Positive Reinforcement and Threshold Contrast Sensitivity Testing in Alert, Task Oriented Rhesus Monkeys

Abstract

Abstract: : Purpose: We have developed a non–human primate (NHP) model to study the effects of acute laser injury on visual function in alert task oriented Rhesus monkeys. Using a custom developed positive reinforcement behavioral task we can rapidly assess optotype contrast sensitivity threshold. Methods: Through a multi–step program of operant conditioning, the behavior of three adult Rhesus monkeys has been shaped to perform a modified Reaction Time Observing Response task. The Landolt ring stimuli for this task were presented on a computer driven CRT screen at 1 meter viewing distance and varied in size and contrast. The primary positive reinforcement was small (1.3ml) aliquots of flavored water (Tang). The use of a variable reward ratio with increasing probability of reward for increasing difficulty of the task (decreasing size and/or contrast) is designed to push the subject’s performance and improve motivation near threshold. Contrast thresholds for spatial frequencies ranging from 0.5–10 cycles per degree(CPD) were measured. Results: Contrast thresholds were reliably measured at 5 different spatial frequencies (Gap sizes) in both eyes for all subjects. The variable reward ratio paradigm produced sustained motivation at threshold. Contrast thresholds begin to decrease below 2 CPD. Inter and intra–subject variability in behavior existed. Conclusions: Simplicity and ease of training make this technique well suited for NHP visual assessment.

Keywords: laser • lesion study • visual acuity