Over the last twenty years his work has concentrated on questions in visual perception, particularly regarding the perception of colour and material properties, visual attention, and visual experience. Many of the questions he addresses arose from work with neurological patients with blindsight and cerebral achromatopsia (cortical colour blindness) which he has gone on to test using psychophysical and neuroimaging methods in normal observers. His work on visual attention without visual awareness and on colour processing in cerebral achromatopsia have formed the basis of fruitful exchanges with philosophers in recent years.