Abstract
Purpose::
To determine whether convergence angle in the dark can be sensed (registered).
Methods::
Fifteen naive and normally binocular subjects were recruited and asked to observe a luminant near target in dark surrounds, shown at one of four randomized distances. The target was then extinguished, initiating a decay of convergence. Following a variable delay after darkness onset, an auditory cue prompted subjects to indicate either (1) where their eyes were looking at that instant (sensed distance task) or (2) where they last remembered the target before it disappeared (remembered distance task). Judged distance was assessed by kinesthetic matching, or ball tossing, depending on the magnitude of the judged distance. Informed consent was obtained prior to experimentation.
Results::
The relationship between judged distance, vergence angle in darkness, and remembered target distance was analyzed with ANCOVA analysis for repeated measurements. For the remembered distance task, average judged distance was larger for greater target distances (p<0.0001), but was unrelated to vergence angle in darkness. For the sensed distance task, judged distances were also significantly influenced by target distance (P< 0.001). There was great variability between subjects' responses in the sensed vergence task. As a group, however, subjects were able to slightly detect change of vergence angle in darkness. Judged distance increased 0.1 prism diopters with every 1 prism diopter increase in convergence in the dark (p<0.01). This weak but statistically significant population effect was due to the judged distance responses of two subjects who showed a strong correlation of judged distance with vergence angle in darkness. The other thirteen subjects showed no such correlation.
Conclusions::
Judged viewing distance in darkness is largely based on the remembered locations of previously viewed targets. Only two out of fifteen subjects displayed the ability to sense convergence angle in the dark. We conclude that for the most part, normal individuals do not have the ability to sense convergence angle in the dark.
Keywords: vergence • binocular vision/stereopsis