Arthur Jensen was a big name in intelligence research, and liked to look for "g" - the underlying general intelligence factor. It was identified that "g" was strongly correlated with reaction time. I was in grad school where he taught (Berkeley) and took part in research to look at what underlay reaction time - they were investigating speed of nerve conductivity. Hilariously, the research assistant was an on again, off again boyfriend, and my speed of nerve conductivity was like 1/3 of the average.
I'll just emphasize that PSI is a really different ability than abstract reasoning, say, physiologically. Math says students "should" regress to the norm on different measures. I used to do special ed. testing for a charter school, and practically every student I tested was gifted with low PSI. I got really interested in the causation of PSI deficits, but I can't say it's clear.
