Stereomotion Scotomas: An impairment of velocity-based mechanisms revealed by variation of stimulus speed

In otherwise typical observers stereomotion scotomas can occur, an intriguing impairment restricted to specific locations in the visual field in the perception of motion-in-depth based on binocular cues. Potential causes for the impairment are failures to process changes in binocular disparity over time (CD) and interocular velocity differences (IOVD). Previous work using cue-isolating stimuli has suggested that IOVD failures are the primary contributor (Barendregt, Dumoulin, Rokers; 2016). Here we explored a method to identify the source of failure using stimuli that always contained both cues. Participants discriminated the direction (toward/away) of a field stereomotion of dots moving for 1 second within circular apertures (1.5 deg radius) distributed across 32 locations in the visual field. Stimuli contained both cues, but moved at either slow (0.25 deg/sec) or fast (4.0 deg/sec) speeds. These speeds primarily targeted CD and IOVD mechanisms respectively. We conducted multiple behavioral sessions with a group of typical participants. Our results confirmed that around 50% of the participants experienced impairment in stereomotion processing. For fast motion, the location of impairment varied between participants but remained stable across sessions. For slow motion, while impairments were also observed, their location was not stable over time. Furthermore, we did not find a correlation in the distribution of perceptual impairment between the two speeds. Varying the speed of stimuli containing both CD and IOVD cues supports prior work suggesting that stereomotion scotomas are based on an impairment in velocity-based mechanisms underlying motion-in-depth perception.