Accuracy of estimating time to collision using only monocular information in unilaterally enucleated observers and monocularly viewing normal controls

Since individuals who have lost an eye in early life rely on monocular information, one asked if they would better estimate the time to collision (TTC) with an approaching object based on the monocular cue [(θ/(dθ/dt), i.e. tau] than a control group using only monocular information. Estimates of TTC were measured with a simulated approaching textured object using a staircase procedure. Seven adult observers who were unilaterally enucleated at an early age were compared with 18 normally sighted control observers who viewed the stimuli monocularly. Consistent with previous findings, the majority of the controls (13/18) underestimated TTC. Three enucleated observers had larger estimation errors than the 95% confidence interval of the mean of the control group. One enucleated observer was unable to give reliable results. These results suggest that unilaterally enucleated observers cannot estimate TTC more accurately (and may even be worse) than normal controls when estimates are based on monocular information alone. Further, the majority (83%) of enucleated observers were influenced by perceived distance information derived from the object's initial size when estimating TTC with an approaching object. The use of this other optical variable could account for their reduction in performance. It was suggested that in every day life enucleated individuals make use of as many optical variables as possible to partially compensate for the lack of binocularity.