Dissociation of Memory for Order and Memory for Items by Concurrent Cognitive Load in Rhesus Monkeys (Macaca mulatta)

Memory of the order in which events occurred addresses the "when" component of episodic-like memory in animals. Seminal work demonstrated that normal rats remember the order in which they encountered odors, and hippocampal lesions selectively impaired this memory for order while leaving recognition performance intact. This work inspired us to develop an image-order paradigm to model episodic memory in monkeys. Monkeys saw five images in sequence and at test reported which of two images appeared first. Unlike the case with rats, hippocampal lesions did not impair memory for order, raising questions about which memory systems monkeys used in this task. Because the memory intervals were short in the image-order task, monkeys may have used nonhippocampal-dependent working memory instead of long-term memory. To test this possibility, we introduced a concurrent cognitive load between the study and test phases of the image-order task. We hypothesized that if monkeys were using working memory to remember the order of the images, concurrent cognitive load would impair performance. We also tested recognition performance as a control, because familiarity-based recognition is relatively resistant to concurrent cognitive load. Memory for order was much more attenuated by concurrent cognitive load than recognition. These results indicate that monkeys used working memory to solve this image-order task, making it a poor model of long-term episodic memory. The findings also provide clear evidence for a dissociation between working memory-dependent order memory and familiarity-based recognition memory.