Dynamic aphasia: an inability to select between competing verbal responses?

In this study we report a patient (A.N.G.) who, following a malignant left frontal meningioma impinging upon Brodmann area 45, presented a 'pure' dynamic aphasia. Her spontaneous speech was markedly reduced in the absence of any syntactical impairment. Her naming, repetition and reading skills were completely normal. Two experimental investigations were carried out. The first investigation found that A.N.G. had a profound impairment in phrase and sentence generation tasks given a verbal context. However, her verbal generative skills were normal when she was asked to describe pictorial scenes and complex actions. Moreover, it was found that A.N.G. had no difficulty ordering the constituent words of a sentence. Thus, it was concluded that her verbal planning skills were intact. The second investigation tested a hypothesis that dynamic aphasia is due to an inability to select a verbal response option whenever the stimulus activates many competing verbal responses. Predictions based upon this hypothesis were confirmed on three different verbal generation tasks. It was found that our patient's grave verbal generative impairment was present for tasks involving stimuli which activate many potential responses. However, it was absent for tasks involving stimuli which activate few or only a single 'prepotent' response. The findings are discussed with reference to traditional interpretations of dynamic aphasia and more general interpretations of prefrontal cortex functioning. On the basis of a computational model of prefrontal cortex functioning, we propose that pure dynamic aphasia may be caused by damage to a 'context' module containing units responsible for selection of verbal response options. Moreover, it is suggested that our findings support the view that Brodmann area 45 is involved in verbal response generation to stimuli which activate many potential response options.