Abstract
An important goal for models of motion processing is to recover object speed, yet few psychophysical studies have investigated the importance of speed to the visual system. Using static fixation, Reisbeck & Gegenfurther (1999, Vision Research, 39, 3267-3285) showed speed dominated the discrimination of moving gratings, as opposed to spatial and temporal cues. But observers typically pursue moving objects, so I investigated whether speed remained dominant for pursued targets by measuring discrimination contours in the distance-duration plane. If speed dominates, stimuli moving over different distances and durations should be more difficult to discriminate when they have the same speed. Resulting discrimination contours (ellipses) should therefore be oriented obliquely along iso-speed lines, as opposed to parallel to the distance-duration axes. Because extra-retinal signals and retinal-flow signals both help to estimate the speed of pursued stimuli, contours were measured with and without visible static backgrounds. Trials consisted of a horizontally-moving dot shown without background (no flow) or with horizontal lines (reduced flow) or vertical lines (high flow). Two standard stimuli (4deg/s, 1s or 2s, 4 or 8deg) and one test were shown in random sequence, with the speed, distance and duration of the test determined by a fixed orientation in the distance-duration plane. Sixteen orientations were investigated, and discrimination thresholds determined using a staircase procedure. For 3 observers, discrimination ellipses were oriented obliquely along iso-speed lines, suggesting that speed was the dominant cue in all conditions. Furthermore, ellipses became less elongated as the salience of flow increased. This suggests that visible backgrounds enhance distance cues not speed cues, because adding flow should have enhanced the latter and so elongated the ellipses (note duration cues were identical across conditions). Nevertheless, speed dominated performance throughout, underscoring the primacy of this cue for the processing of motion with or without pursuit.
Meeting abstract presented at VSS 2014