The Theta Cycle and Spike Timing During Fixation in Rat Hippocampal CA1
In this paper, we report the first evidence that the spikes of CA1 neurons shift approximately half of the theta cycle: The shift occurs from the positive peak of the cycle recorded in the CA1 cell layer at the beginning of fixation, to the negative troug
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Abstract In this paper, we report the first evidence that the spikes of CA1 neurons shift approximately half of the theta cycle: The shift occurs from the positive peak of the cycle recorded in the CA1 cell layer at the beginning of fixation, to the negative trough at the end of fixation. This happens during a one-second period of immobile fixation, while the rat is fully alert and waits for the next event. It is known that the strongest input from entorhinal cortex occurs at the positive peak of the CA1 cell layer theta, whereas the strongest input from CA3 is associated with the trough of the theta. We hypothesize that CA1 pyramidal cells perform a function as an adaptive filter between entorhinal cortex and CA3, propagating relevant information depending on task requirements.
1 Introduction The theta rhythm in hippocampus is thought to play an important role in learning and memory. Many rodent studies have focused on the hippocampal theta rhythm that can be observed during translational movements (i.e., Type 1 theta [1].) However, less is known about the function of the theta rhythm during alert immobility (i.e., Type 2 theta [1].) Several studies pointed out that the hippocampus involves M. Takahashi () • J. Lauwereyns Graduate School of Systems Life Sciences, Kyushu University, Fukuoka 819-0395, Japan Brain Science Institute, Tamagawa University, Tokyo 194-8610, Japan e-mail: [email protected] Y. Sakurai Department of Psychology, Graduate School of Letters, Kyoto University, Kyoto 606-8501, Japan Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology Agency, Kawaguchi 332-0012, Japan Y. Isomura • M. Tsukada Brain Science Institute, Tamagawa University, Tokyo 194-8610, Japan Y. Yamaguchi (ed.), Advances in Cognitive Neurodynamics (III), DOI 10.1007/978-94-007-4792-0 101, © Springer ScienceCBusiness Media Dordrecht 2013
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intensive information processing during the delay period when the subject stays immobile while being fully alert [2]. Therefore, to understand the neural basis of learning and memory it is essential to elucidate the hippocampal mechanisms during this period. During the period of active exploration, hippocampal place cells exhibit a phenomenon called “theta phase precession,” firing spikes late in the ongoing theta cycle when a rat first enters the place field of the cell, but firing spikes earlier in the theta cycle as the rat progresses through the field [3, [4]. Some theoretical studies claimed an importance of this phenomenon to the hippocampal information processing [4–8]. Here we investigated the relationship between the theta rhythm in the local field potentials (LFPs) and the spike timing of the CA1 area during immobile fixation when Type 2 theta oscillation can be observed [9].
2 Methods Four male adult Wistar/ST rats were trained to perform a delayed (memory-guided) spatial alternation task using a nose-poking paradigm (Fig. 1) [9, 10]. A hyperdrive assembly with 14 tetrodes was chronically implanted on the rig
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