On perceptual learning, categorical perception and perception and perceptual expertise.

The empirical work reported in the current thesis set out to explore the relationship between perceptual expertise, categorical perception (CP) and perceptual learning. Evidence to support the idea that the way people organise the world into categories can qualitatively affect their perception of it has been provided by CP research. Recent work indicates that categorisation experience can lead to enhanced sensitivity to diagnostic stimulus features and is consistent with the possibility that, as experts have learned to distinguish among objects, they have also acquired new ways of perceptually structuring the objects to be categorised. Nevertheless, there is debate about whether these effects are really perceptual and if so about the mechanisms and locus of learning. Here, experiments were designed to test whether the process of acquiring perceptual categories drives a perceptual learning process that enhances the discrimination of category relevant features thereby contributing to the development of perceptual expertise. The work therefore sought to test the possibility that category learning could drive changes to early stages of perceptual processing. Two classes of stimuli were used to address these issues. Initial experiments showed that learning to categorise Gabor patches can lead to learned CP effects that are specific to the trained spatial frequency, orientation and retinal location. Experiments using morphed cervical cell stimuli showed that expert cervical screeners have acquired heightened discrimination to cells that cross the normal/abnormal category boundary and that training novices to categorise cells as normal or abnormal can also lead to retinotopically specific learned CP effects. Taken together, the results reported in the current thesis support a general explanation of CP effects arising from categorisation driven perceptual learning at early stages of visual processing. Furthermore, the work speculated that modifications to intra-cortical connections at this stage of processing may underpin the learned CP effects observed.