Isodirectional tuning of adjacent interneurons and pyramidal cells during working memory: Evidence for microcolumnar organization in PFC

Studies on the cellular mechanisms of working memory demonstrated that neur ons in dorsolateral prefrontal cortex (dPFC) exhibit directionally tuned ac tivity during an oculomotor delayed response. To determine the particular c ontributions of pyramidal cells and interneurons to spatial tuning in dPFC, we examined both individually and in pairs the tuning properties of regula r-spiking (RS) and fast-spiking (FS) units that represent putative pyramida l cells and interneurons, respectively. Our main finding is that FS units p ossess spatially tuned sensory, motor, and delay activity (i.e., "memory fi elds") similar to those found in RS units. Furthermore, when recorded simul taneously at the same site, the majority of neighboring neurons, whether FS or RSI displayed isodirectional tuning, i.e., they shared very similar tun ing angles for the sensory and delay phases of the task As the trial entere d the response phase of the task, many FS units shifted their direction of tuning and became cross-directional to adjacent RS units by the end of the trial. These results establish that a large part of inhibition in prefronta l cortex is spatially oriented rather than being untuned and simply regulat ing the threshold response of pyramidal cell output. Moreover, the isodirec tional tuning between adjacent neurons supports a functional microcolumnar organization in dPFC for spatial memory fields similar to that found in oth er areas of cortex for sensory receptive fields.