Categorization of biologically significant objects, food and gender, in rhesus monkeys. I. Behavioral study.

Macaque monkeys have a highly evolved visual system comparable to that of humans. One of the important visual functions is performing discriminations among biologically significant objects such as food or heterosexual partners. In the present study, we examined whether rhesus monkeys could categorize two-dimensional images related to food or gender using a visual discrimination task. Three rhesus monkeys were trained to make distinctions of food from non-food items, and between male and female monkeys, using 60 or more different pictures of each category. After more than 9 months of training, the monkeys discriminated a variety of foods from non-food and different males from females with more than 80% accuracy, even when the stimuli were used for the first time or presented only once in a session. The proportion of correct responses and response latencies showed better performance in discrimination of food/non-food than that of gender. The results suggest that rhesus monkeys are able to perform visual discrimination of highly abstract biologically significant categories with better performance in a food-related category than a gender-related one, using two-dimensional visual information.