Multi-item Memory in the Primate Prefrontal Cortex

The ability to retain multiple items in short-term memory is fundamental for cognition, yet almost nothing is known about its neural basis. To explore the mechanisms underlying this ability, we trained two monkeys to remember a sequence of two images across a short delay. We then recorded the activity of neurons from the lateral prefrontal cortex during task performance. We found that the majority of neurons showed delay activity that depended on the identity of both images (a minority reflected just one image), and that activity related to a given combination of images was only partially predictable from each neuron’s activity to individual images. A model to predict the resultant neural activity was tested. We also examined the effect of task demands on the neural representation of multiple images. Our first experiment showed that each of the two images in memory was represented with a certain strength, and that this strength was dependent on how long the image had been in memory; image strength decayed as time progressed. We found that changing the way that the memory of the images was reported, from a bar release to a sequence of eye movements, changed the relative strength of the image representations. In the eye-movement version of the task the strength of the representation of the image did not decay with time; in fact the strength of older images could even surpass the strength of newer images, depending on how frequently the tasks were switched. Further experiments showed that when the monkey switched between the two tasks individual neurons could turn their image coding on and off. We also found a substantial population of cells that directly represented the task that the animal was performing. Thesis Supervisor: Earl K. Miller Title: Picower Professor of Visual Neuroscience Associate Director, Picower Institute for Learning and Memory