The relationship of hippocampal theta oscillations to neurophysiology, attention, and motor behavior in the rat

The theta rhythm is an oscillation of 3–12 hz that has been recorded from the hippocampus of several mammalian species. This thesis examines the relationship of this rhythm with certain neurophysiological properties and components of behavioral tasks within the rat. Within urethane anesthetized rats, it was found that there is a consistent relationship between the phase of theta and the amplitude of evoked synaptic potentials. This data contributes to a number of theta-phasic parameters found by other investigators and informs computational models of hippocampal function (Hasselmo, Bodelon and Wyble, 2002). A second experiment examined the possibility of phase reset during acquisition of a conditional visual discrimination task, a phenomenon previously found in working memory tasks (Givens, 1996). There was no observed relationship between theta phase and bar presses or the illumination of the cue light. Using the same data, theta power was examined and it was found that during periods of chance (50%) accuracy, center lever presses to initiate a trial were accompanied by an increase in theta power. During periods of successful discrimination, theta power decreased slightly during the same bar press and increased about 200 msec prior to the cue light. Further analysis of theta power in this task found a large decrease in theta power during the choice bar presses, in contrast to the increase in theta power during bar presses to initiate a trial. This result stands counter to results that predict consistent relationships between motor actions and theta power (Vanderwolf, 1968, Whishaw, Bland and Vanderwolf, 1972).; A model of theta is suggested in which Type-2 cholinergic theta adds to an underlying baseline of motor-related, noncholinergic type-1 theta. To further investigate the discordance between theta power and motor activity a runway task was devised in which rats ran back and forth between two ends of a linear track, one of which was always rewarded and the other never rewarded. Theta power decreased 240 msec before movement ended on the rewarded end but not the unrewarded end of the track. Results are discussed in the context of a possible suppressive effect of consummatory behavior on theta oscillations.