Abstract
There are several asymmetries between the S-cones and the L- and M-cones (e.g. S-cones are much rarer; are absent from the central foveola; and have a peak spectral sensitivity well separated from the L and M peak sensitivities). But perhaps the most significant asymmetry is that, while the L- and M-cones have access to a variety of post-receptoral channels, the S-cone signal is largely confined to S-opponent pathways (e.g. Dacey & Lee, 1994; Sun et al., 2004). Only under special conditions is a small S-cone input to luminance observed psychophysically.
We exploited the functional asymmetries between S-cone vision and L- and M-cone vision to determine psychophysically the lights required to isolate the S-cone response. We evaluated three ways of finding the tritan line that depend on differences in: (i) spatial resolution; (ii) recovery from extinction of a yellow (−S) adapting field, and (iii) sensitivity changes caused by a blue (+S) adapting field. All three methods, if properly applied, converge to a common estimate of the tritan line for a given observer and retinal eccentricity. Using reaction time measurements and temporal masking, we estimated the latency of response to such tritan stimuli to be no more than 20–30 msec greater than latency of response to L/M-opponent stimuli, a result that agrees with earlier temporal phase measures (Stromeyer et al., 1991). Other evidence for larger differences may reflect luminance contamination in the L/M stimulus.
Tritan stimuli are particularly attractive in teasing apart visual processing, for they minimise contributions of the magnocellular (and parvocellular) pathways. Furthermore, since the S-cones exhibit little or no projection to superior colliculus, a brain area thought to be important for shifts of attention, tritan stimuli can be a powerful tool in psychological research (Sumner, Adamjee & Mollon, 2002).
Supported by the Wellcome Trust and a Royal Society Conference Grant