Category-effects and stimulus characteristics in visual perception

Abstract

This thesis describes a number of experiments that aimed to investigate the role of relatively low-level visual input factors in category-specific effects in object identification and colour perception. In the object recognition experiments, using picture-name or name-picture verification tasks, as well as object-naming tasks, clues to the causal factors contributing to such effects were obtained. It was found that category-specific effects in normal object identification, both living things advantages and living things disadvantages can occur even when nuisance variables like familiarity and complexity are well controlled. Task demands on perceptual differentiation and stimulus presentation conditions can influence and even reverse category-specific effects (Report I). When identification has to rely mostly on global shape visual information, the living things advantage in identification is enhanced compared to when visual detail is available in stimulus pictures. Furthermore, a lack of visual detail induces a left hemisphere disadvantage for identification, but only for nonliving things (Report II). In an experiment utilising eye movement methods, it was found that when rotating objects in depth, which presumably causes changes in outline shape, changes in participants' eye movement strategies could be observed. Specifically, participants tended to focus more on the objects' centres of gravity when rotations went from canonical to noncanonical views. This effect was, however, only reliably observed for nonliving things. (Report III). In a study examining differential interference effects in Stroop performance, it was found that the amount of interference is smaller for non-opponent compared to opponent colours. An artificial neural network that coarsely implements a trichromatic input coding scheme can simulate this reduced opponent colour interference. Additionally, it was found that individual differences in colour discrimination ability are associated with individual differences in Stroop performance. (Report IV).