Global shape processing in human visual cortex

The research described in this thesis used a combination of neuroimaging, brain stimulation and psychophysical methods to explore the representation of global shape in human visual cortex. Global shape processing mechanisms integrate over local orientation information to form an abstracted representation of an object’s shape; an important processing step for accurate object recognition in the complex visual scenes of everyday life. This thesis presents evidence for a global shape processing stream in human lateral occipital cortex comprising brain areas LO1, LO2 and object-selective lateral occipital complex (LOC). First, brain responses to shape were shown to be more global and less dependent on task in LO2 compared to LO1. Second, a global shape aftereffect was used to demonstrate that global representations of object shape are formed by a series of integrative mechanisms, where each successive mechanism integrates over outputs from the preceding mechanisms to form increasingly global shape representations. Third, brain stimulation was used to determine whether area LO2 is causally important for global shape adaptation and shape discriminations, however brain stimulation had no effect on shape perceptions meaning conclusions about a causal role for LO2 in global shape processing could not be drawn. Finally, neural tuning to radial frequency, one possible dimension of a representational shape space, was modelled using Gaussian neural modelling. Tuning towards radial frequencies that are processed globally was localised to lateral occipital cortex, and shifted from local to global frequencies in LO2 and LOC. Overall, this thesis provides novel contributions to the current understanding of how the analysis of an object’s shape facilitates object recognition, and the nature and locus of global shape representations in the human brain.