Pattern discrimination following higher harmonic adaptation

Abstract

Linear filtering theory in spatial vision suggests that, near threshold, a square-wave grating can be distinguished from a sine-wave grating of the same spatial frequency and contrast when the third harmonic component of the square-wave reaches its own threshold. This implies that prior adaptation at the third harmonic frequency should severely impair the ability of the visual system to make a square-wave/sine-wave discrimination. We tested this hypothesis by measuring discrimination thresholds, using two-interval forced-choice methods, at 1.33, 2.66, and 5.33 cycles/deg in subjects before and after 4 min of adaptation to a high contrast (0.4) sine-wave grating at 4, 8, and 16 cycles/deg. Thus, the frequency of the adapting grating was equal to the third harmonic of the square-wave test grating. To control for the magnitude of the adaptation effect at and near the adapting frequency, elevation in contrast thresholds was also measured for sine-wave test gratings, whose frequency ranged from −2 to +2 octaves about the adapting frequency. Results indicate that adaptation has a substantial effect at the third and fifth (0.6 and 0.4 log unit, respectively) harmonic frequencies but had only a slight effect (~0.15 log unit) on discrimination thresholds. This increase in discrimination thresholds is a mere fraction of that expected if discrimination thresholds were being determined either by a channel tuned to the third harmonic alone or by a linear summation of higher harmonic components of the square-wave test grating. The implications of this nonlinearity are discussed.