Sensitivity to velocity- and disparity- based cues to motion-in-depth with and without spared stereopsis in binocular visual impairment

Purpose: Two binocular sources of information serve motion-in-depth (MID) perception: changes in disparity over time (CD), and interocular velocity differences (IOVD). While CD requires the computation of small spatial disparities, IOVD could, in principle, be computed with a much lower-resolution signal. IOVD signals therefore might still be available under conditions of binocular vision impairment (BVI) with limited or no stereopsis, e.g. amblyopia.
Methods: Sensitivity to CD and IOVD was measured in adults who had undergone therapy to correct optical misalignment or amblyopia in childhood (n=16), as well as normal vision controls with good stereoacuity (n=8). Observers selected the interval containing a smoothly- oscillating MID “test/signal” stimulus from a “control/noise” stimulus in a two-interval forced choice (2IFC) paradigm.
Results: Of the BVI observers with no static stereoacuity (n=9), one displayed evidence for sensitivity to IOVD only, while there was otherwise no sensitivity for either CD or IOVD in the group. Generally, the BVI observers with measurable static stereoacuity thresholds (n=7) displayed a pattern resembling the control group: showing a similar sensitivity for both cues. A neutral-density (ND) filter placed in front of the fixing eye in a subset of BVI observers did not improve performance.
Conclusions: In one BVI observer we found evidence for preserved sensitivity to IOVD but not CD, though overall only those individuals with BVI and at least gross static stereopsis were able to detect disparity-based or velocity-based cues to MID. The results imply that IOVD signals are not necessarily processed in a manner remotely different to CD.