Troubled letters but not numbers. Domain specific cognitive impairments following focal damage in frontal cortex

Abstract

A well educated right-handed woman developed severe and stable alexia and agraphia following a circumscribed surgical lesion in the left premotor cortex. The lesion was located in Brodmann's field 6, above Broca's area, in the region traditionally referred to as Exner's area. The alexia and agraphia occurred in a pure form, that is, in the setting of otherwise normal cognitive and neurological function. She was not aphasic or hemiparetic and her visual perception, intellect, memory, oral spelling and drawing were normal. The patterns of impairment of reading and writing closely paralleled one another. Reading of single words and letters was severely impaired, and she was entirely unable to read sentences. She was virtually unable to write recognizable letters, could write no words, and her writing attempts were severely distorted spatially. By contrast, she could easily read all numbers and nonverbal symbols, and she was equally able to write numbers and perform written calculations without difficulty. These striking dissociations provide further evidence of the domain specificity of cognitive/neural representations. They also point to the possible role of premotor cortices in the coactivation of precise sequences of motor and sensory activity patterns involved in reading and writing.