The Effects of Speed and Acceleration on the Theta and Delta Band Oscillations in the Hippocampus and Medial Entorhinal Cortex
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The theta band oscillation in the hippocampus and medial entorhinal cortex carries a strong correlation with free movement throughout an environment. Recently several models of these oscillations and their relationship to spatially modulated cells in the hippocampus and entorhinal cortex have started to emerge. One area of focus in theta research has been how the speed of movement modulates the theta frequency. Previous attempts to determine this relationship in the hippocampus and medial temporal lobe have failed to either engage the subject in prolonged running at a constant velocity, or they have not been able to facilitate natural movements during the recordings. Using a novel apparatus that can provide strict control over the speed of a freely moving rat, this study examines the relationship between movement related variables such as running speed and acceleration and oscillations in the theta (7-12Hz) and delta (1.5-6Hz) band in the medial entorhinal cortex and hippocampus during both constant running and during transitions in speed. The results showed that there was no relationship between running speed and medial entorhinal theta oscillations, as had previously been reported in the open field experiments. Interestingly the modulation of hippocampal and entorhinal theta was related to the magnitude of the acceleration of the animals’ movements. This novel finding questions the current opinion attained from open field recordings of instantaneous constant running as well as the models of grid cell firing and theta phase precession that assume a linear relationship between running speed and theta frequency.