Visual perception is highly variable across healthy individuals and increasing evidence suggests that this inter-individual variation could be due to differences at the genetic, neurochemical, structural and neurophysiological level. Specifically, variation in the GAD1 gene (responsible for synthesising the majority of cortical GABA) has been associated with differences in the level of the inhibitory neurotransmitter GABA. In addition, differences in GABA and cortical structural parameters (surface area and thickness) have been shown to predict differences in neural gamma oscillations. However, these findings have not been replicated in large independent studies. Hence, Chapter 3 and 4 of this thesis combines the non-invasive neuroimaging tools MRI, MRS and MEG with genetic data to investigate the relationship between variations in genes, GABA, structure and gamma oscillations in the visual cortex of a large cohort of healthy individuals. Group differences in GABA, structure and gamma oscillations have also been reported between psychiatric populations (schizophrenia and bipolar disorder) and healthy individuals. However, differences in the direction of effect (increase or decrease) and no group differences have been found. Thus, Chapter 5 aims to further study these inconsistent findings by exploring group differences in GABA, structure and gamma oscillations between a healthy group and a schizoaffective bipolar disorder group. Lastly, inter- individual variation is also present in auditory perception but has received much less attention into the factors driving this variation. As in the visual system, similar links between neurochemical, structural and neurophysiological measures could be present in the auditory domain. Chapter 6 investigates the association between auditory gamma oscillations and auditory structural parameters in a healthy cohort.