FIGURE 1.

Representative examples of VEP waveform to onset (300 milliseconds) – offset (700 milliseconds) horizontal sinusoidal gratings (spatial frequency two cycles per degree, field size 14 ×16 degrees) with different levels of pure chromatic contrast (RG: red-green; BY: blue-yellow; BW: black-white); two traces are superimposed for each stimulating pattern to ensure reliability. The upper line show responses to 90% contrast and the lower line to 25% contrast for all the three types of stimuli. Note that chromatic responses are represented mainly by a negative-positive complex (N1-P1;) whose amplitude and latency change substantially with contrast. Vertical lines indicate upper normal limits using 2 SD. Positivity is up.