A non-invasive method for studying an index of pupil diameter and visual performance in the rhesus monkey

Background A non-invasive model has been developed to estimate gaze direction and relative pupil diameter, in minimally restrained rhesus monkeys, to investigate the effects of low doses of ocularly administered cholinergic compounds on visual performance. Methods Animals were trained to co-operate with a novel device, which enabled eye movements to be recorded using modified human eye-tracking equipment, and to perform a task which determined visual threshold contrast. Responses were made by gaze transfer under twilight conditions. 4% w/v pilocarpine nitrate was studied to demonstrate the suitability of the model. Results Pilocarpine induced marked miosis for > 3 h which was accompanied by a decrement in task performance. Conclusions The method obviates the need for invasive surgery and, as the position of point of gaze can be approximately defined, the approach may have utility in other areas of research involving non-human primates.