Yale researchers have found that the brain can unconsciously differentiate between social stimuli (pictures of faces), and non-social stimuli (pictures of objects), using a primitive visual pathway that terminates in the amygdala.
"These results provide the best evidence to date that this pathway can process unseen images that have social relevance," said senior author Robert T. Schultz, associate professor in the Yale Child Study Center.
To measure how the brain responds to images of great social importance that are not consciously seen, Schultz and his co-authors presented pictures of houses to a group of healthy young adults 18 to 34, during functional MRI measurement of brain activity. The pictures were simultaneously superimposed with pictures of emotionally expressive faces or a neutral non-social object such as a chair. Because the participants wore 3-D glasses with colored lenses, pictures of the faces were randomly projected to just one eye.
While the brain's cortex mirrored the participants' experience and just saw the houses, a phylogenetically older pathway involving the amygdala registered the unconscious representation of the human face.
"This shows that the brain's response to pictures of faces were not consciously seen and that older visual structures of the brain can differentiate social from non-social stimuli without input from the cortex," said Schultz.
Schultz said the study proves that the amygdala and related sub-cortical visual structures can differentiate social from non-social stimuli without cortical input. The approach can also now be used as a model to understand infant perception, as this is heavily reliant on the sub-cortical visual system during the first weeks and perhaps months of life, while the cortical system is still maturing and coming on line.
These study methods are now being adapted to research various disorders, such as autism, where there is a known role for the amygdala in the pathophysiology, but where the relative importance of sub-cortical vs. cortical inputs have not until now been able to be disentangled for lack of an appropriate method.
Other authors on the study included Brian N. Pasley and Linda C. Mayes at Yale School of Medicine .
Citation: Neuron, April 2004, Vol. 42, 163-172.
Contact
Karen N. Peart
203-432-1326