Long-term declarative memory depends on pattern separation, which reduces the degree of overlap between similar representations, to maintain memory specificity, and on pattern completion, which occurs when a degraded cue is used to retrieve a previously stored memory. Previous studies aimed at evaluating the underlying neuronal substrates of these computational processes have used a mnemonic discrimination paradigm and fMRI to focus on the hippocampus, to the exclusion of cortical processing. We aim to investigate the influences extra-hippocampal processes have on pattern separation in the following two studies. Study 1. Computational models of pattern completion suggest it occurs cortically and results in generalized memories whereas pattern separation occurs in the hippocampus and results in memory specificity. It is unknown how the incongruity of these two neuronal processes is resolved. Many studies evaluating the neuronal correlates of pattern separation have used fMRI to evaluate activity in the hippocampus. The sluggish time resolution of fMRI and the restricted spatial focus leave room for considerable differences between pattern completion and pattern separation to go undetected. Here, we use encephalography (EEG) and an event-related potential (ERP) analysis to examine neuronal activity during pattern separation and pattern completion to investigate whether or not cortical processing is employed to resolve the discrepancy between these two neuronal processes. We largely did not observe differences between the ERPs associated with pattern separation and pattern completion. Failure to identify neuronal differences could result from the bulk of neuronal processing differentiating between the two processes occurring deeper in the brain than can be measured by ERPs. Study 2. Extrinsic rewards contingent on memory performance can boost memory and learning. However, the effects of extrinsic rewards on memory specificity, particularly in regards to the process of pattern separation, are not well understood. In this behavioral study, we evaluate how extrinsic rewards affect behavioral performance in a task that taxes pattern separation. Our data show that rewards given for participation at the time of encoding boost mnemonic discrimination between target-lure pairs while rewards given for memory performance at the time of retrieval do not. We hypothesize this is because pattern separation is an encoding dependent process. This boost in discriminability is only seen when the rewarded stimuli are blocked together in separate blocks from the non-rewarded stimuli. When the rewarded and non-rewarded stimuli are interspersed within blocks, discriminability does not significantly differ between the rewarded and non-rewarded trials. Overall, performance was better when rewards were contingent on performance than when rewards independent of performance, although this difference is eliminated when attention during encoding is controlled.



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Life Sciences; Physiology and Developmental Biology



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pattern separation, mnemonic discrimination, hippocampus, extrinsic rewards, attention, episodic memory, event-related potentials



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Physiology Commons