For unknown reasons, the human brain distinctly separates the handling of images of living things from images of non-living things, processing each image type in a different area of the brain. For years, many scientists have assumed the brain segregated visual information in this manner to optimize processing the images themselves, but new research shows that even in people who have been blind since birth the brain still separates the concepts of living and non-living objects.
The research, published in the Cell Press journal Neuron, implies that the brain categorizes objects based on the different types of subsequent consideration they demand—such as whether an object is edible, or is a landmark on the way home, or is a predator to run from. They are not categorized entirely by their appearance.
"If both sighted people and people with blindness process the same ideas in the same parts of the brain, then it follows that visual experience is not necessary in order for those aspects of brain organization to develop," says Bradford Mahon, postdoctoral fellow in the Department of Brain and Cognitive Sciences at the University of Rochester, and lead author of the study. "We think this means significant parts of the brain are innately structured around a few domains of knowledge that were critical in humans' evolutionary history."
Previous studies have shown that the sight of certain objects, such as a table or mountain, activate regions of the brain other than does the sight of living objects, such as an animal or face—but why the brain would choose to process these two categories differently has remained a mystery, says Mahon. Since the regions were known to activate when the objects were seen, scientists wondered if something about the visual appearance of the objects determined how the brain would process them. For instance, says Mahon, most living things have curved forms, and so many scientists thought the brain prefers to processes images of living things in an area that is optimized for curved forms.
To see if the appearance of objects is indeed key to how the brain conducts its processing, Mahon and his team, led by Alfonso Caramazza, director of the Cognitive Neuropsychology Laboratory at Harvard University, asked people who have been blind since birth to think about certain living and non-living objects. These people had no visual experience at all, so their brains necessarily determined where to do the processing using some criteria other than an object's appearance.
"When we looked at the MRI scans, it was pretty clear that blind people and sighted people were dividing up living and non-living processing in the same way," says Mahon. "We think these findings strongly encourage the view that the human brain's organization innately anticipates the different types of computations that must be carried out for different types of objects."
Mahon thinks it's possible that other parts of the human brain are innately structured around categories of knowledge that may have been important in human evolution. For instance, he says, facial expressions need a specific kind of processing linked to understanding emotions, whereas a landmark needs to be processed in conjunction with a sense of spatial awareness. The brain might choose to process these things in different areas of the brain because those areas have strong connections to other processing centers specializing in emotion or spatial awareness, says Mahon.
Mahon is now working on new experiments designed to further our understanding of how the brain represents knowledge of different classes of objects, both in sighted and blind individuals, as well as in stroke patients.
The data for the study were collected at the Center for Mind/Brain Sciences at the University of Trento in Italy.
- Bradford Z. Mahon, Stefano Anzellotti, Jens Schwarzbach, Massimiliano Zampini, Alfonso Caramazza. Category-Specific Organization in the Human Brain Does Not Require Visual Experience. Neuron, 2009; DOI: 10.1016/j.neuron.2009.07.012
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