Stability of phase recognition in complex spatial waveforms

Observers viewed 200 msec presentations of gratings containing first (0.5 c/deg) and third (1.5 c/deg) harmonic components. The phase of the third harmonic and the absolute position of the grating on the screen varied randomly from trial to trial. Classification of the phase relation (0, 90, 180 or 270 deg was 99% perfect. When a 2 sec period of inspection of the grating or its fundamental preceded the test presentation, strong shifts in perceived waveform were observed that depended on the test grating's position relative to the inspection grating, and resembled the effects seen during continuous viewing. No phase-specific effects were obtained. The pattern of results was exactly that predicted by negative afterimage. Phase recognition at low contrast, and at a high frequency, was also good. Triangular-wave gratings were misperceived (the "square-wave illusion") only when real or simulated afterimages were present. We conclude that recognition of phase relations in a complex waveform is stable when the predictable variation due to afterimages is eliminated.