Behavioral evidence for working memory of sensory dimension in macaque monkeys

For animals, which lack language codes, there might be limitations in the use of abstract concepts in guiding their behavior. We trained two macaque monkeys in a computerized version of Wisconsin Card Sorting Task (WCST) with two dimensions (shape and color) to test their capability for memorizing, updating and using representation of sensory dimension. A sample had to be matched with one of three test items by either matching in color or shape. The monkeys had to find the relevant sensory dimension and its sudden changes by trial and error only based on reward and error signals. Both monkeys succeeded in more than ten shifts in the rules within a daily session. Several probe tests showed that the monkeys in fact used the dimension-based matching rules, because the rule shift was generalized from the experience of some samples to all of the others in the sample set, and because the rules were immediately applied to newly introduced samples. The frequent shifts can be overcome only with representation of the sensory dimension in working memory. These results, thus, suggest that nonhuman primates can store sensory dimension such as color and shape in working memory, dynamically update it based on the behavioral outcomes, and use it to guide behavior, despite their lack of apparent language.