The study of visual processing and abnormalities due to lesions of cortical structures sheds light on visual awareness/consciousness and may help us to better understand consciousness. We report on clinical observations and psychophysical testing of achromatopsia/prosopagnosia, visual agnosia, and blindsight. Achromatopsia and prosopagnosia reveal that visual cortices have functionally specialized processing systems for color, face perception, and their awareness, and that furthermore these systems operate independently. Dysfunction is limited to some aspects of visual perception; someone with achromatopsia, although not conscious of color, is aware of the objects’ form, motion, and their relationship with sound and other sensory percepts. Perceptual awareness is modular, with neuronal correlates represented by multiple separate specialized structures or modules. Visual agnosia shows that awareness of a complete visual percept is absent, though the subject is aware of single visual features such as edges, motion, etc., an indication that visual agnosia is a disruption of the binding process that unifies all information into a whole percept. Blindsight is characterized by the subject’s ability to localize a visual target while denying actually seeing the target. Blindsight is mediated by residual islands of the visual cortex, which suggests that sensory modules responsible for awareness can function only when structurally intact. We conclude (1) that perceptual awareness (consciousness?) is modular, and (2) that perceptual integration is also modular, which suggests that integration among distinct cortical regions is a parallel process with multiple communication pathways. Any hypothesis about consciousness must include these observations about the presence of multiple parallel, but spatially and temporally different, mechanisms.
What do models of visual perception tell us about visual phenomenology?
