A SOMATOSENSORY LATENCY BETWEEN THE THALAMUS AND CORTEX ALSO CORRELATES WITH LEVEL OF INTELLIGENCE

As part of a study on speed of information processing and intelligence , 205 young adult postsecondary students were tested for somatosensory evoked potential (SEP) latencies and intelligence. Following stimulat ion at the wrist, latencies of three SEPs were determined: N13, genera ted in the cervical spinal cord/medulla region; N19, generated in the thalamus; P22, generated in the arm region of the somatosensory (parie tal) cortex. These latencies and two latency differences, N19 - N13 an d P22 - N19, were tested for correlation with a nonverbal measure of i ntelligence; only P22 - N19 significantly correlated (r = - .217; p = .0 13, two-tailed). Comparing this latency difference in students in t he first IQ quartile (mean IQ = 103.4) with that of students in the fo urth quartile (M = 131.2) showed mean differences of 4.13 ms versus 3. 21 ms, respectively (p = .0034, two-tailed). P22 - N19 measures time f or signal transmission from the thalamus to the sensory cortex. These results agree with considerably more extensive data on visually evoked potentials showing a negative correlation between IQ and the latency for a visual stimulus of the retina to produce a signal at the visual cortex (most of this latency is between the thalamus and the cortex; R eed & Jensen, 1992). The findings here agree with the visual results a nd strongly suggest that the IQ-latency correlation occurs because the latency indexes cortical nerve conduction velocity, an important comp onent of information-processing speed.