Discrimination of spatial phase changes: contrast and position codes

Changing the relative phase of the frequency components of a stimulus usually also produces local contrast variations. Using stimuli composed of the product of a sinusoid (carrier) and a spatial envelope, an attempt was made to distinguish between the visual system's ability to code spatial phase on the one hand and local contrast and position cues on the other. The experiments assess the ability of observers to detect which of two stimuli is farther to the left. In the main experiments a large, easily detectable, envelope shift is presented on every trial and performance is measured as a function of the size of a carrier shift in the same direction. Increasing the size of the carrier shift gradually increases the size of the phase difference between the two stimuli in a trial but simultaneously reduces the contrast change in the bars of the stimulus. If the visual system can code phase directly the ability of observers to detect a change in location should improve as the size of the carrier shift increases but if local contrast is coded performance should be poorer over a small range of carrier shifts than that obtained without a carrier shift. It is shown that a region of poorer performance is obtained and therefore it is concluded that the visual system does not code spatial phase explicitly.