When size matters: how the layout of primary visual cortex (V1) shapes higher cognitive functioning

Human beings are endowed with a wide range of highly developed cognitive abilities. Which factors have led to this level of intellectual capacity in the course of evo- lution? In cross species studies, overall brain size is a reliable predictor of cognitive performance, but more specific information about how the brain���s structural and functional organization may be linked to these enhancements is lacking. The aim of this thesis was to investigate the role of the anatomy and function of the primary visual cortex (V1) in behaviour. V1 is the earliest sensory stage for vision in the cerebral cortex, and is responsible for processing low-level visual features such as spatial orientation, location and frequency. Interestingly, V1 size displays huge variance between individuals; this offers an amazing opportunity to study the behavioural effect of brain enlargement within a human sample while avoiding the various confounds that can distort the results in cross species studies. Previous research has shown that a larger V1 is linked to higher perceptual sensitivity; but to what extent these size differences may also affect higher cognitive functions is unknown. Using a combined approach of behavioural testing and fMRI brain imaging methods, we find that the acuity of two cognitive functions, namely the precision of visual imagery and visual working memory storage, are significantly positively correlated with the surface size of V1: individuals with a larger V1 tended to have more precise imagery and greater visual working memory storage. In contrast, the strength of visual imagery was negatively related to V1 surface size. In addition, we find that other indices of neural function, such as the concentration of the inhibitory neurotransmitter GABA in the medial occipital cortex and V1 spontaneous functional activity levels, covaried significantly with differences in V1 surface size. Our findings support the notion of a positive relationship between brain (areal) size and cognition and show how even very low-level sensory areas are involved in shaping our intellect. As a last point, we discuss the potential limitations of the positive link between brain size and cognitive ability.