Topic shifts elicit costs in naturalistic discourse: evidence from healthy adults and individuals with traumatic brain injury

Individuals with traumatic brain injury (TBI) often report difficulties with following conversation topics. It remains unclear how healthy adults manage topic shifts, making it difficult to determine why these abilities break down after TBI. We analyzed conversations from healthy adults and TBI patients, identified topic boundaries, and compared responses following new subtopics (“shift”) versus continuing subtopics (“no shift”) using measures of language production. Across groups, post-shift responses were shorter and simpler (fewer words, verbs, and instances of repetition or rephrasing) and required more planning (more filled pauses), reflecting “shift costs”. TBI patients produced overall shorter and syntactically simpler responses, though shift costs did not differ between groups. Within the TBI group, more severe injuries were associated with smaller shift costs, possibly reflecting reduced sensitivity to topic structure. These findings suggest that the cognitive demands of tracking topic changes affect language production and that these effects are modulated by injury severity.