Processing of luminosity in the visual system: Is luminosity a basic feature?

Light-emitting (luminous) objects are perceived as qualitatively different from reflecting objects, and this difference seems to be linked to differential cortical activity for light-emitting and reflect- ing objects in the occipito-temporal cortex (Leonards et al, 2005 Cognitive Brain Research 24 173 ^ 176). However, it is still unclear whether object luminosity is treated as an independent visual feature (a building block of visual perception) comparable to orientation, motion, colour, or even faces. In a series of visual-search experiments, targets perceived as light-emitting amongst perceptually reflecting distractors of similar luminance led to efficient search, while reflecting targets within light-emitting distractors led to inefficient search. However, control experiments revealed that search efficiency for light-emitting targets was induced by the presence of luminance gradients producing the percept of luminosity, rather than by luminosity itself. It appears that the perceptual quality of luminosity does not reach feature status, questioning earlier hypotheses about specific sensory mechanisms to detect light-emitting objects. The role of gradients in visual perception, as yet relatively unexplored, remains to be resolved. [Supported by the British Academy (SG-38465).]