Properties of spike train spectra in two parietal reach areas

In the lateral intraparietal area (LIP), a saccade-related region of the posterior parietal cortex (PPC), spiking activity recorded during the memory period of an instructed-delay task exhibits temporal structure that is spatially tuned. These results provide evidence for the existence of 'dynamic memory fields' which can be read-out by other brain areas, along with information contained in the mean firing rate, to give the direction of a planned movement. We looked for evidence of dynamic memory fields in spiking activity in two parietal reach areas, the parietal reach region (PRR) and area 5. Monkeys made center-out reaches to eight target locations in an instructed-delay task with a memory component. Neurons in both areas exhibited sustained activity during the delay period that was spatially tuned. Many single cell PRR spectra exhibited spatially tuned temporal structure, as evidenced by a significant and spatially tuned peak in the 20-50 Hz band. The PRR population spectrum of spike trains was also tuned, with the peak power centered on approximately 25 Hz. In contrast, area 5 spiking activity did not exhibit any significant temporal structure. These results suggest that different mechanisms underlie sustained delay period activity in these two areas and that dynamic memory fields, as revealed by our techniques, are more prominent in PRR than in area 5. Temporal structure in the spike train and local field potential (LFP) are related in at least one other brain area (LIP). The present results suggest then that LFP activity obtained from PRR may be better suited than area 5 LFP activity for use in neural prosthetic systems that incorporate analysis of temporal structure as part of a decode mechanism for extracting intended movement goals.