Brain doesn’t sort by visual cues alone

The bird and the feather apparently do not flock together, at least not in the visual processing centers of the brain. That holds true even in blind people, new research indicates.

A functional MRI study shows that congenitally blind people’s brains distinguish between living and nonliving things in the same visual areas that sighted people use, researchers report August 13 in Neuron.

“I think the results are clear and [the researchers] make a good case,” says cognitive neuroscientist Irving Biederman from the University of Southern California, who was not involved in the study. The results imply that people’s brains may be hard-wired to tell living things from nonliving things without visual cues, he says.

Previous studies have suggested that the ability to recognize objects by sight makes use of a neural pathway called the ventral stream, found on the underside of the brain. This ventral stream is involved in the early steps of vision processing as well as in object recognition that relies on other senses.

Scientists already knew that, in sighted humans at least, different areas of the brain seem to specialize in recognizing certain categories of objects, such as animals, faces, nonliving objects and even inanimate living objects such as fruits and vegetables. But it was not clear whether people were born with an ability to separate things into these categories or developed the categories through visual experience. By looking at blind people, the study authors hoped to see whether visual experience with objects was necessary to create these patterns of organization.

Study author Bradford Mahon of Rochester University in New York and colleagues from the University of Trento in Italy used fMRI to test brain activity in 20 sighted people and three who had been blind since birth. All of the blind and seven of the sighted subjects listened to a series of words representing objects in a single category — either living or nonliving — and were asked to compare the sizes of those objects. Dogs, spiders and other animals that the participants were likely to have encountered went into the category of living things, while nonliving things consisted of tools, such as hammers and forks, or larger objects that cannot be manipulated by hand, such as cars and tables. All subjects, blind and sighted alike, kept their eyes closed.

All of the sighted people also took the fMRI test while looking at pictures of the objects instead of listening to spoken words. As the researchers expected, both groups of sighted volunteers showed greater activity near the midline of the ventral stream when thinking about nonliving objects and closer to the back of the brain when thinking about animals.

Researchers had not previously tested how blind people’s brains sort the living from the nonliving. In the new tests, blind subjects showed the same activation patterns as sighted people.

If there were no innate organization of the brain, the blind participants’ brains should have shown random activations for living and nonliving things, or entirely different patterns of activation, Biederman says.

“Probably the default assumption most people had who work with the ventral pathway was that organization is driven by shape — your encounter with objects,” Biederman says. “And this is saying ‘no,’ that may be part of the story, but that’s not the whole story.”

Study coauthor Mahon agrees that experience is clearly critical to the ability to sort objects. But the ways a person interacts with a hammer and a cat are so entirely different that it makes sense that these types of objects might be wired through evolution to be recognized in different areas of the brain.