Authors:

Price, C.J., Howard, D., PATTERSON, K., Warburton, E.A., Friston, K.J. & Frackowiak, R.S.J.

Reference:

Year of publication:

1998

CBU number:

3677

Abstract:

Deep dyslexia is a striking reading disorder that results from left hemisphere brain damage and is characterised by semantic errors in reading single words aloud (e.g. reading SPIRIT as “whisky”). Two types of explanation for this syndrome have been advanced. One is that deep dyslexia results from a residual left hemisphere reading system that has lost the ability to pronounce a printed word without reference to meaning. The second is that deep dyslexia reflects right hemisphere word processing. Although previous attempts to adjudicate between these hypotheses have been inconclusive, the controversy can now be addressed by mapping functional anatomy. In this study, we demonstrate that reading by two deep dyslexic patients (CJ and JG) involves normal or enhanced activity in spared left hemisphere regions associated with naming (Broca’s area and the left posterior inferior temporal cortex) and with the meanings of words (the left posterior temporo-parietal cortex and the left anterior temporal cortex). In the right hemisphere homologues of these regions, there was inconsistent activation within the normal group and between the deep dyslexic patients. One (CJ) showed enhanced activity (relative to the normals) in the right anterior inferior temporal cortex, the other (JG) in right Broca’s area and both in the right frontal operculum. Although these differential right hemisphere activations may have influenced the reading behaviour of the patients, their activation patterns primarily reflect semantic and phonological systems in spared regions of the left hemisphere. These results preclude an explanation of deep dyslexia in terms of purely right hemisphere word processing.