The third task was performed by 50 participants who reported distortion in only one eye, or not in either eye (this was confirmed with the computer-based Amsler grid) and 10 control observers. The task was an adapted version of binocular correspondence perimetry
25 except with shutter glasses (nVidia, Santa Clara, CA, USA) instead of anaglyph glasses (to reduce crosstalk and color rivalry) and eye movement monitoring (to ensure compliant fixation). The same apparatus was as in the previous experiments. Although it was possible to track eye position through the shutter glasses, the reduction in signal strength made the eye tracker more vulnerable to noise so that the gaze was lost more frequently. Consequently, we also ensured compliance by having the experimenter observe the participant's eye position via the infrared image recorded by the eye tracker. Sixteen red circular points, each 0.35° in diameter falling on a 4 × 4 grid subtending 4.5 × 4.5° were presented only to the unaffected eye, one at a time in random sequence (
Fig. 1C, left panel). A green crosshair (whose line width was adjusted to be easily visible to the participant) was presented only to the more impaired eye, and the participant controlled its location with the computer's mouse (
Fig. 1C, central panel). The participant was required to fixate a central stationary white point and use the mouse to move the green crosshair so it was aligned with the red circular point (
Fig. 1C, right panel). Again, points were presented one at a time at all 16 locations. It was desirable to have the dynamic/moveable crosshair presented to the impaired eye and the static alignment point presented to the better eye because it helped overcome any intraocular suppression so that participants were able to see both on screen at the same time.
26 A background of 1/F noise (noise whose amplitude spectrum was filtered to resemble that of natural images) was used to minimize the impact of any crosstalk between the two panels of the shutter-glasses, which could otherwise allow the reference dots and crosshair to be visible to either eye, and potentially cause an underestimation of the magnitude of distortion.
25 An example of this stimulus is shown in
Figure 1C. Each participant completed two runs of the task (where each of the 16 points were presented once per run) to estimate variability around each reference location. The entire task took approximately ten minutes.
Surprisingly, even with movement of the cross hairs, 36% (18/50) of maculopathy participants with monocular impairment could not see the cross hairs and so were unable to successfully complete this task. We occluded their less-affected eye to confirm that the crosshairs were potentially visible to their impaired eye under these conditions. However, as soon as the better eye was not occluded, they became unable to see the crosshairs even after prolonged periods of moving the crosshairs with the mouse. We conclude that the better eye chronically suppresses the output of the impaired eye.
16 This suppression was not observed in any of the control participants.