Invisible influences reveal unconscious pathways used in visual perception

Mar 28 2005

Our conscious awareness is underpinned by activity in our brain, but there is far more going on in our brain at any one time than we can possibly be aware of. So what aspects of our brain's activity are we actually aware of?

In a paper published recently in Current Biology, University of Sydney scientists Dr. Colin Clifford and Dr. Justin Harris investigated just how far visual signals from our eyes can penetrate into our brain's processing network without being consciously registered. They found that visual signals can actually travel farther under the radar of consciousness than many scientists had previously thought.

Vision begins with the formation of an image on the back of the eye, an event that in turn stimulates a cascade of nerve impulses sending signals deep into the brain. It is in the brain's visual cortex that these signals are interpreted. In the study, Clifford and Harris examined the influence on conscious vision of an "invisible" image--in this case, an image that had been experimentally masked and could be perceived by the brain but not consciously detected. The researchers showed that such an invisible image received in one eye can influence the appearance of an image in the other eye, thus demonstrating that signals from the invisible image must travel into the brain at least as far as the point at which signals from the two eyes are combined. To reach this point, signals from each eye must be relayed through the mid-brain to the brain's occipital lobe at the back of the head. While it has long been known that the occipital lobe is specialized for visual processing, many scientists believe that it is also the seat of visual consciousness. The results of the Clifford and Harris study challenge this belief, showing instead that activity in the occipital lobe can occur in the absence of conscious visual perception.

Colin W.G. Clifford and Justin A. Harris: "Contextual Modulation outside of Awareness"