JN Journal of Neurophysiology
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J Neurophysiol (May 6, 2009). doi:10.1152/jn.91159.2008
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Submitted on October 20, 2008
Revised on April 24, 2009
Accepted on April 28, 2009

A Code for Spatial Alternation during Fixation in Rat Hippocampal CA1 Neurons

Muneyoshi Takahashi1*, Johan Lauwereyns2, Yoshio sakurai3, and Minoru Tsukada4

1 Tamagawa University Brain Science Institute
2 Victoria University of Wellington
3 kyoto university
4 Tamagawa University

* To whom correspondence should be addressed. E-mail: taka{at}lab.tamagawa.ac.jp.

The classical notion of hippocampal CA1 "place cells," whose activity tracks physical locations, has undergone substantial revision in recent years. Here, we provide further evidence of an abstract spatial code in hippocampal CA1, which relies on memory and adds complexity to the basic "place cell". Using a nose-poke paradigm with four male Wistar rats, we specifically concentrated on activity during fixation, when the rat was immobile and waiting for the next task event in a memory-guided spatial alternation task. The rat had to alternate between choosing the right and left hole on a trial-by-trial basis, without any sensory cue, and relying on an internal representation of the sequence of trials. Twelve tetrodes were chronically implanted for single-unit recording in the right CA1 of each rat. We focus on 76 single neurons that showed significant activation during the fixation period as compared to baseline activity between trials. Among these 76 fixation neurons, we observed 38 neurons that changed their fixation activity systematically as a function of the alternation sequence. That is, even though these rats were immobile during the fixation period, the neurons fired differently for trials in which the next spatial choice should be left (i.e., RIGHT-TO-LEFT trials) as compared to trials in which the next spatial choice should be right (i.e., LEFT-TO-RIGHT trials), or vice versa. Our results imply that these neurons maintain a sequential code of the required spatial response during the alternation task, and thus provide abstract information, derived from memory, that can be utilized for efficient navigation.







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