Analysis of lexical-semantic processing and extensive neurological, electrophysiological, speech perception, and language evaluation following a unilateral left hemisphere lesion: Pure word deafness?

Abstract

Background : Theoretical models of lexical-semantic processing that explicate the relationships between spoken word comprehension, repetition, and writing to dictation are discussed in relation to pure word deafness (PWD). Various theories relative to the neuroanatomical sites and pathophysiological/electrophysiological mechanisms that cause the speech perception deficit seen in PWD are reviewed, as well as case studies that have identified variables that facilitate comprehension of spoken language for individuals with PWD. Aims : A case study of an individual with an extensive and complex neurological history and a constellation of symptoms suggestive of PWD is presented. Extensive evaluation, including experimental tasks developed to test a theoretical lexical processing model, was undertaken to determine whether this case fitted the criteria for a diagnosis of PWD. Methods & Procedures : Extensive neurological, electrophysiological, speech perception, and language testing was undertaken to confirm site of lesion, assess neurophysiological processes underlying speech perception, and determine overall language status. Experimental tasks based on a lexical-semantic processing model assessed word and nonword repetition and writing to dictation under two conditions: presentation with and without facial cues. Outcomes & Results : MRI confirmed a lesion in the participant's left frontoparietal area. Electrophysiological measures indicated his auditory nerve and auditory brainstem functions were intact, middle latency response (MLR) was normal over his right hemisphere and absent over his left hemisphere, and a significant mismatch negativity (MMN) response was elicited by pitch, but not phonetic, differences. Speech perception testing indicated he had difficulty with spectral or spectro-temporal discrimination and combining phonological and semantic information, as well as decreased ability to use formant motion in sentence perception and to perceive speech in noise. Language testing revealed relatively intact spontaneous language production and reading comprehension. The participant's performance on experimental tasks indicated an effect of syllable length and presentation mode for both words and nonwords. That is, as the number of syllables increased, his performance decreased; however, repetition and writing to dictation reflected superior performance with facial cues regardless of word and nonword length when compared to presentation without cues. Relative to the lexical processing model, the participant's performance suggested impoverished phonetic processing in the auditory analysis system. Conclusions : The results of extensive evaluation revealed a selective impairment in acoustic phonetic perception that is consistent with a diagnosis of PWD. The findings of this study support other studies that have demonstrated that speech perception involves cortical areas other than the temporal lobes.