Home page


A face that rings a bell ... and fires a neuron

Why do specific nerve cells respond to certain objects or images?


By Tom Siegfried / The Dallas Morning News

NEW YORK – At first glance, the human brain seems to be ruled by mob mentality. Billions of nerve cells shout to one another by firing electrical impulses, controlling how the brain's owner behaves.

But within that mob of nerve cells, or neurons, are specialists that do their own thing. Some cells ignore all but one specific sort of stimulus, then fire at will when encountering their specialty.

Many neurons respond best to faces, others to animals, some to houses. Some single neurons tune in to specific facial expressions; others fire for familiar faces but not for strangers.

And one neuron fires like crazy at the sight of an angry female face. For many men, that neuron works overtime.

It's not easy, though, to find out what any one nerve cell's job is. Brain waves measured from outside the scalp reflect the signals from many cells, as do brain scans that measure brain activity by recording blood flow. Much of what scientists know about single neurons, therefore, is acquired from tiny electrodes implanted in the brains of animals, especially macaque monkeys or other primates.

Since the 1950s, scientists have also occasionally placed electrodes into the brains of people, typically epilepsy patients. The electrodes help identify the misbehaving part of the brain in order to guide corrective surgery.

Those same electrodes can reveal information about normal cognitive processes such as remembering and learning. For many years, though, the electrode approach was used only for medical purposes.

"Its application to cognition has been much more limited," says University of Washington neuroscientist George Ojemann.

But lately more researchers have given willing patients simple tests to study such mental processes as face recognition, memory and use of language. Scientists discussed some of their findings last month in New York City at the annual meeting of the Cognitive Neuroscience Society.

Such research makes use of ultratiny "microelectrodes" to probe brain tissue and record the electrical signals that neurons fire. By analyzing the recordings, scientists can determine what kind of stimuli will trigger a neuron to fire more often than it does in its unstimulated, or "resting," state. Resting firing rates are generally a few times per second; a stimulated neuron might fire 20 or 30 times a second.

The new human findings confirm reports from monkey studies showing that many single neurons react most vigorously to photos or drawings of faces.

"A face is a very powerful stimulus," says neurosurgeon Itzhak Fried of the UCLA School of Medicine in Los Angeles.

'Grandmother' cells

Neuroscientists sometimes refer to face-specialized neurons as "grandmother" cells, for a neuron that would fire only at the sight of a certain person – say, your grandmother. Most face-sensitive neurons are not quite that specific, though.

"We don't find many neurons that respond to a particular face and not 10 other faces," Dr. Fried said. "Those neurons are rare."

Yet neurons do show a wide range of face-specific behaviors. Some neurons are excited only by faces of famous people. Some respond strongly to female faces, but weakly to males. Others respond only to faces showing particular emotions.

Dr. Fried and colleagues conducted the study identifying a neuron that fired most rapidly when viewing a woman's face expressing anger.

"You might wonder what's the use of a neuron that responds only to angry females," Dr. Fried said at the neuroscience meeting. He suggested that childhood experiences might offer an explanation.

Faces are not the only images that elicit specialized neuron activity. Some neurons prefer pictures of houses, for example, and some will respond only to pictures of animals, Dr. Fried and collaborators Gabriel Kreiman and Christof Koch, of the California Institute of Technology in Pasadena, have found.

In a study reported last fall in Nature Neuroscience, they measured neuron responses in 11 patients from electrodes positioned in parts of the brain important for processing memories and emotions. Of 427 single neurons measured, 14 percent showed sensitivity to visual images in specific categories.

In another study of nine patients, reported by Dr. Koch at the neuroscience meeting, researchers tested patients who were shown pictures and then were asked to close their eyes and imagine what they'd seen.

Some neurons responded only to the real images, while some responded only to the imagined images, Dr. Koch said.

Other studies of specific neurons reveal curious aspects of learning and memory, Dr. Fried said. Some neurons, for example, will fire more in response to an image that has been seen previously, but not to a new image.

In fact, sometimes a person may forget having seen the image, but the neuron doesn't, and fires anyway.

"The subject is saying I haven't seen it, but the neuron is saying that the stimulus has actually been presented to the senses," Dr. Fried said.

Further complications arise because some neurons respond to a specific stimulus not by firing more, but by firing less. When viewing an object for the first time, neurons that respond in the amygdala, involved in emotion, always increase their firing rate, Dr. Fried said. But in the hippocampus, a nearby part of the brain linked to memory formation, some neurons respond by firing less often than usual. For the task of recognizing a previously seen object, even more neurons are likely to inhibit their firing rates.

Apparently, Dr. Fried said, a balance of excitation and inhibition must be necessary for recognizing objects.

Memory predicting

Other tests of memory suggest that the prospect of remembering specific words can be predicted by monitoring neuron activity, Dr. Fried said. One neuron in the hippocampus, for example, responded more strongly when presented with words that would soon be forgotten.

Besides helping understand memory, vision and other mental processes, the single-neuron studies provide scientists improved knowledge of the differences between human and other primate brains. And a different sort of study discussed at the New York meeting has found a peculiar type of neuron that may aid the understanding of human brain diseases.

Comparing various primates, researchers have found one type of unusually long and large neuron only in humans, chimpanzees, gorillas and orangutans. Called a spindle neuron because of its shape, it is found in the anterior cingulate cortex, a brain region involved in decision making and numerous other aspects of brain function, including regulating heart rate and blood pressure.

One of the researchers, Patrick Hof of Mount Sinai School of Medicine in New York City, said that the spindle neuron is a relatively recent evolutionary addition to animal nervous systems.

"You can date that neuron back to 16 million years," he said at the meeting.

In most primates, he said, the spindle cells are scattered in the anterior cingulate cortex. But in bonobo chimps and in humans, the spindle cells congregate in clusters.

Evidence suggests, Dr. Hof said, that the spindle cells may be implicated in some degenerative brain diseases, such as Alzheimer's.

"It's in the human you find more of these spindle cells," he said. "There is a very serious loss of these neurons in Alzheimer's disease."

Other great apes, in which the spindle cells do not cluster, seem not to acquire Alzheimer's, Dr. Hof said.

So it may be that continuing to study neurons in apes, as well as in humans, may be helpful in understanding such diseases.

"There's a lot to be learned," Dr. Hof said, "from great apes."


Feature index

Subscribe to The Dallas Morning News Archives

© 2001 The Dallas Morning News
Privacy policy