Is the Prefrontal Cortex Important For Fluid Intelligence? A Neuropsychological Study Using Matrix Reasoning

Abstract

Patients with prefrontal damage and severe defects in decision making and emotional regulation often have a remarkable absence of intellectual impairment, as measured by conventional IQ tests such as the WAIS/WAIS-R. This enigma might be explained by shortcomings in the tests, which tend to emphasize measures of “crystallized” (e.g., vocabulary, fund of information) more than “fluid” (e.g., novel problem solving) intelligence. The WAIS-III added the Matrix Reasoning subtest to enhance measurement of fluid reasoning. In a set of four studies, we investigated Matrix Reasoning performances in 80 patients with damage to various sectors of the prefrontal cortex, and contrasted these with the performances of 80 demographically matched patients with damage outside the frontal lobes. The results failed to support the hypothesis that prefrontal damage would disproportionately impair fluid intelligence, and every prefrontal subgroup we studied (dorsolateral, ventromedial, dorsolateral + ventromedial) had Matrix Reasoning scores (as well as IQ scores more generally) that were indistinguishable from those of the brain-damaged comparison groups. Our findings do not support a connection between fluid intelligence and the frontal lobes, although a viable alternative interpretation is that the Matrix Reasoning subtest lacks construct validity as a measure of fluid intelligence.

ACKNOWLEDGMENT

We thank Drs. Rob Jones and Wes Houston, and two anonymous reviewers, for comments on earlier versions of this paper. The work is supported by Program Project Grant NINDS NS19632 and NIDA R01 DA022549.

Notes

¹For the purposes of the current study, it is important to distinguish the various versions of the WAIS test: the original WAIS, published in 1955; the revised WAIS-R, published in 1981; and the WAIS-III, published in 1997. We will use these designations carefully throughout the manuscript.

Age and Education are in years; Chronicity is years between lesion onset and collection of WAIS-III data. DLPC = dorsolateral prefrontal cortex; BDC = brain-damaged comparison group.

Matrix Reasoning and Vocabulary are age-corrected scaled scores. VIQ = Verbal IQ; PIQ = Performance IQ; FSIQ = Full Scale IQ; DLPC = dorsolateral prefrontal cortex; BDC = brain-damaged comparison group.

²“Points” for the Matrix Reasoning and Vocabulary subtests refers to age-corrected scaled-score points; “points” for the IQ scores refers to IQ points.

³It should be understood that for any individual participant, the “normality” of an IQ score or an IQ subtest score depends on a contrast between the obtained score and the expected score based on the participant's estimated premorbid level (typically derived primarily from educational and occupational background). Within each of the DLPC and BDC groups, there could be individuals who have IQ and/or IQ subtest impairments. But the fact that the two groups are well matched on education and occupation makes highly unlikely the possibility that there is any systematic group difference regarding patients with IQ impairments.

Age and Education are in years; Chronicity is years between lesion onset and collection of WAIS-III data. VMPC = ventromedial prefrontal cortex; BDC = brain-damaged comparison group.

Matrix Reasoning and Vocabulary are age-corrected scaled scores. VIQ = Verbal IQ; PIQ = Performance IQ; FSIQ = Full Scale IQ; VMPC = ventromedial prefrontal cortex; BDC = brain-damaged comparison group.

Age and Education are in years; Chronicity is years between lesion onset and collection of WAIS-III data. DLPC and DL = dorsolateral prefrontal cortex; VMPC and VM = ventromedial prefrontal cortex; BDC = brain-damaged comparison group.

Matrix Reasoning and Vocabulary are age-corrected scaled scores. VIQ = Verbal IQ; PIQ = Performance IQ; FSIQ = Full Scale IQ; DLPC and DL = dorsolateral prefrontal cortex; VMPC and VM = ventromedial prefrontal cortex; BDC = brain-damaged comparison group.

Matrix Reasoning and Vocabulary are age-corrected scaled scores. VIQ = Verbal IQ; PIQ = Performance IQ; FSIQ = Full Scale IQ; DLPC and DL = dorsolateral prefrontal cortex; VMPC and VM = ventromedial prefrontal cortex; BDC = brain-damaged comparison group.

Age and Education are in years. Matrix = Matrix Reasoning; Vocab. = Vocabulary; both are age-corrected scaled scores. VIQ = Verbal IQ; PIQ = Performance IQ; FSIQ = Full Scale IQ.