Inspection time (IT), an information-processing correlate of psychometric intelligence, has been extensively studied. Previous research has shown that IT is a reliable correlate of psychometric intelligence across different developmental periods, mirroring developmental trends of fluid intelligence. Despite this extensive previous literature, very little is known about the biological basis of IT. In the present review, we discuss recent results from our laboratories examining the neurochemical determinants of IT. In this review, we outline the significance of several studies in which performance on the IT task is measured before and after modulating key human central nervous system (CNS) neurotransmitters and receptor systems (e.g., cholinergic, serotonergic, noradrenergic, and dopaminergic systems). The results of these studies indicate a primarily cholinergic basis for IT, although other aspects of psychometric intelligence may have serotonergic and dopaminergic determinants in addition to a cholinergic basis. The results are consistent with data reporting cholinergic depletion and impaired IT performance in dementia of the Alzheimer's type (DAT). Speculatively, we propose that compounds that enhance the release of the neurotransmitter acetylcholine (Ach) will improve IT and the variance that IT shares with IQ test performance.
- inspection time