Neural mechanisms of visual awareness

The conscious perception of visual patterns and objects is thought to derive from their impact upon specialized neurons in the brain’s visual areas. Physiological recordings in animals have demonstrated that a given image striking the retina leads to a characteristic activity pattern among neurons in many subcortical and cortical brain structures. Such sensory ‘encoding’ of a stimulus is sometimes considered to lead automatically to its perception. Yet such a simple view does not account for the fact that conditions exist in which perception can be entirely dissociated from sensory stimulation. A visual pattern can, for example, impact the brain without leading to any perception. Conversely, a vivid visual percept can arise in the absence of any sensory stimulation whatsoever. Investigating such conditions may be important for understanding how sensory and perceptual signals are integrated in the brain, and may provide clues as to the neural underpinnings of conscious visual awareness. We have approached such issues with a series of neurophysiological studies in monkeys viewing multistable visual patterns. Such patterns, when continually present, lead to an unstable perceptual experience, in which the visual impression oscillates between two or more alternatives many times each minute. I will describe neurophysiological experiments in which monkeys viewing such patterns. The monkeys were trained to report their subjective experience while neurons in several visual cortical areas were continually monitored. The results suggest that the activity of a subset of neurons throughout the visual cortex is directly related to the subjective perception of a visual stimulus, while others in the same areas are more dedicated to scrutinizing the sensory pattern. I will discuss the sensory vs. perceptual nature of the neural representations in these areas. In addition I will speculate on possible origins of perceptual alternation, focusing upon what such past and future studies might reveal about mechanisms of normal conscious vision