The aim of this study was to shed light on some of the neurophysiological mechanisms behind visual perception and specifically look into feedback processes that may be taking place during visual processing and also inhibition processes of the visual cortex. The oblique effect is a preference of the visual system for cardinal orientations rather than oblique ones. A recent MEG study (Koelewijn, et al. 2011) finds in V1 an initial inverse oblique effect (80 msec from stim onset) which however later (120msec from stimulus onset) showed a trend towards the classical oblique effect and feedback processes here are suggested taking place from the extrastriate cortex. We look into this using fMRI and interestingly we do manage to find an inverse oblique effect, which indicates that the initial MEG “inverse” effect in V1 is detectable with fMRI even though fMRI does not have the temporal resolution of MEG. Unfortunately in this fMRI study the extrastiate region was not localized. In the 3rd experimental chapter (following up on a study by Edden et al.) we look into the relation of behavioural thresholds and gamma activity in the visual cortex. Here we found in the SAM analysis) for the oblique condition, a positive correlation of the oblique main effect in a cortical location in the medial visual cortex (at a frequency range of 30--‐70 Hz) to behavioural thresholds. However here we did not detected an oblique effect when we compared oblique to cardinal condition. In the final experimental chapter we look into the relation between GABA and training effects using however two GABA scanning protocols (with and without macromolecule suppression). Here we find that training effects depend on GABA concentration (as found in unpublished findings by Edden et al.). Additionally here we find negative correlations with behavioural thresholds and GABA however these are strongest for the untrained sessions.