Phase-reversal discrimination in one and two dimensions: performance is limited by spatial repetition, not spatial frequency content.

Lawden [(1983) Vision Research, 23, 1451-1463] used vertical gratings containing two frequencies (F, nF) in phase discrimination (F + nF against F - nF) and compound detection (F + nF against F) experiments, where thresholds were measured by manipulating the contrast of the nF component. When n was varied, Lawden found a phase-plateau of moderate breadth where phase discrimination thresholds were about half of those measured in compound detection. I present the results of similar experiments, using one-dimensional (gratings) and two-dimensional (plaids). In a sine-plaid condition, the 1F grating was split into two 1F plaid components at +/- 45 deg from vertical while the nF component remained a vertical grating. In a square-wave plaid (SqW-plaid) condition the plaid components were square waves. For each of these conditions, the horizontal spatial repetition (SR) of the plaid is given by (F/square root of 2); it is half an octave lower than the spatial frequency (SF) of the oblique components but it is not represented in the stimulus spectrum. By plotting phase discrimination relative to compound detection a phase-plateau was found for all three conditions. When these data were plotted as a function of SF ratio (nF/F) the curves describing the two plaid conditions were found to be leftward translations of that describing the grating condition. However, when the results were plotted as a function of SR ratio (nF/SR), the three functions lay on top of each other. The finding that phase-reversal discrimination is not governed by the Fourier attributes of the stimulus per se, rules out an explanation in terms of a linear, broad-band, phase-sensitive mechanism. Rather, the results imply that information is combined across the set of SF- and orientation-tuned mechanisms before the decision variable. These interactions appear to be governed by the spatial (not Fourier) attributes of the luminance profile of the stimulus. A modified version of Bennett's [(1993) Perception & Psychophysics, 53, 292-304] phase discrimination model is presented as a post-hoc account of the data.