Neural Switch Processes Guide Semantic and Phonetic Foraging in Human Memory

Humans may retrieve words from memory by exploring and exploiting in linguistic "space" similar to hownon-human animals forage for resources in physical space. This has been studied using the verbalfluency test (VFT), in which participants generate words belonging to a semantic or phonetic category in alimited time. The foraging in mind model proposes that individuals performing VFT monitor their responseproduction rate as they search through and deplete a local patch (subcategory) of items in memory andthen switch to a new patch in another part of semantic or phonetic space. An alternative model holds thatparticipants use a random walk process, and switches are merely epiphenomenal long steps reflectingthe tail of the random walk step size distribution. This study tests these competing theories by examiningwhether there is distinct neural activity during exploring between ("switching") versus exploiting within("clustering") related response groupings (foraging), or no neural differences between search phases(random walk). Thirty participants performed category and letter VFT during functional magneticresonance imaging. Responses were categorized as cluster or switch events based on computationalmetrics of similarity and participant evaluations. Findings provide neural evidence of a cognitive foragingprocess, with greater hippocampal and cerebellar activation during switching compared to clustering,even while controlling for greater semantic and phonetic distance and response times. Furthermore,these regions exhibited ramping activity leading up to switch events. These results clarify how neuralswitch processes may guide memory searches in a manner akin to foraging in patchy spatialenvironments.