Searching for the index code in the neocortex - Learning-induced conjunctive representation in the medial prefrontal cortex.
ARL NSMA, Univ. Arizona, Tucson, AZ
The hippocampus is presumed to contribute to retaining memories by storing a temporary index of the activated neocortical areas until the memories are consolidated as cortico-cortical connections; however, the nature of these cortico-cortical connections remains an open question. One possibility is that new, perhaps more efficient index codes develop in the medial prefrontal cortex (mPFC), which is important for retrieval of remote memory, after consolidation. We explored this hypothesis by searching for conjunctive representations of the sensory stimuli constituting behavioral and spatial context that underlie the ability to retain a specific index code for each episode and analyzing the time-course of their development.
We recorded multiple single-unit activity from the prelimbic area of the mPFC of rats that received pairings of a tone and a periorbital shock with a fixed temporal pattern (trace eyeblink conditioning: EBC) in one spatial context and those with a random interval (pseudoconditioning: PC) in the other spatial context.
The rats acquired the eyeblinks to the tone (CR) preferentially during EBC, but not during PC. Approximately 75% of recorded neurons responded to the tone presentation, and half of them differentially responded to the same tone depending on the behavioral and/or spatial context. Population activity to the tone was similar in both behavioral contexts at the beginning of conditioning; however, clear tonic responses developed specifically during EBC as the rat acquired the CR and gradually strengthened to a level that was maintained for a 6 week over-training period. Population activity to the tone alone presentation in the training contexts and that during PC did not change through all the conditioning sessions.
These results suggest that the mPFC can encode conjunctive representations of sensory stimuli and their spatial and/or behavioral context, if the stimuli are behaviorally significant in that context. Such conjunctive activity could potentially serve as a cortical index code for the consolidated memory.
Grant/Other Support: MH046823; Human Frontier Science Program
Keyword (Complete): consolidation; episodic memory; multielectrode; classical conditioning