The effect of strabismus on object detection in the ring scotoma of a monocular bioptic telescope

Purpose People with reduced visual acuity are permitted to drive with the aid of bioptic telescopes in the USA, the Netherlands, and Canada. When viewing through a monocular bioptic telescope, suppression of the deviating eye in strabismus may reduce the ability of the non-telescope eye to detect objects whose images fall in the ring scotoma area of the telescope eye, which could impair detection of traffic-relevant events. This ability to detect stimuli in the ring scotoma area was compared for strabismic and non-strabismic patients. Methods Ten strabismic and six non-strabismic patients with bilaterally reduced visual acuity (0.30-1.0 logMAR, 6/12 to 6/60) participated. A dichoptic perimeter presented stimuli to the fellow (non telescope) eye in the area of the ring scotoma under binocular viewing. Fellow-eye detection rates were determined with and without a bioptic, on uniform and patterned backgrounds, while performing passive (viewing a cross) and active (reading letters) fixation tasks. Results All strabismic patients were found to have anomalous retinal correspondence. Both non-strabismic and strabismic patients had lower fellow-eye detection rates on patterned than on uniform backgrounds, and while performing the active task. In addition, strabismic patients had lower detection with than without the bioptic on the patterned background. They also had a larger decrease in detection from the uniform to the patterned background than non-strabismic patients (26% vs 8%). Depending on the angle and direction of the deviation relative to the stimulus side, strabismus either increased or decreased fellow-eye stimulus eccentricity on the retina. Larger detection rate reductions between the uniform and patterned backgrounds were associated with more eccentric stimulus locations (=0.61, p=0.013). Conclusions Both strabismic patients and non-strabismic patients were able to detect stimuli with the fellow eye in the ring scotoma area, demonstrating successful bi-ocular multiplexing. However, strabismic patients generally had a greater reduction in detection performance from the uniform to the patterned background than non-strabismic patients, which was accounted for in part by differences in stimulus eccentricities on the retina (that varied with the angle and direction of the strabismus). However, a study with a larger sample, including participants with strabismus and normal retinal correspondence, is needed before our findings can be generalized.