One of the most striking examples of our brain's capacity to subconsciously process visual input is the phenomenon called "blindsight." Individuals with damage to the primary visual cortex are effectively blind—they're not consciously aware of objects in their visual field. But, remarkably, they remain capable of navigating around obstacles they can't actually "see."

Now, research with other primates has started to untangle the pathways that enable this blindsight ability. A paper describing the findings will appear in today's issue of Nature.

The authors describe how the primary visual cortex, V1, is largely responsible for processing focus-driven visual information. In both humans and other animals, loss of this area leaves the brain consciously unaware of visual information; smaller lesions may leave similar deficits for a subset of the visual field.

But there are a number of other areas of the brain that appear to be active in processing visual information (V2 through V5 have been identified), and it's possible that these contribute to the blindsight phenomenon.

The authors worked with two macaques that have small lesions in their primary visual cortexes, which leave them unable to respond to visual cues in a subset of their visual field. A fair amount of work went in to defining precisely the areas within the visual field that were no longer effective, and confirming that stimuli in those areas could still induce activity (measured via functional MRI) in the remaining visual cortexes.

The authors then focused on a structure called the lateral geniculate nucleus (LGN), which acts as a relay point for signals travelling between the retina and the primary visual cortex. Other work had shown that the LGN also has projections to a number of secondary visual areas, suggesting that it may serve as a major hub in the visual system.

To test this suggestion, the authors injected the LGN with a chemical that activates the receptor for a major inhibitory signaling molecule (the chemical, THIP, is what's termed a "GABAA-receptor agonist"). When the chemical is present, nerve cells receive a signal telling them to stop signaling, so this this injection has the effect of shutting the LGN down entirely.

The treatment was highly effective. With the LGN shut down, visual stimuli that normally induce a blindsight response didn't elicit any response from the visual centers of the macaques.

Although blindsight is a (fortunately) rare condition, this finding will ultimately help us understand how a single input, originating in nerves that relay information from the retina, can be distributed to a number of areas in the brain simultaneously. We may think the actions of the primary visual cortex are the most essential, but the mere phenomenon of blindsight—navigating around obstacles without expending the resources involved in consciousness to do so—provides just one indication of the importance of the other functions.

Nature, 2010. DOI: 10.1038/nature09179 (About DOIs).