Abstract
Studies in the macaque Lateral Prefrontal Cortex (LPFC) have shown that single neurons encoded spatial working memory (WM) signals. The majority of these studies have used oculomotor delay response (ODR) tasks that do not dissociate remembered locations in different frames of reference (e.g., retina-centered vs space-centered). Here we used a variation of the ODR task that allowed us to dissociate these two frames of reference, while recording the activity of neurons in the LPFC with two microelectrode arrays (10×10 Utah arrays) implanted dorsally (dLPFC) and ventrally (vLPFC) from the principal sulcus (areas 8A and 9/46), respectively. During task trials, animals fixated a dot that appeared at one of 16 different positions on the screen, then a target transiently appeared for 1000ms. The animal was required to maintain fixation for another 1000ms and upon extinction of the fixation point to make a saccade to the remembered target location to obtain a reward. The systematic variations in the initial fixation position allowed us to analyze the data for remembered locations relative to the fixation point (retinal frame), and relative to the screen (screen or space-centered frame). We found that 20% of all recorded dLPFC neurons and 12% of the vLPFC neurons were tuned for the remembered location in a retinotopic frame. Notably, 15% of dLPFC neurons as well as 9% of vLPFC neurons were tuned for the remembered location in a spatiotopic frame of reference. Our results show that the LPFC encodes working memory in both retinotopic and spatiotopic reference frames, with a bias for the former. Moreover, they show that dLPFC contains a larger proportion of neurons tuned for both retino-topic and spatiotopic frames than the vLPFC.
Acknowledgement: Canadian Institutes of Health Research, National Science and Research Council of Canada, German Primate Center