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. 2023 Oct 17;120(42):e2312462120.
doi: 10.1073/pnas.2312462120. Epub 2023 Oct 12.

Neural evidence of switch processes during semantic and phonetic foraging in human memory

Nancy B Lundin  1   2   3 , Joshua W Brown  1   2   4 , Brendan T Johns  5 , Michael N Jones  1   4 , John R Purcell  1   2   6 , William P Hetrick  1   2   7 , Brian F O'Donnell  1   2   7 , Peter M Todd  1   4
Affiliations

Neural evidence of switch processes during semantic and phonetic foraging in human memory

Nancy B Lundin et al. Proc Natl Acad Sci U S A. .

Abstract

Humans may retrieve words from memory by exploring and exploiting in "semantic space" similar to how nonhuman animals forage for resources in physical space. This has been studied using the verbal fluency test (VFT), in which participants generate words belonging to a semantic or phonetic category in a limited time. People produce bursts of related items during VFT, referred to as "clustering" and "switching." The strategic foraging model posits that cognitive search behavior is guided by a monitoring process which detects relevant declines in performance and then triggers the searcher to seek a new patch or cluster in memory after the current patch has been depleted. An alternative body of research proposes that this behavior can be explained by an undirected rather than strategic search process, such as random walks with or without random jumps to new parts of semantic space. This study contributes to this theoretical debate by testing for neural evidence of strategically timed switches during memory search. Thirty participants performed category and letter VFT during functional MRI. Responses were classified as cluster or switch events based on computational metrics of similarity and participant evaluations. Results showed greater hippocampal and posterior cerebellar activation during switching than clustering, even while controlling for interresponse times and linguistic distance. Furthermore, these regions exhibited ramping activity which increased during within-patch search leading up to switches. Findings support the strategic foraging model, clarifying how neural switch processes may guide memory search in a manner akin to foraging in patchy spatial environments.

Keywords: cerebellum; foraging; hippocampus; memory search; verbal fluency.

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Conflict of interest statement

The authors declare no competing interest.

Figures

Fig. 1.
Fig. 1.
Verbal fluency responses produced by two selected participants over 3-min task periods. Separate plots illustrate different methods of designating switch- and cluster-related responses.
Fig. 2.
Fig. 2.
Significant fMRI activation during verbal fluency test (VFT) performance from Switch–Cluster contrasts. Separate rows illustrate results from models which differ by method of cluster and switch designation.
Fig. 3.
Fig. 3.
Significant fMRI activation during verbal fluency test (VFT) performance which loaded onto parametrically modulated regressors (PMRs) that increased linearly during cluster events and reset at switch events. Separate rows illustrate results from models which differ by method of cluster and switch designation.
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