Abstract
When we track an object moving in depth, our eyes rotate in opposite directions. This type of “disjunctive” eye movement is called horizontal vergence. The sensory control signals for vergence arise from multiple visual cues, two of which, changing binocular disparity (CD) and inter-ocular velocity differences (IOVD), are specifically binocular. While it is well known that the CD cue triggers horizontal vergence eye movements, the role of the IOVD cue has only recently been explored. To better understand the relative contribution of CD and IOVD cues in driving horizontal vergence, we recorded vergence eye movements from ten observers in response to four types of stimuli that isolated or combined the two cues to motion-in-depth, using stimulus conditions and CD/IOVD stimuli typical of behavioural motion-in-depth experiments. An analysis of the slopes of the vergence traces and the consistency of the directions of vergence and stimulus movements showed that under our conditions IOVD cues provided very little input to vergence mechanisms. The eye movements that did occur coinciding with the presentation of IOVD stimuli were likely not a response to stimulus motion, but a phoria initiated by the absence of a disparity signal.
Highlights
When we track an object moving in depth, our eyes rotate in opposite directions
While the changing binocular disparity (CD) mechanism first computes the disparities between the retinal images in the left and right eye and determines how those disparities change over time, the inter-ocular velocity differences (IOVD) mechanism first determines the velocities of the retinal images separately for the left and the right eye and compares the two resulting monocular velocity vectors
The assumptions for using these methods are that in an aIOVD stimulus anti-correlation disrupts the computation of static disparities from paired elements[30,31,32] whereas in a dIOVD stimulus disparities cannot be computed because there are no interocular pairs
Summary
When we track an object moving in depth, our eyes rotate in opposite directions This type of “disjunctive” eye movement is called horizontal vergence. Previous studies on vergence eye movements have used a wide variety of different types of stimuli, e.g., isolated light points, lines, gratings or random-dot stereograms. To effectively isolate velocity information an IOVD stimulus must provide a consistent monocular motion signal, whilst not generating consistent changes in disparity. For aIOVD it is not clear whether the disruption of the perception of static disparities results in a disruption of motion-in-depth perception[35,36,37], and in dIOVD stimuli random pairings between dots can potentially introduce a disparity signal[7,38]. By directly comparing the different types of cue-isolating stimuli with each other and with a stimulus that contains both types of information (FULL cue), it is, possible to estimate the contributions of the different cues
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