A Refined Spatiotemporal Analysis of Extracellular Waveforms From Neurons Within the Lateral Intraparietal Area of the Macaque Posterior Parietal Cortex

Abstract

Extracellular spike waveforms recorded from neurons are known to reflect the intracellular process of action potential generation through the activation and inactivation of ion channels. The variability between the shape of spike waveforms is thought to be a direct result of the differential ion channel expression between these subtypes. In cerebral cortex, spike waveforms recorded from pyramidal neurons are known to be ‘broader’ than the ‘narrow’ spike waveforms exhibited by interneurons. To capture this temporal variability in primarily biphasic waveforms, where a negative deflection occurs before a positive deflection, researchers have used the spike width—the time between the trough and peak of the waveform. However, numerous accounts have shown that this approach is too simplistic, failing to robustly discriminate putative pyramidal neurons from interneurons, as well as account for further nuanced classes of neurons. Continuing efforts are therefore being made to refine classifications with firing statistics and other parameters of the spike waveform shape. In this thesis, I report on the defining characteristics of neurons recorded from the lateral intraparietal area, for which there exists only three other studies, within the posterior parietal cortex of one rhesus monkey. I significantly refined the standard approach to calculating the template mean waveform, which I used to extract previously reported spatiotemporal parameters as well as several new parameters. Using unsupervised classification, I derived a three-class provisional model comprised of one narrow-spike class robustly separated from two broad-spike classes. Consolidating this classification, I identified a phenomenon observed exclusively in all broad-spike neurons, a knee-like change in the voltage close to the waveform peak. In addition, narrow-spike neurons were characterized by high firing rates, more irregular firing, and generally lacking bursting. While displaying converse firing statistics from narrow-spike neurons, there was no identifiable difference in the discharge properties between the two identified broad-spike classes. Nevertheless, several waveform features involving the knee-like change were highly discriminating between these two classes. Moreover, the class with the broader spikes exhibited more highly unusual troughs, which has been associated with multiple Na+ influxes thanks to larger dendritic tree arborization. Hence, I speculate that this phenomenon is predominant in pyramidal neurons within infra-granular layers. My detailed analysis provides a fresh perspective and new tools to help identify distinct spatiotemporal and functional signatures of three different types of cortical neurons.