Modulation of Hippocampal Memory

In studying learning and memory mechanisms, we make an important distinction between conditions required for learning (i.e., modulation of learning), and the actual stored content of learning. The conditions of pattern separation can be studied from the perspective of survival systems where emotional arousal is involved, and also from the perspective of reinforcement signals where feedback and prediction error are involved. We use both perspectives to examine the conditions that optimize hippocampal pattern separation and how these conditions change as a function of age.

Emotion arousal and hippocampal pattern separation

We examined the role of emotional arousal in modulating hippocampal function (cf. McGaugh, 2004) using an emotional pattern separation task (see figure) where valence and interference (i.e., similarity) were parametrically varied. We found that emotion enhanced target recognition but impaired lure discrimination, suggesting that emotion’s enhancing effect was specific to gist information and not details. We also demonstrated that emotional pattern separation was associated with emotional modulation signals in the amygdala and memory signals in the dentate (DG)/CA3. We also found that adults with depressive symptoms were better at discriminating negative lures and worse at discriminating neutral lures, an effect that was associated with a network imbalance where the amygdala’s response was heightened and the DG/CA3 response was suppressed. These results have appeared in two recent papers led by graduate student Stephanie Leal. The first is a behavioral paper in Neurobiology of Learning and Memory and the second is a high-resolution fMRI paper in Hippocampus.

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Emotional discrimination task. During encoding, participants rated images according to their emotional valence (using three buttons for negative, neutral, and positive). Each image was presented for 2500ms with a 500ms inter-stimulus-interval (ISI). After a 5- minute delay, participants underwent a surprise recognition test where they viewed negative, neutral, and positive targets (exact repetitions), foils (new items), and lures (similar items) and were asked to indicate whether items were “old” or “new”.