ON and OFFS-cone pathways have different long-wave cone inputs

Three experiments compared thresholds for S-cone increments and decrements under steady and transient adaptation conditions, to investigate whether st imuli of both polarities are detected by the same cone-opponent psychophysi cal mechanism. The results could not be accounted for by a standard model o f the S-cone detection pathway [Polden & Mellon (1980) Proceedings of the R oyal Society of London, B, 210, 235-272]. In particular, a transient tritan opia detection paradigm that measured threshold elevation following the off set of long-wavelength fields produced different field sensitivities for S- cone increment and decrement tests. The decrement field sensitivity functio n was shifted to shorter wavelengths relative to the increment function. L- cone opponency is apparently stronger for S-cone increments than for decrem ents. The most plausible substrates of the two different psychophysical det ection mechanisms are the ON and OFF channels. The results suggest that S-O N (bistratified) and S-OFF ganglion cells receive different relative amount s of L- and M-cone input. (C) 2000 Published by Elsevier Science Ltd.