20 - Cerebral evoked potentials and language

Summary

Technological advances have not only greatly increased our ability to document the sites of lesions associated with aphasias through CT scans and other modern radiological techniques, but have also enabled us to record electrical activity in the brain that is correlated with language functions. Electrical activity in the brain can be measured through the use of electrodes placed on the scalp. Electroencephalographic (EEG) recordings have a major role in the assessment of neurological functioning in many patients. Abnormalities in the EEG can be used to help diagnose the location of neurological disease and its nature in cases such as epilepsy. Many changes in the physiological activity of the brain, such as those associated with sleep, are associated with changes in the EEG. However, because it reflects the activity of millions of neurons, all of which generate electrical charges, the EEG itself does not change in relation to specific cognitive functions in any way that can be reliably measured. For instance, the EEG changes dramatically when a person resting quietly with his eyes closed opens his eyes. In these conditions, the “alpha-rhythm” in the posterior portions of the brain disappears, and the EEG becomes “de-synchronized”. Though this change reflects the fact that the subject is attentive to visual stimuli and more aroused with his eyes open than with his eyes closed, the particular visual stimuli or the particular content of a person's thoughts do not induce further detectable changes in the general EEG record.

We do know, however, from physiological studies in animals that patterns of electrical activity in individual neurons are altered in specific ways by the presence of particular stimuli.