9 The SVR latency ranges from 50 to 250 ms. The potentials are distributed over the frontal cortex, and they became known as the ‘vertex potential’ because they were best recorded from the scalp at the vertex (Davis & Zerlin 1966). Topographical studies have shown that contralateral stimulation produces potentials of a larger amplitude than does ipsilateral stimuli (Wolpaw & Penry 1977). The anatomical and physiologic origins of the SVR are not known, and various waves have several sources. It is thought that SVR is generated in the cerebral cortex, but it is not a primary response because of its latency (Davis 1976). Knight et al (1980) have studied the effects of cortical lesions in man. They have found that frontal lesions do not affect N1 or P2 components; however, extensive temporo-parietal lesions eliminate N1, but not P2. In cortical evoked-response audiometry, the patient is tested with eyes open in order to reduce EEG alpha activity, which may interfere with SVR recording. EEG theta activity can be prominent in some children and also cause problems in SVR recording. Reading is tolerable, but gross body movement, especially muscle activity of the scalp, jaw, and neck, should be avoided, because this causes gross bio-electrical artefacts in ongoing EEC The patient should be relaxed, reclining in a chair or in a supine position.
The slow vertex response (SVR)
GENERATORS OF SLOW-LATENCY RESPONSES
METHODS
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