Semantic Diversity Accounts for the “Missing” Word Frequency Effect in Stroke Aphasia: Insights Using a Novel Method to Quantify Contextual Variability in Meaning

摘要

Word frequency is a powerful predictor of language processingnefficiency in healthy individuals and in computationalnmodels. Puzzlingly, frequency effects are often absent in strokenaphasia, challenging the assumption that word frequency influencesnthe behavior of any computational system. To address thisnconundrum, we investigated divergent effects of frequency in twoncomprehension-impaired patient groups. Patients with semanticndementia have degraded conceptual knowledge as a consequencenof anterior temporal lobe atrophy and show strong frequencyneffects. Patients with multimodal semantic impairmentsnfollowing stroke (semantic aphasia [SA]), in contrast, show littlenor no frequency effect. Their deficits arise from impaired controlnprocesses that bias activation toward task-relevant aspects ofnknowledge. We hypothesized that high-frequency words exertngreater demands on cognitive control because they are more semanticallyndiverse—they tend to appear in a broader range ofnlinguistic contexts and have more variable meanings. Using latentnsemantic analysis, we developed a new measure of semantic diversitynthat reflected the variability of a wordʼsmeaning across differentncontext. Frequency, but not diversity, was a significantnpredictor of comprehension in semantic dementia, whereas diversitynwas the best predictor of performance in SA. Most importantly,nSA patients did show typical frequency effects but onlynwhen the influence of diversity was taken into account. These resultsnare consistent with the view that higher-frequency wordsnplace higher demands on control processes, so that when controlnprocesses are damaged the intrinsic processing advantages associatednwith higher-frequency words are masked.