Mechanisms underlying perseveration in aphasia: evidence from a single case study.

Aphasic individuals often inappropriately and unintentionally repeat recent responses, errors termed recurrent perseverations. In a series of picture naming experiments, we investigated the impact of manipulating stimulus factors on the number of perseverations produced by an aphasic patient (E.B.) with markedly impaired naming skills. E.B. produced significantly more perseverations to pictures with low frequency names and following stimulus repetition. In contrast, semantic relatedness and presentation rate failed to influence perseveration. Our results are considered in the context of theories that relate recurrent perseverations to intact priming mechanisms [Brain 121 (1998) 1641; Aphasiology 12 (1998) 319; J. Exp. Psychol. Learn. Mem. Cogn. 19 (1993) 243]. We conclude that these theories can correctly predict some but not all aspects of E.B.'s perseverations. In particular, they failed to predict that: (1) perseverations often appeared to reflect the earlier sequential proximity of stimuli and responses; and (2) perseverations became less likely as more experimental trials intervened, a trend that did not interact with presentation rate. We review evidence relating recurrent perseverations to neuromodulatory deficits and we conclude that a theory of the functional role of neuromodulation in the cerebral cortex proposed by Hasselmo [Neural Netw. 7 (1994) 13] is capable of accounting for all aspects of E.B.'s perseverative behavior.