateral onset P105 component did not have a comparable component in the reversal mode.