Chapter 4 - Cortical disorders of speech processing: Pure word deafness and auditory agnosia

Abstract

Selective disorders of auditory speech processing due to brain lesions are reviewed. Over 120 years after the first anatomic report (Dejerine and Sérieux, 1898), fewer than 80 cumulative cases of generalized auditory agnosia and pure word deafness with documented brain lesions are on record. Most patients (approximately 70%) had vascular lesions. Damage is very frequently bilateral in generalized auditory agnosia, and more frequently unilateral in pure word deafness. In unilateral cases, anatomical disconnection is not a prerequisite, and disorders may be due to functional disconnection. Regardless of whether lesions are unilateral or bilateral, speech processing difficulties emerge in the presence of damage to the superior temporal regions of the language-dominant hemisphere, suggesting that speech input is processed asymmetrically at early stages already. Extant evidence does not allow establishing whether processing asymmetry originates in the primary auditory cortex or in higher associative cortices, nor whether auditory processing in the brainstem is entirely symmetric. Results are consistent with the view that the difficulty in processing auditory input characterized by quick spectral and/or temporal changes is one of the critical dimensions of the disorder. Forthcoming studies should focus on detailed audiologic, neurolinguistic, and neuroanatomic descriptions of each case.