Neural correlates of individual differences in personality and intelligence : Studies using an isochronous timing task and voxel based morphometry

Abstract: It is easy to imagine that humans have always noticed the differences between themselves and others and perhaps even speculated about the origins of these differences. The study of individual differences as a formal field encompasses variations in intelligence and personality as well as other dimensions. Studies of the neural basis of individual differences are by their nature correlational. Two approaches that have proven fruitful in the investigation of intelligence and personality are the examination of relations between traits and performance on timed elementary cognitive tasks as well as between traits and structural features of the brain. This literature has shown that high intelligence is related to faster and less variable performance in reaction time tasks and other tasks requiring temporal processing, as well as larger volumes of neural tissue in widespread cortical areas. The personality trait neuroticism has, similar to low intelligence, been related to more variable performance in reaction time tasks. Finally, personality differences have been related to neural structural differences as well, in regions involved in social and emotional processing. The overall aim of this thesis was to further investigate the neural underpinnings of intelligence and personality, in particular the relations of these variables to the accuracy of timing and neuroanatomy. A main question investigated in Study I was whether a relation between intelligence and temporal variability is also seen in a simple, automatic timing task. We studied performance variability on a simple finger tapping task, isochronous serial interval production (ISIP). As predicted, we found a negative correlation between intelligence and ISIP variability. Furthermore, this relation was strongest for local interval-to-interval variability using tapping intervals in the range 500-1000 ms, and there was no difference in the strength of the correlation between more and less variable trials. These findings support a bottom-up model of the relation between timing variability and intelligence, where temporal variability of neural activity plays a functional role for cognition. In Study II we investigated whether ISIP variability and intelligence also share neuroanatomical correlates. Regional white and gray matter volumes were quantified using voxel based morphometry (VBM) from structural MR images. A main finding was that both low ISIP variability and high intelligence are related to larger white matter volume in overlapping prefrontal brain regions. This provides further support for a functional relation between temporal variability and cognitive ability, and suggests that these variables may have common neural substrates in the frontal lobe. In Study III we investigated the relation between ISIP variability and neuroticism. A main question was whether neuroticism is related to a different component of temporal variability than intelligence, since these traits are essentially uncorrelated. A positive relation between neuroticism and ISIP variability was found and, unlike the correlation with intelligence, this relation was strongest for non-local variability, i.e. drift. This suggests that while intelligence and neuroticism are both related to variability, the underlying mechanisms of these relations may differ. In the final study (Study IV) we examined relations between personality and neuroanatomy, using VBM. We found that extraversion is negatively related to regional brain volume in cortical gray matter regions known to be involved in the behavioral inhibition and socialemotional processing, as well as to global white matter volume.