Different encoding mechanisms for phase and contrast

Phase sensitivity was assessed over a large range of exposure durations by means of a 2AFC staircase procedure where the observer had to detect the relative position of sinusoidal gratings relative to a superimposed thin, dark line. Phase discrimination thresholds decreased as a function of exposure duration although the contrast of the stimuli was weighted for equal detectability at all durations. Phase sensitivity improved markedly with contrast, as opposed to the degradation with contrast seen in contrast discrimination paradigms. The contrast and time functions of phase sensitivity both support the hypothesis that phase is processed separately from contrast by a pathway with different temporal and contrast characteristics. We propose a model where phase sensitivity depends on a luminance subtraction process with a time constant of about 130 msec.