In addition to their expected chromatic contribution, the S-cones make a small contribution to luminance, revealed when the S-cone response is relatively enhanced by intense long-wavelength adaptation. Under such conditions, the S-cone contribution is negative (-S) and also substantially delayed. This slow input is not unique to the S-cones: we and others have identified M- and L-cone inputs with comparable temporal properties. These inputs appear as spectrally-opponent pairs, which can be either +L-M or +M-L, depending on the adaptation conditions. This raises the interesting possibility that the –S input found under long-wavelength adaptation might be complemented by a +S input under other conditions, a speculation reinforced by recent physiological evidence for a +S (or S-ON) input to magnocellular cells. To date, however, we have been unable to find any psychophysical evidence for a positive S-cone contribution to luminance.

One concern is that the negative S-cone input to luminance can be misidentified as +S due to its substantial delay. Such a mistake is easy to make: depending on the adaptation conditions, the phase delay of the -S input can cause it to add to luminance at frequencies as low as 5 Hz and as high as 25 Hz despite its negative sign. To be certain of the sign of the S-cone input, measurements must be extended to temporal frequencies that are low enough for the phase delay of the S-cone signal to be extrapolated to 0 Hz, where its sign is unambiguously revealed.