Historical Context

The association between aphasia and damage to a particular region of the left cerebral hemisphere in the vast majority of right-handed individuals (and, to a lesser extent, among left-handers) has, since it was first noted by the neurologist Paul Broca in the late 1800s, generated an explosion of research into neural bases of language organization and functioning. Much of the early research took the form of clinical case reports of aphasia following various kinds of brain injury. Other clinical research, such as that carried out in the 1950s by the Montreal-based neurosurgeon Wilder Penfield, studied patients scheduled to undergo neurosurgery for treatment ofsevere cases ofepilepsy. These patients first underwent cortical electrical stimulation to map sites responsible for language production. Another technique used with these patients, the Wada technique named after its founder, involves injection of sodium amytal into the left or right cerebral artery, which has the effect of rendering the patient temporarily aphasic, but only when the language-dominant hemisphere has been injected. In contrast to these invasive techniques, which are restricted to patient populations, many noninvasive methods emerged in the 1960s that allowed questions about brain organization of language to be addressed in neurologically healthy individuals. Laterality techniques were used to draw inferences about brain functional asymmetry from performance asymmetries in judgments on dichotic listening or tachistoscopic half-field viewing tasks. These behavioral measures have been supplemented in the past two decades by electrophysiological and neurobehavioral measures based on brain imaging technologies that provide snapshots of cortical and subcortical functioning while participants are engaged in some cognitive task.

The vast majority of clinical reports described aphasic deficits in only a single language. However, as early as 1919, the neurologist Bychowski noted that

"it seems quite improbable that the many aphasies whose extensive ease histories have been published should have spoken and understood only the one language of the researcher.'' What was not known until fairly recently is that many eases were indeed being published doeumenting aphasia in speakers of two or more languages. Although by no means as numerous as reports of aphasia in (ostensibly) monolingual speakers, reports of aphasia in polyglots have appeared as far back as the 1860s. The fact that as many as 70% of the first 250 cases culled from largely European journals appeared in languages other than English (mostly German or French) made this body of research largely inaccessible to a North American audience until recently, when certain investigators, such as Martin Albert, Loraine Obler, and Michel Paradis, began to make this literature more widely known within the United States through comprehensive monographs and anthologies of this research in English translation.

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