Development of Neurophysiological Indices of Retardation: Interhemispheric Asymmetry of the Visual Evoked Cortical Response

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Abstract
Visual evoked cortical responses (VECRs) were recorded from the left (O1) and right (O2) hemispheres at varying interstimulus intervals (ISIs) in children who were either: Normal (dextral), Dextral Retardates, or Sinistral Retardates. Latencies of N1 and P2 components of the VECR were stable and similar for both hemispheres in all groups. There was an asymmetry between the hemispheres for N1-P2 amplitude: right > left in Normals; this direction of asymmetry was also present in Sinistral Retardates, but was reversed in Dextral Retardates. The direction of normal VECR asymmetry was the reverse of that previously established for the auditory evoked cortical response (AECR) amplitude (Richlin. et al., 1973; Matsumiya, et al., 1972; Morrell & Salamy, 1971; Wood, et al., 1971). It is suggested that some neural mechanism of the retardates may allow excessive non-verbal stimulation of the normally “verbal” hemisphere. The electrical activity of the brain consequent to auditory or visual stimulation appears to depend upon: the modality of stimulation, the mental status of the individual, and his handedness. These factors have led to the establishment of an additional index to provide differential diagnosis of the mental status of the child.