Anomalous responses: Disturbing the order in V4 and human visual cortex

The anatomical location and retinotopic organisation of human V4 has been debated almost from the outset of fMRI. Initially, a large amount of the debate centred around whether V4 organisation in humans mimicked the organisation seen in macaque monkeys and other non-human primates. In these animals, V4 is split into dorsal and ventral components, representing the lower and upper visual quarterfields respectively. In humans however, it appeared from early on that V4 was instead organised into a continuous hemifield retinotopic map on the ventral surface, adjacent to ventral V3. The biggest argument in favour of this was the simple fact that locating a dorsal component to V4 in humans has never been done. In many hemispheres, an obvious hemifield map of V4 is identifiable, but often this is not the case. Instead, V4 sometimes appears to map only half to three quarters of the contralateral hemifield, with the last quarter – including the lower vertical meridian – not being present. With the majority of the visual neuroscience community accepting that V4 is indeed located solely on the ventral surface, the question became why can we not identify a complete map in up to half of hemispheres? The line of inquiry has focussed on the possibility that artefact from nearby draining veins disrupt the signal along the lower boundary, obscuring this region of the map. There is some support that this is the case in at least some hemispheres. The current work aims to resolve incomplete V4 maps in humans. In Chapter 2, we examine the possibility that obscured regions can be revealed by correcting time courses of voxels contaminated with venous artefact. Chapter 3 considers the effect of the phase encoding direction of the MRI scanner itself as a potential cause for incomplete V4 maps, and Chapter 4 examines V4 maps and venous artefact across cortical depth. We show that in some instances V4 maps are recoverable using correction procedures, that the phase encoding direction can alter the appearance of retinotopic maps of V4 and early visual cortex, and how retinotopic maps and venous artefact change across cortical depth