MC neurons have higher firing rates to luminance contrast than do neurons in the PC pathway. MC neurons react nonlinearly to contrast, whereas PC units show almost linear characteristics. The contrast gain of a typical MC neuron is about 10 times higher than that of PC neurons and their contrast response function usually saturates at lower contrasts (Kaplan & Shapley,
1982; Shapley, Kaplan, & Soodak,
1981). The amplitude of a luminance contrast evoked VEP is often linearly related to the log of contrast (Campbell & Maffei,
1970); however, these contrast amplitude response curves (CR) evoked by different spatial frequency gratings show some nonlinearities. Several authors have reported either a straight-line relation at low contrast followed by saturation at high contrast or a double-slope straight-line relation in their studies (Bobak, Bodis-Wollner, Harnois, & Thornton,
1984; Rudvin, Valberg, & Kilavik,
2000; Valberg & Rudvin,
1997). These nonlinearities or multi-slope CRs can be associated with different parallel visual pathway sensitivities (Souza, Gomes, Saito, da Silva Filho, & Silveira,
2007). Sinusoidally modulated 11-Hz grating stimulation (MC dominant stimulus) showed high sensitivity to contrast, the best fit to the CR was the sigmoid function leveling off at around 30% (Alexander, Rajagopalan, Seiple, Zemon, & Fishman,
2005).