Enhanced Pattern Separation Memory in Adults Diagnosed with ASD

Background:  Jocelyne Bachevalier and others have demonstrated in non-human primates the importance of tracking how specific timing and placement of lesions in the medial temporal lobe differentially affect autism symptoms. Behavioral studies by Bowler and colleagues have shown that atypical hippocampus function in ASD may underlie relational difficulties as well as some specific memory deficits. To date, however there have been few functional neuroimaging studies of explicit memory in ASD that can bridge these separate but converging lines of evidence. Memory tasks involving pattern separation can highlight mechanisms involved in interference reduction among similar memory representations and are important for memory accuracy. Pattern separation is thought to specifically involve dentate gyrus and CA3 regions of the hippocampus (Holden et al., 2013) that have been implicated in at least one mouse model of autism (Takeuchi et al., 2013). 

Objectives: To characterize the neural correlates of performance on a pattern separation memory task in adults diagnosed with an ASD compared to age- and IQ-matched healthy controls.

Methods: While undergoing high-resolution fMRI scans, participants completed a continuous recognition task in which they were shown a series of everday objects one at a time. For each object, participants indicated whether it was new (a novel object that had never been seen), old (the exact same object had been seen before), or similar (similar, but not identical to one seen previously).  The key comparison is the ability to identify “similar” but not identical objects.

Results: Participants diagnosed with ASD performed better than controls on the similar condition of the pattern separation task, indicating increased accuracy for detail overall. FMRI comparisons did not show significant differences in hippocampus activity but the ASD group showed some areas of greater prefrontal cortex activation during the similar condition, suggesting more effortful and/or more focused attention to the memory stimuli. This pattern is in line with other reports of better detail-oriented performance in high-functioning ASD and may contribute to difficulties tracking other complex stimuli, such as social interactions, because of extra neural resources devoted to lower-level tasks. 

Conclusions: There is a need for more paradigms to investigate more specific brain regions—such as subregions of the hippocampus—in order to align human neuroimaging research with focused animal studies. Pattern separation tasks can offer such detailed investigations of the hippocampus including regions implicated in autism. These findings indicate that superior performance in some areas, such as memory for details, involves a cost in neural activity that may contribute to performance deficits in complex situations such as social interactions.