Ricco's area, contrast sensitivity, and positional acuity in normal and strabismic monkeys

Purpose. Numerous studies in humans and monkeys have shown a relationship between positional acuity and sensitivity to contrast. We have previously shown thai the degree of vernier acuity loss in amblyopic monkeys is similar to the grating acuity deficit when vernier stimuli are equated for effective contrast. To understand more clearly the nature of the relationship between contrast sensitivity and positional acuity we conducted an experiment, after Levi and Klein ( 1992), designed to directly compare positional sensitivity and contrast sensitivity to the positional cue. Klein and Levi have shown that Ricco's crilical area predicts vernier acuity for normal observers and some amblyopes. Methods. We tested normal macaque monkeys and monkeys with experimentally-induced strabismic amblyopia. We measured positional acuity for an edge vernier stimulus composed of an edge with a shorter, abutted thin line of variable width. The size of the vernier offset (positional cue) was therefore the width of the thin line. We measured positional acuity over a range of contrast from 0.03-1.0, and independently measured threshold for detection of the edge and contrast sensitivity for the thin line alone. Results. We calculated Ricco's integration area and compared it to edge vernier acuity at edge detection threshold. If positional acuity were determined by the observer's ability to detect the positional cue, then these two measures would be equal. Our data support this prediction for both amblyopic and normal eyes. Ricco's integration area is large for amblyopic eyes but the proportional relationship to positional acuity at threshold is maintained. Conclusions. Ricco's integration area predicts positional acuity at Ihreshold equally well for strabismic and normal monkeys. Levi and Klein (1992) Neuroscience Letters 136: 63-66.