Figure 11.

A scheme as to how the 2F response may be generated. An achromatic receptive field yields a 1F response to sinusoidal modulation. (The example shown for the upper panels is that expected for L cone modulation alone.) Two chromatic receptive fields of opposite cone polarities (+L − M and +M − L) yield responses in counterphase. An accelerating (squaring) nonlinearity is applied to these waveforms; the resulting sum shows a 2F response. When added to the achromatic response in different conditions (lower panels), the pattern of responses seen is similar to that observed empirically. Rectification of response histograms has been indicated by dashed lines for negative firing rates.