Intelligence as a factor in evoked potential studies of psychopathology. I. Comparison of low and high IQ subjects.

To evaluate possible influences of differences in intelligence on evoked potential (EP) relations to psychopathology, EPs of subjects with high and low scores on Raven's Progressive Matrices were compared. EPs to left median nerve (LSEP), right median nerve (RSEP), visual (VEP), and auditory (AEP) stimul were recorded from 15 leads. EPs of age- and sex-matched high and low IQ nonpatients (7 of each) and psychiatric patients (40 of each) were compared with respect to group mean EPs, amplitude, latency, and wave-shape stability measures; high and low IQ patients were also matched for diagnosis. Certain differences between high and low IQ subjects were common to both patients and nonpatients; lower IQ was associated with (i) higher early (before 100 msec) LSEP and RSEP amplitudes; (ii) lower later LSEP amplitudes; (iii) less late epoch LSEP wave-shape stability; (iv) less VEP wave-shape stability. Since these EP characteristics resemble deviations from normal reported for psychotics, intelligence differences could account for part, but by no means all, of the psychopathology-related EP differences. The relationship to IQ of the EP contour length ("string") measure, reported by Hendrickson and Hendrickson to be highly correlated with IQ, was also assessed; results were negative.