Can what we know influence how the brain reconstructs a memory?

Details

When recalling breakfast from last Wednesday, you might not remember the exact meal but confidently select foods you usually eat. This illustrates how semantic knowledge—what you usually eat—can contribute to reconstructing an episodic memory of what you actually ate.

Researchers used fMRI and a highly realistic virtual environment to test the influence of semantic knowledge on episodic memory retrieval. During the task, 60 participants actively encountered task-relevant or passively encountered task-irrelevant everyday objects, placed either in expected locations, such as a rubber duck in a bathroom, or unexpected ones, such as a toaster in a bathroom. This created a conflict during retrieval between the episodic memory trace of a toaster in the bathroom and semantic information about a toaster in the kitchen. The researchers used multivariate analyses to examine the neural basis of this semantic bias. Before retrieval, half of the participants received cortisol, which is typically associated with impaired episodic memory retrieval, to manipulate the balance between correct episodic and incorrect semantic retrieval.

In the lateral occipital cortex (LOC), unexpected task-relevant objects showed greater similarity to their expected semantic counterparts than task-irrelevant objects did. Spatial memory also tended to be reflected in similarity patterns in the LOC. Unexpected objects showed greater pattern reorganization, described as pre- and post-encoding similarity, than expected objects, reflecting a difference in neural representation for objects encountered in conflict with prior knowledge.

Contrary to the researchers’ hypotheses, cortisol before retrieval had no effect on semantic bias. However, the cortisol group showed greater pattern reorganization in the posterior hippocampus. Higher confidence was linked to similarity patterns in the LOC and lingual gyrus in the placebo group, but not in the cortisol group. The researchers said these findings indicate an effect of cortisol on the reinstatement of memory traces during retrieval.

The findings help explain how the human brain constructs past episodes from episodic memory traces and suggest that prior semantic knowledge influences this process, as reflected in neural similarity patterns.