Amplitude of the transient visual evoked potential (tVEP) as a function of achromatic and chromatic contrast: contribution of different visual pathways.

We investigated how the stimulation mode influences transient visual evoked potentials (tVEP) amplitude as a function of contrast of achromatic and isoluminant chromatic gratings. The chromatic stimulation probed only responses to the red-green axis. Visual stimuli were monocularly presented in a 5 degrees diameter circle, achromatic and chromatic horizontal gratings, 1 Hz pattern reversal stimulation, and achromatic and chromatic gratings, 300 ms onset per 700 ms offset stimulation. For the achromatic pattern reversal stimulation, a double slope function describes how the P100 amplitude varied as a function of log contrast which had a limb at low-to-medium contrasts and another limb at high contrasts. For the achromatic onset/offset stimulation, C2 amplitude saturated at the highest contrast tested and a single straight line described how it changed along most of the contrast range. Both presentation modes for chromatic gratings resulted in amplitude versus log contrast relations which were well described by single straight lines along most of the contrast range. The results may be interpreted as if at 2 cpd, achromatic pattern reversal stimulation evoked the activity of at least two visual pathways with high and low contrast sensitivity, respectively, while achromatic onset/offset stimulation favored the activity of a pathway with high contrast sensitivity. The neural activity in the M pathway is the best candidate to be the high contrast mechanism detected with pattern reversal and pattern onset/offset VEPs. The activity of color opponent pathways such as the P and K pathways either combined or in isolation seems to be responsible for VEPs obtained with isoluminant chromatic gratings at both presentation modes. When the amplitudes of chromatic VEPs were plotted in the same contrast scale as used for achromatic VEPs, chromatic contrast thresholds had similar values to those of the achromatic mechanism with high contrast sensitivity.