Dependence of visual latency on wavelength: predictions of a neural counting model.

Simple reaction time (RT) was determined as a function of wavelength for equally visible, near-threshold stimuli. The stimuli were 5-deg spectral onsets of 1,000-ms duration presented on a 100-Td spatially coincident white background. All three subjects manifested the same result: RTs were fastest in the region of 590 nm. These data were analyzed in the context of a counting model of visual latency. This model predicts that for equally visible stimuli a transient detector will result in shorter visual latencies than will a more sustained detector. On the basis of this analysis, it is concluded that although most long duration, near-threshold, spectral step onsets are detected by the sustained parvocellular pathway, an exception occurs in the region of 590 nm: these stimuli are detected by a relatively transient pathway, presumably the magnocellular pathway.