Hippocampal theta activity related to elicitation and inhibition of approach locomotion

This study determined if the hippocampal theta rhythm showed phase relationships or changes in amplitude and frequency with the onset of stimuli and locomotion in a task in which auditory cues initiated and suppressed approach locomotion. Rats with electrodes in the dorsal hippocampus lapped at a milk dipper and were presented a tone which predicted the delivery of a food pellet. In some trials the pellet cue tone was negated by 60-Hz clicks beginning 0.3 s after onset, and no pellet was delivered. A video capture system (20-ms sampling) synchronized to the hippocampal recording system (10-ms sampling) was used to determine the onset of locomotor approach to the pellet area. The findings failed to support proposals that phase-related mechanisms play a role in encoding and retrieval of movement-related information. Neither the pellet cue nor the negating cue reset the theta rhythm, and they did not produce differential evoked potentials. During milk lapping, theta amplitude increased in the 1/2 s prior to all pellet cues regardless of their locomotor effect. Frequency also rose but only when a non-negated pellet elicited short-latency locomotion. During locomotor execution, theta peak amplitude peaked earlier than theta frequency by approximately one period. In general during performance of this task, increasing theta amplitude reflected a general preparation to process the cue and increasing theta frequency reflected the readiness to respond to the cue with locomotion.