Alternating orientation of the chromatic pattern visual evoked potential improves signal, even in the absence of contrast adaptation.

The visual evoked potential (VEP) to chromatic pattern reversal is greatly reduced compared to VEPs to pattern onsets. Chromatic pattern onsets produce large and stereotypical waveforms that reliably differ from standard achromatic pattern reversal VEP waveforms used in clinical applications. Rapid contrast adaptation for sustained chromatic but not transient achromatic mechanisms has been suggested as one explanation for these observations. Here we first examined changes in the magnitude of response during recordings to reversing and onset grating patterns that preferentially modulate the L-M, S, and achromatic pathways. VEPs were recorded for 60 s with 2 onsets/reversals per second using both fixed and alternating (horizontal/vertical) orientations. We hypothesized that contrast adaptation may be reduced by changing the orientation and/or phase of the pattern for each reversal or onset. However, responses for 6-second windows did not reveal evidence of adaptation for chromatic or achromatic onsets or reversal patterns over the 60-second recording period. Despite this, alternating pattern orientation and/or phase improved the signal amplitudes for chromatic onset and achromatic reversal conditions. Onset responses were larger for all conditions, even for achromatic stimuli. Additionally, alternating pattern orientation and/or phase produced shorter response latency for all onset conditions. The responses to achromatic and LM reversals and S onset conditions with orientation or phase were larger than responses to horizontal or vertical fixed patterns. In most cases, both orientation and phase changes improve the chromatic onset responses equivalently. Mechanisms other than contrast adaptation must be invoked to explain the results.