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THE NEURAL BASIS OF GENERAL INTELLIGENCE

Abstract

Professor John Duncan Educated at the University of Oxford (1970–1976) and spent two years at the University of Oregon working with Michael Posner before taking up a research position with the Medical Research Council. Currently he is a Programme Leader at the MRC Cognition and Brain Sciences Unit in Cambridge, with an adjunct appointment at the University of Oxford. Trained in cognitive psychology and physiology, he now has research programmes in neuropsychology, neuroimaging, and single cell electrophysiology, addressing problems of attention, intelligence, cognitive control, impairment and recovery following brain damage, and frontal lobe functions. He is a Fellow of the Royal Society and the British Academy, and winner of the 2012 Heineken Prize in Cognitive Science.

Deficits in psychometrically-measured “intelligence” and in “executive functions” are common in neurological and neuropsychiatric diseases. In this talk I shall address a number of interrelated questions. What is the link between intelligence and executive function? How should executive functions be defined, differentiated and measured? What do executive tests measure, and which are likely to be useful in research and clinical practice?

Evidence from functional imaging suggests a core network of frontal and parietal regions, active during many different types of cognitive demand. Included among these “multiple-demand” or MD regions are cortex along the inferior frontal sulcus, in the anterior insula/frontal operculum, dorsomedial frontal cortex including dorsal anterior cingulate, and along the intraparietal sulcus. In all organized cognition, I suggest, the MD system breaks complex problems into simpler parts, resulting in a structured series of attentional episodes. Importance in all kinds of behaviour is reminiscent of the psychometric concept of Spearman's g, and indeed, conventional tests of fluid intelligence produce strong MD activity. Loss of fluid intelligence, furthermore, is predicted by volume of damage within the MD system.

Following focal frontal lobe lesions, many conventional tests of executive function show deficits that are entirely explained by loss of fluid intelligence. Once fluid intelligence is partialled out, all deficit compared with healthy controls is removed. Included in this list are Wisconsin card sorting, Trails B, verbal fluency and more. These tests, I suggest, measure only the common MD function of structuring any complex behaviour, with little importance attaching to their specific surface form. A second group of tests behaves differently, with frontal lobe deficits remaining even after partialling fluid intelligence. Included are tests of social cognition and complex multitasking. Plausibly, such tests concern frontal lobe regions outside the MD system, with some evidence, for example, linking multitasking to the frontal pole.

The dissociation between these two groups of executive tasks applies very widely, with replications in Parkinson's disease, frontotemporal dementia, and schizophrenia. The results suggest a principled, empirical basis for distinguishing different kinds of executive deficit, and for linking these to separate frontal lobe systems. Acknowledging the core role of psychometric intelligence and the MD system, I suggest, may clarify much thinking in the study and measurement of frontal lobe functions.

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