October 10, 2004
Ferret Visual Cortex 80% Active Even In The Dark

While a popular myth holds that only about 10% of the neurons in our brains actually do anything Michael Weliky, associate professor of brain and cognitive sciences at the University of Rochester, investigated ferrets and found that young ferret brains may be less able to organize and make sense of visual stimuli, that young ferret brains are less busy in the dark than adult ferret brains, and that even in the dark 80% of adult ferret brain visual cortexes are still busy.

The test was then to see if there was any relationship between the statistical motion of the movie and the way visual neurons in the ferrets fired. Each visual neuron is keyed to respond to certain visual elements, such as a vertical line, that appears in a specific area of the ferret’s vision. A great number of these cells combine to process an image of many lines, colors, etc. By watching the patterns of how these cells fired while watching The Matrix, Weliky could describe the pattern statistically, and match those statistics of how the ferret responded to the film with the statistics of the actual visual aspects of the film.

Weliky found two surprises. First, while the neurons of adult ferrets statistically seemed to respond similarly to the statistics of the film itself, younger ferrets had almost no relationship. This suggests that though the young ferrets are taking in and processing visual stimuli, they’re not processing the stimuli in a way that reflects reality.

“You might think of this as a sort of dyslexia,” explains Weliky. “It may be that in very young brains, the processing takes place in a way that’s not necessarily disordered, but not analogous to how we understand reality to be. It’s thought that dyslexia works somewhat like this—that some parts of the brain process written words in an unusual way and seem to make beginnings of words appear at their ends and vice versa. Infant brains may see the entire world the same way, as a mass of disparate scenes and sounds.” Weliky is quick to point out that whatever way infant brains may interpret the world, just because they’re different from an adult pattern of perception does not mean the infants have the wrong perception. After all, an adult interpreted the visual aspects of the film with our adult brains, so it shouldn’t be such a surprise that other adult brains simply interpret the visual aspects the same way. If an infant drew up the statistics, it might very well match the neural patterns of other infants.

The second, and more surprising, result of the study came directly from the fact that Weliky’s research is one of the first to test these visual neurons while the subject is awake and watching something. In the past, researchers would perhaps shine a light at an unconscious ferret and note which areas of the brain responded, but while that method narrowed the focus to how a single cell responds, it eliminated the chance to understand how the neural network of a conscious animal would respond. Accepting all the neural traffic of a conscious brain as part of the equation let Weliky get a better idea of the actual processing going on. As it turned out, one of his control tests yielded insight into neural activity no one expected.

When the ferrets were in a darkened room, Weliky expected their visual neurons to lack any kind of activity that correlated with visual reality. Neurologists have long known that there is substantial activity in the brain, even in darkness, but the pattern of that activity had never been investigated. Weliky discovered that while young ferrets displayed almost no patterns that correlated with visual reality, the adult ferrets’ brains were humming along, producing the patterns even though there was nothing to see. When watching the film, the adult ferrets’ neurons increased their patterned activity by about 20 percent.

“This means that in adults, there is a tremendous amount of real-world processing going on—80 percent—when there is nothing to process,” says Weliky. “We think that if you’ve got your eyes closed, your visual processing is pretty much at zero, and that when you open them, you’re running at 100 percent. This suggests that with your eyes closed, your visual processing is already running at 80 percent, and that opening your eyes only adds the last 20 percent. The big question here is what is the brain doing when it’s idling, because it’s obviously doing something important.”

Since the young ferrets do not display similar patterns, the “idling” isn’t necessary for life or consciousness, but since it’s present in the adults even without stimulus, Weliky suggests it may be in a sense what gives the ferret its understanding of reality. The eye takes in an image and the brain processes the image, but 80 percent of the activity may be a representation of the world replicated inside the ferret’s brain.

There's an obvious math error in how this press release is written. If the brain is operating at 80% of capacity in the dark and then increases to 100% capacity then from the reference point of the brain's activity level in the dark the amount of increase going into a richer visual environment is actually 25%. But that is just a quibble.

A more basic problem is with the 100% figure for what is implied to be some sort of maximum activity level. Is that supposed to be the absolute maximum level of activity of a ferret brain's visual cortext? Isn't it possible that there are conditions under which a ferret visual cortex might become twice as active as the highest level of activity that this researcher ever measured? There has to be some absolute maximum level of activity because there is a limit to how much oxygen the bloodstream can deliver to neurons. Also, some neurons are going to be less active because the suppression of some neurons combines with the excitation of other neurons to form a representation of any one image.

All of these results are from ferrets and it is possible that human brains do not exhibit similar behavior. But my guess is that while the absolute percentages may differ from the numbers reported above human brains probably do have similar differences between babies and adults. The ability of adult brain visual cortexes to stay so active in the dark likely is the result of the development of a fairly complex model of the visual world. That model is always running and mulling over older images even when no new images are being presented to it.

What this suggests about babies and children is especially interesting. They can't make as much sense of the world. They do not know as many logical relationships between objects in an image field and therefore can't create as many higher level meanings from what they are seeing. So the visual world must seem far more random and unpredictable to them. They may not even have as great an ability to track temporal order of changes in elements in a visual field.

Share |      Randall Parker, 2004 October 10 01:07 PM  Brain Development

Brock said at October 10, 2004 1:28 PM:

Just as its hard to think of a thing without thinking of the word, it's also hard to think of it without visualizing it. I imagine dreams, plans and memories stimilate the visual cortex as well - and that's what the non-sighted 80% is.

Jody said at October 10, 2004 8:15 PM:

I wonder what the idling statistics are for those blind from birth (BFB). I also wonder what's going on when the BFB think about the world as "visualizing it" probably means something very different for them.

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