Most psychophysiological output variables display marked individual differences in the maximum and often in the minimum levels of which S is capable. Since such variations in range are generally unrelated to the underlying variable of interest, measures of tonic level or of changes in level should be corrected so as to remove their influence. Formulas for this correction are provided together with experimental evidence showing that such range-corrections may accomplish marked reductions in error variance.1 Supported by grants from the Graduate School of the University of Minnesota and from the National Institute of Mental Health. We are grateful to the psychiatry departments of the Hennepin County General Hospital and the Minneapolis Veterans Administration Hospital for providing subjects and facilities.2 Now with the department of psychology, University of Illinois.
Within the broader context of our investigations into the heredity of the human EEG, we analysed the EEGs of 28 pairs of monozygotic and 21 pairs of dizygotic twins who were separated as infants and reared apart. The principal goal of this study was to determine the degree to which environmental factors possibly influence the development of a person's EEG. Monozygotic twins reared apart were, with respect to their EEGs, only slightly less similar to each other (if there is any difference at all) than the same person is to himself over time. For dizygotic twins reared apart, we verified the findings of our previous study, namely, that the average within-pair similarity of EEGs estimated from a sufficiently representative sample of fraternal twins was significantly higher than the average inter-individual similarity of EEGs obtained from unrelated persons. The results on both monozygotic and dizygotic twins, yielded conclusive proof that the individual EEG pattern is predominantly determined by hereditary factors.
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