Ozgen, E.
Bilkent University
Recent evidence suggests that categorical perception (CP) effects in colour result from the activity of a linguistic code, rather than perceptual re-organisation (warping) as previous research suggested. I will review recent evidence from our labs that suggest otherwise. The first point of consideration is the erroneous use of colour metrics to test hypotheses. In one study, performance on low-level discrimination and a “high-level” task has been compared, to reach the conclusion that low-level discrimination does not show categorical effects, while high-level tasks, where linguistic codes can and must be used, do. But this study used two very different colour metrics to equate stimuli on the two tasks. We show that these results can be entirely attributed to the confound of colour space, and that depending on the space used, "CP effects" can also be observed in low-level discrimination. We also suggest a less commonly used metric that gives good results. Another line of support for the verbal coding hypothesis comes from the study of hemispheric asymmetries, which suggests that CP effects are lateralised to the left hemisphere:, providing a direct link with language processing. However, evidence from our labs, suggests that at least on low-level discrimination, there are no hemispheric asymmetries in categorical effects. If anything, any hemispheric asymmetry seems to have more to do with cone sensitivity: there is a right hemisphere advantage for blue region discrimination, while no asymmetry is observed in the green region or on the blue-green boundary. It is possible that this “blue” effect is falsely interpreted as evidence for hemispheric asymmetry in CP. Finally, I will review evidence from our labs on the effects of category learning on colour discrimination thresholds. Category learning seems to selectively improve discrimination on the category boundary. This is consistent with a perceptual change account of CP.