A failure of high level verbal response selection in progressive dynamic aphasia

Abstract

Different theoretical interpretations have been offered in order to account for a specific language impairment termed dynamic aphasia. We report a patient (CH) who presented with a dynamic aphasia in the context of nonfluent progressive aphasia. CH had the hallmark of reduced spontaneous speech in the context of preserved naming, reading, and single word repetition and comprehension. Articulatory and grammatical difficulties were also present. CH had a very severe verbal generation impairment despite being able to describe pictorial scenes and action sequences well. In the experimental investigations CH was severely impaired in word, phrase, and sentence generation tasks when many competing responses were activated by a stimulus. By contrast, he could generate verbal responses satisfactorily when a dominant response was activated by a stimulus. For the first time, we demonstrated that the verbal generation impairment was specific to the production of language. Strikingly, our patient was unimpaired on a number of nonverbal generation tasks (e.g., design fluency, gesture fluency, and motor movement generation). MRI revealed focal left frontal atrophy that predominantly affected Brodmann's Areas 44 and 45. Our findings are discussed with reference to alternative accounts of dynamic aphasia and models of speech production. We interpret our patient's impairment as being underpinned by an inability to select between competing verbal response options. This interpretation converges with evidence from the neuroimaging literature, which implicates the left inferior frontal gyrus in the selection of a response among competing information. We conclude that the left posterior inferior frontal gyrus is involved in the generation of verbal output, and specifically in the selection between competing verbal responses.