A watershed model of individual differences in fluid intelligence

Rogier A Kievit,Simon W Davis,John Griffiths,Marta M Correia,Cam-Can Cam-Can,Richard N Henson

doi:10.1016/j.neuropsychologia.2016.08.008

Abstract

Fluid intelligence is a crucial cognitive ability that predicts key life outcomes across the lifespan. Strong empirical links exist between fluid intelligence and processing speed on the one hand, and white matter integrity and processing speed on the other. We propose a watershed model that integrates these three explanatory levels in a principled manner in a single statistical model, with processing speed and white matter figuring as intermediate endophenotypes. We fit this model in a large (N=555) adult lifespan cohort from the Cambridge Centre for Ageing and Neuroscience (Cam-CAN) using multiple measures of processing speed, white matter health and fluid intelligence. The model fit the data well, outperforming competing models and providing evidence for a many-to-one mapping between white matter integrity, processing speed and fluid intelligence. The model can be naturally extended to integrate other cognitive domains, endophenotypes and genotypes.

Highlights

Fluid intelligence is a crucial cognitive ability that predicts key life outcomes across the lifespan
The relationship between fluid intelligence, processing speed (PS) and white matter (WM) is hierarchical, such that WM influences PS, which in turn affects performance on tests of FI. We show that this model naturally accommodates a wide and disparate range of empirical findings, integrates a series of relatively well-established findings into a single larger model, and, most importantly, can be formally tested using Structural Equation Modelling (SEM)
If we examine the predictions by Cannon and Keller described above, we can see that both the phenotype and the proposed endophenotypes are highly heritable - FI (Deary et al, 2010), PS (e.g. Vernon, 1989) and whole-brain Fractional Anisotropy (FA) (Chiang et al, 2009) - yet there has been a notable lack of success in establishing replicable genetic markers for FI (Chabris et al, 2012)

Summary

Introduction

Fluid intelligence is a crucial cognitive ability that predicts key life outcomes across the lifespan. We propose a watershed model that integrates these three explanatory levels in a principled manner in a single statistical model, with processing speed and white matter figuring as intermediate endophenotypes We fit this model in a large (N 1⁄4555) adult lifespan cohort from the Cambridge Centre for Ageing and Neuroscience (Cam-CAN) using multiple measures of processing speed, white matter health and fluid intelligence. Processing speed refers to the general speed with which mental computations are performed It has been considered a central feature of higher cognitive functioning since the development of the first formalized models of (fluid) intelligence (Salthouse, 1982; Spearman, 1927). Finkel et al (2007) used bivariate latent change score models in older adults to show that processing speed was a leading indicator of cognitive changes, including in abstract reasoning tasks Together, these behavioural findings suggest a strong relationship between processing speed and fluid reasoning ability

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