Online research in philosophy

Awareness is a personal experience, which is only accessible to the rest of world through interpretation. We set out to identify a neural correlate of visual awareness, using brief subliminal and supraliminal verbal stimuli while measuring cerebral blood flow distribution with H215O PET. Awareness of visual verbal stimuli differentially activated medial parietal association cortex (precuneus), which is a polymodal sensory cortex, and dorsolateral prefrontal cortex, which is thought to be primarily executive. Our results suggest participation of these higher order perceptual and executive cortical structures in visual verbal awareness.

Stoerig links blindsight to lesions between the primary visual cortex and the extravisual cortex. A parallel 'blindsight' occurs when input from the primary visual cortex is blocked during eye movements, convergence and blinks. At such moments (i) conscious vision based upon retinal input is blocked, (ii), however, like in blind sight retinal input can be used in motor tasks. The main difference to blindsight is that we are not only blind but cannot even with deliberate attention bring this blindness into awareness. We are doubly unaware: unaware of being blind and unaware that in spite of this that what we can see is created by the posterior parietal cortex substituting output (for that temporarily not coming from the primary visual cortex) for higher areas of the cerebral cortex.

A central issue in philosophy and neuroscience is the problem of unified visual consciousness. This problem has arisen because we now know that an object's stimulus features (e.g., its color, texture, shape, etc.) generate activity in separate areas of the visual cortex (Felleman & Van Essen, 1991). For example, recent evidence indicates that there are very few, if any, neural connections between specific visual areas, such as those that correlate with color and motion (Bartels & Zeki, 2006; Zeki, 2003). So how do unified objects arise in visual consciousness? Some neuroscientists propose that neural synchrony is the mechanism that binds an object's features into a unity (e.g., see Crick, 1994; Crick & Koch, 1990; Engel, 2003; Roelfsema, 1998; Singer, 1996; von der Malsburg, 1996, 1999). I argue, on both empirical and philosophical grounds, that neural synchrony fails to explain the unity of visual consciousness.

Some patients with a lesion to the striate cortex (V1), when assessed through forced-choice paradigms, are able to detect stimuli presented in the blind field, despite reporting a complete lack of visual experience. This phenomenon, known as blindsight, strongly implicates V1 in visual awareness. However, the view that V1 is indispensable for conscious visual perception is challenged by a recent finding that the blindsight subject GY can be aware of visual qualia in his blind field, implying that V1may not be critical under all circumstances. This apparent contradiction raises the following question: if V1 is not always necessary for phenomenal awareness, why do V1 lesions have such a detrimental effect on conscious perception? It is suggested here that this contradiction can be resolved by considering the impact of V1 lesions on the functioning of the whole visual cortex.

What is the relationship between a visual percept and the underlying neuronal activity in parts of the brain? This manifesto reviews the theoretical framework of Crick and Kochfor answering these questions based on the neuroanatomy and physiology of mammalian cortex and associated subcortical structures. This evidence suggests that primates are not directly aware of neural activity in primary visual cortex, although they may be aware of such activity in extrastriate cortical areas. Psychophysical evidence in humans supporting this hypothesis is discussed.