Timing of visual events for motion discrimination

The observer's task was to identify the temporal order of the two adjacent luminance excursions one of which was a step-function and the other a linear increase in luminance starting from zero and reaching various final amplitude A after some period of time D. The interstimulus delay, delta t, between these two transitions was determined at which they appeared isochronous. The point of the subjective equality (PSE) depended on both ramp parameters, the rise-time duration D and its amplitude A. All of the data can be accounted for by supposing that judgements about the temporal order are based on the comparison of a simple attribute of these luminance excursions, the time moments when the luminous energy concentrated on low temporal frequencies exceeds some level. The perceived temporal order, which was experienced as a leftward or rightward displacement of the whole pattern, was determined by the sequence in which low-frequency portion of these two luminance excursions reached the threshold value. The implications of this simple contrast detection explanation for theories of motion analysis are discussed.