How much of this world does your mind actually see? Potentially more than you think, according to series of studies on a blind man whose brain can still record and respond to the facial expressions from others without him being aware of it. These observations, published today in the Journal of Neuroscience, suggest the existence of visual brain pathways that register hostile or unhappy visages without our conscious knowledge.
The man in question, who the study's authors refer to as Patient TN, suffered two strokes in 2003 that almost completely eradicated his primary visual cortex. This brain region, located at the back of the skull, is responsible for processing visual input from the eyes and shipping it to the rest of the brain. Thus, Patient TN's blindness is caused by a faulty brain circuitry rather than eye damage. Indeed, one could assume that his eyes are still transmitting visual information to his brain, but "nobody is home" to collect the message.
Without his primary visual cortex, you might have predicted that Patient TN should be utterly blind, but follow-up experiments at the University of Geneva suggested the contrary.
In 2005, neuropsychologist Dr. Alan Pegna and colleagues placed a series of pictures with facial expressions in front of Patient TN's eyes and asked him to guess the emotions being portrayed in the photos.
Amazingly Patient TN could accurately distinguish between happy and angry faces 60 percent of the time, which is a success rate that could not be attributed to mere chance.
Brain scans with functional magnetic resonance imaging (fMRI) revealed that these pictures triggered electrical activity in the amygdala, a pocket of neurons that the mind's response to fear. The amygdala also plays a major role in the processing of emotional reactions and memory.
Fast forward to 2013, where today's study shows that Patient TN's amygdala is even stimulated by subtle changes in a person's gaze. An 8-fold increase in brain activity was seen in Patient TN's amygdala when he was shown a face with direct gaze versus a countenance with an averted look.
Control subjects with normal vision exhibited similar patterns on amygdala activation in response to direct/averted gazes.
Unlike with happy and angry faces, Patient TN could not accurately guess when he was facing a direct gaze versus averted eyes.
Patient TN isn't the first person to suffer from visual cortex damage and keep possession of some residual vision. His special condition is known as affective blindness — or 'blindsight' — and suggests the brain is wired to see without consciousness.
Pegna and crew tried to nail down this subconscious network in this study and identified a group of brain regions that were activated in parallel with the amygdala.
Patient TN's affective blindness extends beyond merely reading facial expressions. A 2008 study, conducted by neuroscientist Dr. Beatrice de Gelder of Harvard and Tilburg University in the Netherlands, had Patient TN walk across a hallway without the use of his walking stick. As reported in Scientific American, Patient TN was led to believe the hall was empty, but it was actually filled with an array of obstacles.
Patient TN was able to dodge the objects without "seeing" them (see below).
(source: Scientific American)
Sources: Burra N, Hervais-Adelman A, Kerzel D, Tamietto M, de Gelder B, Pegna AJ. Amygdala Activation for Eye Contact Despite Complete Cortical Blindness. The Journal of Neuroscience. 2013.
de Gelder B, Tamietto M, van Boxtel G, et al. Intact navigation skills after bilateral loss of striate cortex. Current Biology. 2008.
Pegna AJ, Khateb A, Lazeyras F, Seghier ML. Discriminating emotional faces without primary visual cortices involves the right amygdala. Nature Neuroscience. 2005.