Variation in Vernier acuity sensitivity with temporal frequency and relative temporal phase

Purpose. Hyperacuity highlights the visual system's ability to combine inputs in a spatially precise fashion. We studied temporal aspects of hyperacuity by examining how abutting-line vernier performance depends on frequency and phase of the stimulus components. Methods. Vertical displacement thresholds for two horizontal bars (4 x 32 min., 0.95 sec duration on a 4 x 4 deg, 150 cd/m² field) were determined by a 2-AFC staircase. Temporal frequency (0, 1, 4, 16 Hz) and relative temporal phase (0, 0.25, 0.5 cycles) of the bars were varied. Contrast was set to multiples (2x, 4x, 8x, 16x) of each subject's detection threshold determined at each temporal frequency. Results. At each temporal frequency, threshold vs. contrast functions were parallel across relative phase. The threshold for the 0.5-cycle phase shift condition was higher than the threshold for the in-phase condition, with the maximal difference at 4 Hz (2.7-fold). Thresholds in the 0.25-cycle condition were generally similar to thresholds in the 0.5-cycle condition. Conclusions. Sensitivity of displacement threshold to temporal phase was anticipated from studies of static stimuli (e.g. Levi & Westheimer 1987), but the peaking of phase sensitivity near 4 Hz was unanticipated and provides a probe of the dynamics underlying vernier judgements. The phase and frequency-dependence of vernier thresholds appears to require a linear (or half-wave rectifying) pathway and also a full-wave rectifying pathway.