Stereomotion scotomas occur after binocular combination

Stereomotion scotomas are a surprisingly common visual impairment that result in an observer's inability to accurately report the direction of an object's motion in depth in restricted parts of the visual field. In this study we investigated the role of binocular cues to motion in depth. Using stimuli containing only non-stationary cues to stereomotion, we measured sensitivity across the visual field and identified areas of significant impairment in stereomotion processing in over 50% of otherwise healthy observers. These impairments vary idiosyncratically in extent and location between observers. We established that these impairments occur for a variety of visual stimuli, as long as they share the property that stimulus motion is exclusively defined by interocular and velocity differences. We tested for concordant impairments at relatively early stages along the visual pathway, i.e. changes in sensitivity across the visual field to local eye-dominance, monocular motion or instantaneous binocular disparity. Although we find variability in sensitivity across the visual field of our observers for all visual tasks, this variability across visual field locations did not correlate with the impairments in stereomotion processing. We therefore conclude that these stereomotion scotomas are due to impaired processing of dynamic cues after the stage of binocular combination.