Discrimination at threshold: labelled detectors in human vision.

We examined discriminations between small patches of grating that differed in either spatial or temporal frequency. The patches were presented at contrasts near to detection threshold. For certain pairs of stimuli, each was correctly identified as often as it was detected. To explain this result, we hypothesize that the detectors of these stimuli are lube/led, in the sense that the observer can distinguish the response of each detector from that of any other. Under this assumption, we find that the detectors form two non-overlapping sets in their selectivity for temporal frequency. In their selectivity for spatial frequency, the detectors of slowly varying stimuli can be partitioned into 7 distinct sets, but only 3 sets are evident among the detectors of rapidly modulated patterns.