Individual differences in excitation and inhibition in visual cortex

The formation of a visual percept in the human brain involves multiple processes, the extent of which may be related to each other within individuals but may show variability between participants. The aim of this thesis was to investigate the relationships between individual variability in various measures of visual processing. The non-invasive neuroimaging methods of magnetic resonance imaging (MRI), magnetic resonance spectroscopy (MRS), magnetoencephalography (MEG) and functional MRI (fMRI) were employed to measure brain structure, neurotransmitter concentration, neuronal oscillations and haemodynamic activity, respectively. Reductions in haemodynamic activity in non-stimulated areas of visual cortex (negative blood oxygen level-dependent responses) were shown to hold useful information about the stimulus, in addition to the responses in stimulated cortex. In general, there were no strong relationships between increased or decreased functional responses in visual cortex and measures of brain structure or of neurotransmitter concentration. Age was the major determinant of individual variability in the frequency of neuronal oscillations. These findings do not replicate results from previous studies that have shown links between individual differences in these measures. This discrepancy was not due to poor repeatability of MRS measures, since methods for optimisation of MRS were identified in this thesis. Simulations were also conducted to determine the sample sizes required in correlational studies involving neuroimaging measures with associated measurement noise. The lack of replication of relationships between neuroimaging measures of individual differences in visual processing is likely to be influenced by low statistical power, due to the small sample sizes and weak relationships tested. Such studies should therefore be conducted and interpreted cautiously, bearing in mind issues of power, demographic mediators of relationships and the likely strength of relationships inferred from the physiological mechanisms linking the variables tested