Chapter 19: Language & Lateralization

The discussion initiates with the discovery of hemispheric specialization, largely derived from studies of split-brain individuals who have undergone corpus callosotomy, revealing that the left hemisphere generally dominates verbal processing while the right hemisphere specializes in spatial cognition, face recognition, and emotional prosody. Experimental techniques used to study these asymmetries in healthy individuals, including dichotic listening tasks that typically show a right-ear advantage for speech and tachistoscopic presentations, are detailed alongside anatomical markers like the larger left planum temporale. The text explores the correlations between brain lateralization and handedness, citing the Wada test and genetic influences. Significant attention is given to the neurological deficits arising from specific brain lesions, such as astereognosis and prosopagnosia, the latter linked to damage in the fusiform gyrus. The summary rigorously classifies various language disorders or aphasias, contrasting the labored production of nonfluent (Broca's) aphasia with the nonsensical output of fluent (Wernicke's) aphasia, and describing the widespread deficits of global aphasia. Theoretical models of language organization are evaluated, specifically the classic Wernicke-Geschwind connectionist model involving the arcuate fasciculus and the alternative motor theory of language. The utility of brain mapping technologies, from intraoperative electrical stimulation to noninvasive transcranial magnetic stimulation and functional neuroimaging (PET, fMRI), is highlighted in defining cortical language zones. Furthermore, the summary delves into the components of verbal behavior, defining phonemes, morphemes, semantics, and syntax, and discusses the genetic underpinnings of language acquisition, with particular focus on the regulatory gene FOXP2 and parallels found in birdsong learning. The description concludes by examining the neural mechanisms of reading and writing, differentiating between acquired alexias (deep versus surface dyslexia) and developmental dyslexia associated with cortical ectopias, and finally reviewing the brain's capacity for recovery of function through neuroplasticity, embryonic stem cells, and rehabilitation strategies like constraint-induced movement therapy.