Inactivation of parietal and prefrontal cortex reveals interdependence of neural activity during memory-guided saccades.

Dorsolateral prefrontal and posterior parietal cortex share reciprocal projections. They also share nearly identical patterns of neuronal activation during performance of memory-guided saccades. To test the hypothesis that the reciprocal projections between parietal and prefrontal neurons may entrain their parallel activation, the present experiments have combined cortical cooling in one cortical area with single-unit recording in the other to more precisely determine the physiological interactions between the two during working memory performance. The activity of 105 cortical neurons during the performance of an oculomotor delayed response (ODR) task (43 parietal neurons during prefrontal cooling, 62 prefrontal neurons during parietal cooling) was compared across two blocks of trials collected while the distant cortical area either was maintained at normal body temperature or cooled. The mean firing rates of 71% of the prefrontal neurons during ODR performance changed significantly when parietal cortex was cooled. Prefrontal neurons the activity of which was modulated during the cue, delay, or saccade periods of the task were equally vulnerable to parietal inactivation. Further, both lower and higher firing rates relative to the precool period were seen with comparable frequency. Similar results were obtained from the converse experiment, in which the mean firing rates of 76% of the parietal neurons were significantly different while prefrontal cortex was cooled, specifically in those task epochs when the activity of each neuron was modulated during ODR performance. These effects again were seen equally in all epochs of the ODR task in the form of augmented or suppressed activity. Significant effects on the latency of neuronal activation during cue and saccade periods of the task were absent irrespective of the area cooled. Cooling was associated in some cases with a shift in the best direction of Gaussian tuning functions fit to neuronal activity, and these shifts were on average larger during parietal than prefrontal cooling. In view of the parallel between the similarity in activity patterns previously reported and the largely symmetrical cooling effects presently obtained, the data suggest that prefrontal and parietal neurons achieve matched activation during ODR performance through a symmetrical exchange of neuronal signals between them; in both cortical areas, neurons activated during the cue, delay, and also saccade epochs of the ODR task participate in reciprocal neurotransmission; and the output of each cortical area produces a mixture of excitatory and inhibitory drives within its target.