Frequency and phase of hippocampal theta activity in the spontaneously behaving cat.

Abstract 1. 1. The relationship between the activity of hippocampal system structures and behavior was investigated in twenty-two cats with chronically implanted electrodes. 2. 2. The general repetition rate of hippocampal theta rhythm was found to change with phase of behavioral orienting and with type of visual activity during adaptation to a new environment. Both frequency and amplitude were maximal during activated sleep. 3. 3. Simultaneous neocortical-hippocampal desynchronization occurred often during early phases of orienting and during fixed staring with any phase of orienting. On the other hand, in six cats regular theta waves continued during behavioral and EEG sleep in the dorsal hippocampus and/or hippocampal gyrus. 4. 4. Phase relationships between the theta rhythm in different hippocampal structures varied from wave to wave but tended to remain generally similar during all spontaneous activities. Marked phase shifts and occasionally phase reversals accompanied spontaneous changes in theta frequency. 5. 5. Results suggest that changes in theta frequency are a sub-threshold manifestation of the desynchronizing arousal response.

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