Understanding and applying extracellular recordings in awake, behaving animals

Section I of this thesis presents specific projects applying extracellular recordings in macaques. The results demonstrate a novel relative position code employed by individual neurons in the dorsal premotor cortex during reaches and saccades, which may be important for hand-eye coordination. Another project suggests low-frequency coherence between dorsal premotor and posterior parietal cortices participates in making internally guided decisions of where to reach. Section II investigates analysis of data collected with this technique. One project compares methodologies for computing trial-by-trial coherence among neuron ensembles, and another documents how the innate nonstationarity of animal behavior can affect conclusions of certain analyses. Section III investigates the acquisition of extracellular electrophysiological data to understand how these recorded voltages relate to underlying neural activity. One project verifies a physical model of the microelectrode recording circuit using electrodes suspended in saline. Some lower-input impedance head stages used in the field are shown to result in electrode impedance and frequency-dependent amplitude attenuations and phase shifts of recorded signals. Other projects present a theoretical argument that local field potentials (LFPs) recorded from in-depth microelectrodes should be independent of electrode impedance within the range of impedances typically used. A simple physical model shows that if and only if gradients of LFP coherence exist at a scale finer than an electrode's recording site size, lower-impedance electrodes report higher coherence. However this is not expected to occur between different microelectrodes, but could explain differences between microelectrode and EEG recordings. The final project uses simultaneous extracellular and intracellular recordings in corticostriatal rat brain slices to show that frequency-dependent phase shifts and amplitude attenuations occur in neural tissue itself and characterize the transfer function between the intracellular and extracellular voltages. Inhomogenous microscale obstructions inherent in neural tissue are shown to differentially distort current flow depending on the severity of the obstructions. This challenges existing beliefs about the nature of current flow in the brain, and should be considered when interpreting electrophysiological data.