Only two phase mechanisms, ±cosine, in human vision

We evaluated the proposal that there exist detectors of the following four cardinal phases in human vision: +cosine, −cosine, +sine, and −sine. First, we assessed whether there was evidence that these cardinal phases were processed by independent ‘labeled lines,’ using a discrimination at detection threshold paradigm. Second, we assessed whether suprathreshold phase discrimination was best at phases intermediate between these cardinal values. Third, we tried to replicate previous evidence showing that an absence of facilitation occurs only between cosine pedestals and sine tests (or vice-versa). In all three experimental approaches we found no compelling evidence for four cardinal phase groupings. We did however find evidence for independent detectors for pure increments and decrements (±cosine). We suggest that phase discrimination, whether at threshold or suprathreshold, is mediated by mechanisms that encode the relative positions and contrasts of local increments and decrements within the stimulus.