The scientists used photos of houses to stimulate what's called the "parahippocampal place area." And they included pronounceable made-up words rather than actual words to make sure that the meaning didn't affect brain activity.
For each category the participants viewed, they had to press a button to say whether a picture was the same picture as the one before it.
Functional MRI of the brain indirectly measures the activity of firing neurons. The tests gave scientists color-coded maps of brain activity. By comparing each participant's MRI results with his or her twin's, Polk could gauge how similarly their brains worked when tasked with recognizing images in the different categories.
The brain circuits used to recognize chairs or made-up words were no more similar in identical twins than in fraternal twins. That suggests that the neural circuitry underlying these behaviors is not innate. Instead, that circuitry is primarily learned through experience, Polk said.
But in the face and house categories, the scientists saw a different story. The neural pathways used to process these images were more similar in identical than fraternal twins. This suggests that genes play a significant role in this type of brain function. Identical twins are genetic copies of one another. Fraternal twins are as genetically different as regular siblings.
These results cannot be chalked up to greater structural similarity in identical twins' brains, the study says. If they could be, then the brain activity patterns for made-up words and chairs should also have been more similar in identical twins. They weren't.
"Face and place recognition are older than reading on an evolutionary scale," Polk said. "They are shared with other species and they provide a clearer adaptive advantage. It's therefore plausible that evolution would shape the cortical response to faces and places, but not symbols such as words and letters."
He said that this research could help scientists understand what's innate and what is learned. "If we can figure out the extent to which the brain can change as a result of experience and what makes it change, we could potentially develop therapies for people with brain damage," Polk said. When parts of the brain are damaged, other areas often compensate.
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The paper is called "Nature vs. Nurture in Ventral Visual Cortex: An fMRI Study of Twins."
Polk is an associate professor of psychology in the College of Literature, Science, and the Arts, as well as electrical engineering and computer science. He was also selected as an Arthur F. Thurnau professor for his contributions to undergraduate education.
Other contributors to this paper include psychology graduate student Joonkoo Park, psychology research technician Mason Smith and University of Illinois at Urbana-Champaign psychology professor Denise Park.
For more information on Polk, visit: http://www.lsa.umich.edu/psych/people/directory/profiles/faculty/?uniquename=tpolk |
