Figure 2.

Responses of a parafoveal MC cell to phase paradigm and to chromatic, |M – L| modulation. For phase paradigm (phase shift effect), 638 and 554 nm light sources were modulated at different relative phases. Response amplitude (1^st harmonic) and phase are shown (1.22 Hz, 50% LED modulation, mean of 8 cycles). The response minimum is shifted away from ±180°. The solid lines show a fit of a rectified cosine function. The shift is marked with a 4° field (A) but less with a 1° field (B). However, a surround annulus (C) (note 180° phase shift in achromatic response, which is arrowed, for surround condition) fails to evoke a shift. With 638–554 nm chromatic modulation (2F response; 10 Hz, 64 cycles averaged for histograms) there is a response at twice the stimulus frequency (waveforms sketched above). This is strong with a 4° field (A) but less with a 1° field (B), and absent with an annulus. These data are consistent with a chromatic input of intermediate size between achromatic center and surround.