Abstract
Performance of memory tasks is impaired by lesions to either the medial prefrontal cortex (mPFC) or the hippocampus (HPC); although how these two areas contribute to successful performance is not well understood. mPFC unit activity is temporally affected by hippocampal-theta oscillations, with almost half the mPFC population entrained to theta in behaving animals, pointing to theta interactions as the mechanism enabling collaborations between these two areas. mPFC neurons respond to sensory stimuli and responses in working memory tasks, though the function of these correlated firing rate changes remains unclear because similar responses are reported during mPFC dependent and independent tasks. Using a DNMS task we compared error trials vs. correct trials and found almost all mPFC cells fired at similar rates during both error and correct trials (92%), however theta-entrainment of mPFC neurons declined during error performance as only 17% of cells were theta-entrained (during correct trials 46% of the population was theta-entrained). Across the population, error and correct trials did not differ in firing rate, but theta-entrainment was impaired. Periods of theta-entrainment and firing rate changes appeared to be independent variables, and only theta-entrainment was correlated with successful performance, indicating mPFC-HPC theta-range interactions are the key to successful DNMS performance.
Highlights
Working memory tasks require an organism to keep acute, trial specific information online, while simultaneously maintaining longer-term, consistent information about the nature of the task
The process was repeated up to 1000 times and a distribution of correct trial mean resultant length (MRL) was constructed, and we examined whether the error trial MRL fell into one of the two tails of the distribution
Working memory tasks make unique demands on both long term memory and working memory, and it has been speculated that medial prefrontal cortex (mPFC)-HPC interactions mediate the interplay of these two types of information (Winocur, 1992; Doyere et al, 1993; McIntosh, 1999; Wall and Messier, 2001; Koene et al, 2003; Hasselmo, 2005; Ranganath and D’Esposito, 2005; Vertes, 2006)
Summary
Working memory tasks require an organism to keep acute, trial specific information online, while simultaneously maintaining longer-term, consistent information about the nature of the task (including the rules, timing and environment of the task). To be successful the agent, or organism, must combine these two types of information by first encoding and later utilizing the trial specific information at precisely the right time in the task (Zilli and Hasselmo, 2008). It remains unclear how exactly the brain encodes, integrates and implements these two streams of information, but evidence has shown integral involvement of both the hippocampus (HPC) and prefrontal cortex for proper working memory task performance (Jacobsen, 1936; Brown and Bowman, 2002; Chudasama and Robbins, 2004; Eichenbaum, 2004; Lee et al, 2004). FMRI data show simultaneous prefrontal and HPC activation during working memory tasks (Stern et al, 2001; Schon et al, 2005)
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