CCN meeting | Robin Gerrits (Research Group Cognition and Plasticity, Max Planck Institute for Human Cognitive and Brain Sciences, Leipzig), invited by Wim Fias

Beyond left or right: Reconsidering hemispheric specialization as a continuous dimension of intra-individual variability in brain organization.

Many cognitive domains are implemented asymmetrically with a clear population-level alignment in hemispheric dominance. For example, language and motor functions generally lateralize to the left hemisphere, whereas spatial and inhibitory control processes tend to lateralize to the right hemisphere. While brain hemispheric specialization is often conceptualized in a dichotomous way, in this talk I will argue in favor of a framework that approaches the neural implementation of these functions as lateralized but bilateral networks which, crucially, differ in how strongly they are lateralized across individuals along a spectrum. In addition to discussing others’ work, my argument will be based on two of my own projects. The first is an fMRI study in a sample composed of two groups of participants with consistently strong and consistently weak hemispheric asymmetry for verbal phonemic fluency. These groups were constructed by pre-selecting participants based on two independent measurements of task-evoked asymmetries in cerebral blood flow using transcranial Doppler sonography. The variability in this sample was exploited to show that the degree of asymmetry is temporally stable, generalizes to other language tasks – but not to non-language tasks –, and is related to the network economy of the language system. The second study used consecutive TMS-fMRI to test the hypothesis that individuals with less lateralized systems may be less vulnerable to perturbations to the function’s dominant hemisphere. Together, this work shows that we should not adopt a “one-size-fits-all” approach to laterality and lends validity to the notion that the degree of laterality is a trait-like aspect of hemispheric brain organization.