Selective Feature-Based Attention Directed to a Pair of Lines: Psychophysical Evidence and a Psychophysical Model

Abstract

The psychophysical data reported here indicate that observers can attend simultaneously to a pair of test lines while ignoring stimuli between the two test lines, and can extract independently four relationships between the two test lines. We measured discrimination thresholds for the mean orientation of a pair of lines as well as for their orientation difference, separation and mean location. We propose: (a) that all four discriminations were mediated by comparator mechanisms that received inputs from two first-stage narrow receptive fields whose centres were located some distance apart and which were “blind” to stimuli falling between the two receptive fields; (b) that the human visual system contains mechanisms of this kind whose outputs are labelled with the orientation difference, the mean orientation, the mean location, and the separation of the two first-stage receptive fields; (c) that orientation difference, mean orientation, separation, and mean locations are signalled independently. We found that all four discrimination thresholds were independent of test-line contrast for contrasts more than 2–3 times above line-detection contrast threshold. This finding can be understood if each of the four labelled outputs feeds an opponent-process stage. The preceding proposals can account for several previously reported phenomena. More generally, an array of the proposed long-distance comparator mechanisms constitutes a system that may be capable of fully specifying the shape, size, location and implicit orientation of the boundaries of an object’s retinal image.