Relative latencies of cone signals measured by a moving vernier task.

When a bipartite blue and red bar is swept across a bright yellow field, the blue half of the bar appears to lag behind the red and to exhibit a longer persistence (J. D. Mollon & P. G. Polden, 1976). This effect has been taken to reveal the longer time constants of the short-wave channels of the visual system. In Experiment 1, we quantified the effect by a nulling technique: The average latency of the short-wave bar relative to the long-wave was 17.9 ms, and the average value for the relative persistence, i.e. the apparent temporal separation of the trailing edges of the bars, was 48.3 ms. However, the conditions of the original demonstration and those of Experiment 1 place the short-wave and long-wave channels in very different states: The yellow field produces little light adaptation in the short-wave cones, but polarizes opponent channels that carry short-wave signals. In Experiment 2 we selected an adapting field that equally raised the short- and long-wave mechanisms above their absolute thresholds but did not so strongly polarize post-receptoral channels. We adjusted the two stimuli to be equally above threshold on this field. When light adaptation and stimulus conditions were equated in these ways, there was very little difference either in the perceived latency or in the perceived duration of the short-wave and long-wave stimuli. We discuss possible reasons why different estimates of sensory latencies may be obtained from reaction times and from perceptual judgments.