ABSTRACT
Vivian L. Smith Center for Neurologic Research, Department of Neurosurgery,
University of Texas, Health Science Center-Houston
Several methods of investigation have been used to identify the cerebral mechanisms for reading single words. Lesion studies have been used at least since Dejerine (1892) to determine the brain regions necessary for successful reading. Deficits that become apparent following brain damage necessarily imply that part of the lesioned area was critical to successful reading performance, either by hosting neurophysiological processes that support particular component operations involved in reading, or disrupting connections between such brain regions. Cortical stimulation studies in individuals undergoing awake craniotomies have also provided important insights regarding the location of brain areas that serve as components of the brain mechanism for single-word reading. In addition, transcranial magnetic stimulation (TMS) has been used by researchers to identify critical areas for reading by temporarily inhibiting the activity in specific cortical sites. While these techniques rely on the study of deficits caused by either transient or permanent interference with the brain mechanisms for reading, functional brain imaging methods attempt to reveal the outline of the said mechanisms simply by recording “echos” of the healthy, working brain, in the form of various types of electromagnetic signals, while the person engages in reading tasks. From these studies it has become apparent that singleword reading requires a highly integrated network of closely interconnected brain
areas, the majority of which reside in the left cerebral hemisphere. This network or “mechanism” presumably includes separate circuits, each composed of distinct neurophysiological processes that take place in one or more brain regions. A recently proposed model (see for instance Pugh, Mencl, Jenner, Lee, Katz, Frost, Shaywitz, & Shaywitz, 2001) postulates three brain circuits, two of which reside in posterior brain regions while the third comprises anterior brain areas. Of the two posterior circuits, one resides mainly in temporoparietal cortices (roughly corresponding to Wernicke’s area) and the angular gyrus, and is mainly involved in phonological analysis and in establishing associations between word-like stimuli and phonological representations. The second posterior circuit includes higher-order visual association areas in both lateral and ventral occipito-temporal cortices and appears to be primarily involved in the visual analysis of print according to graphemic conventions. The third, anterior circuit involves ventrolateral prefrontal and premotor cortices (in the vicinity of Broca’s area). It appears to be involved in articulatory recoding during both oral and silent reading.
