Neurobiology of Language

with Ronald M. Lazar, PhD  
Professor of Clinical Neuropsychology in Neurology and Neurological Surgery
Director, Levine Cerebral Localization Laboratory
Division of Stroke

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Synopsis

Language distinguishes humans from other species; the human brain makes language possible. Unlike other forms of communication, language is unique in that it relies on rule-governed symbolic relations that are purely arbitrary and determined by cultural context. In his lecture on the neurobiology of language, Dr Ronald M. Lazar examines how the brain gives rise to the complex set of relations known as language. Rather than focusing on language in normals, he examines pathologies of language and describes how these disorders can illuminate the relationship between brain structure and function.

Lazar defines language as a system composed of four major characteristics: semantics, syntax, phonetics, and pragmatics. Semantics relates to the meaning of words and sentences; syntax refers to the structural aspects of language such as grammar; phonetics describes the sounds of language; and pragmatics involves the communicative value of language. He then goes on to show us what parts of the brain control these functions.

Aphasias, as Lazar defines them, are "acquired dysfunction of language from injury in the brain [that] results in disordered use of rule-governed symbolic relations." This lecture focuses on acquired aphasia as opposed to developmental aphasia. A classic characteristic of aphasias are paraphasic errors, which are divided into three types: phonemic, semantic/verbal, or neologisms. First, phonemic paraphasic errors involve substitution of one phoneme for another, such as "silling" instead of "spilling." Second, semantic or verbal paraphasic errors involve the substitution of one word for another. Finally, neologisms are substitutions of meaningless word sounds that are unrelated to the target word. The location of the brain lesion can be used to predict the nature of the aphasia: frontal lobe lesions tend to affect motor language while lesions behind the central sulcus cause disorders of receptive language.

A complete aphasia evaluation has six components: fluency, naming, comprehension, repetition, reading, and writing. Lazar emphasizes that a complete examination takes a considerable amount of time and that only a complete examination will yield accurate diagnosis. Since every language is cultural, if either the patient or the examiner is not fluent in the language of the examination, the validity of the results will be compromised.
Lazar then reviews different types of aphasias, offering detailed explanations of sensory aphasias, such as Wernicke's, and motor aphasias, such as Broca's aphasia. Later he reviews other aphasias such as conduction, transcortical, and thalamic aphasia. For each type of aphasia, he explains its pathophysiology and location in the brain. He also describes how different aphasias affect language functions such as fluency of speech, comprehension, and paraphasic errors.

In conclusion, Lazar offers a note of hope regarding the long-term prognosis of aphasia. There is incredible plasticity in the brain demonstrated by the ability of aphasics to regenerate language function even after devastating lesions. Although most improvement comes in the first months after treatment, patients do continue to improve over time, he insists, well beyond the three- to six-month window conventional wisdom allows. Speech language therapy can have a tremendous benefit for the functioning of aphasic patients. Language function can even relocate to another hemisphere following injury to the brain. As Lazar concludes, "there is hope for us all."