Perceptual signs of parallel pathways.

Previous physiological work has shown that the X and Y cells found in the visual pathways of cats and monkeys have properties that might explain the perceptual distinction between 'sustained' and 'transient' mechanisms. However, when the sensitivities of X and Y cells are measured under conditions comparable with those used in psychophysical experiments, one finds that the properties thought to be relevant to the perceptual dichotomy do not in fact distinguish the two types of cell. One of the most important psychophysical grounds for distinguishing 'sustained' from 'transient' mechanisms is that there appear to be two distinct thresholds for detecting grating patterns, depending upon whether the observer is asked to detect the spatial or the temporal properties of the stimulus. However, if threshold are measured under conditions where the observer's criterion is tigahtly controlled, the two thresholds converge. These experiments question the existence of qualitatively distinct 'sustained' and 'trasient' mechanisms.