When comprehension elicits incomprehension: Deterioration of semantic categorisation in the absence of stimulus repetition

Repetition improves retrieval from memory; however, under some circumstances, it can also impair performance. Separate literatures have investigated this phenomenon, including studies showing subjective loss of meaning following 'semantic satiation', slowed naming and categorisation when semantically related items are repeated and semantic 'access deficits' in aphasia. Such effects have been variously explained in terms of habituation of repeatedly accessed representations, increased interference from strongly activated competitors and long-term weight changes reflecting the suppression of non-targets on earlier trials (i.e., retrieval-induced forgetting). While studies of semantic satiation involve massed repetition of individual items, competition and weight changes at the conceptual level should elicit declining comprehension for non-repeated items: this pattern has been demonstrated for picture naming but effects in categorisation are less clear. We developed a paced serial semantic task (PSST), in which participants identified category members among distracters. Performance in healthy young adults deteriorated with ongoing retrieval for non-repeated words belonging to functional categories (e.g., picnic), taxonomic categories (e.g., animal) and feature-based categories (e.g., colour red - 'tomato', 'post box'). This decline was greatest at fast presentation speeds (when there was less time to overcome competition/inhibition) and for strongly associated targets (which may have accrued more inhibition to facilitate earlier target categorisation). Deteriorating performance was also seen across words and pictures, consistent with a conceptual locus. We observed a release from deteriorating categorisation following a switch to a new category, demonstrating that this was not a general effect of time on task. Patients with semantic aphasia, who have deficient semantic control, maintained their performance throughout the categories, unlike younger adults: this finding is hard to reconcile with accounts of declining performance that propose a build-up of competition, since the patients should have had greater difficulty resolving such competition. These results instead suggest that declining performance on our goal-driven categorisation task was linked to the use of a controlled retrieval strategy by healthy young adults. Patients may not have inhibited related non-target knowledge to facilitate initial categorisation like younger volunteers, and consequently they were less vulnerable to declining comprehension in this paradigm. Together, these results demonstrate circumstances which produce declines in continuous categorisation in healthy adults.