Dissociation of active working memory and passive recognition in rhesus monkeys

Abstract

Active cognitive control of working memory is central in most human memory models, but behavioral evidence for such control in nonhuman primates is absent and neurophysiological evidence, while suggestive, is indirect. We present behavioral evidence that monkey memory for familiar images is under active cognitive control. Concurrent cognitive demands during the memory delay impaired matching-to-sample performance for familiar images in a demand-dependent manner, indicating that maintaining these images in memory taxed limited cognitive resources. Performance with unfamiliar images was unaffected, dissociating active from passive memory processes. Active cognitive control of memory in monkeys demonstrates that language is unnecessary for active memory maintenance.

Figure 1. Memory tests with four levels of concurrent cognitive demand

Monkeys were required to remember an image over a memory interval that was either empty, or filled by one of three tasks: 1) motor: touch a blue square, 2) image: touch a non-classifiable image, or 3) classify: classify a central image as a bird, fish, flower, or person by touching the corresponding symbol. Motor and image stimuli could appear in any of the four screen corners. All three concurrent tasks required the same motor response.

Figure 2. Memory performance for familiar but not unfamiliar images is impaired by concurrent cognitive demand in a demand-dependent manner in monkeys

Proportion correct (±SEM) on the final recognition test is graphed for both the familiar small image set (red dashed line) and the unfamiliar large image set (solid blue line) as a function of the four levels of concurrent cognitive demand imposed during the memory interval. The gray horizontal dashed line represents the proportion correct expected by chance.