Investigations into visual attention have led to the identification of locationand object-based mechanisms of attentional selection. This thesis is specifically concerned with object-based attention and aims to distinguish between two different hypotheses regarding the representations available to the inhibitory mechanisms of object-based selection. According to the global structure hypothesis, inhibitory mechanisms of selection operate over representations that do not have access to object-internal structural properties, such as surface boundaries. According to the local structure hypothesis, inhibitory selection mechanisms operate over representations that do make explicit object-internal structure. The second issue addressed in this thesis concerns the nature of object-shape representations that object-based inhibitory mechanisms operate over. Two candidate shape primitives, surfaces and volumetric components, are examined in order to ascertain whether they modulate object-based attention. These issues are addressed using the inhibition of return (IOR) paradigm (Posner & Cohen, 1984), which has previously been used to examine object-based attention (e. g. Tipper, Driver & Weaver, 199 1). The two aforementioned hypotheses make contrasting predictions about the modulation of object-based IOR by objectinternal structure. The global structure hypothesis predicts that object-based IOR should not be modulated by the object's internal structural properties, irrespective of changes in the object's internal structure. In contrast, the local structure hypothesis predicts that object internal structure will modulate the magnitude of object-based IOR. 2 The results raise a number of interesting issues. First, they show that objectbased IOR is modulated by internal discontinuities in object structure. Second, objectbased IOR operates over representations that make explicit surface properties of volumetric forms. Third, the effect is attenuated when cues and targets appear on the same surface of an object, relative to when the cue and target are separated by an internal structural discontinuity. These findings are consistent with the local structure hypothesis for objectbased attentional selection and provide new evidence (a) to suggest that inhibitory mechanisms of selection can operate over shape representations that make explicit infonnation about object internal structure, (b) about the surface-based nature of these mental representations, (c) to posit new constraints on hypotheses about the distribution of facilitation and inhibition in object-based attention.