Identifying images that excite the brain's neurons
Monday, May 14, 2001
By TOM SIEGFRIED
Special from The Dallas Morning News
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
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
recently in New York City at the annual meeting of the Cognitive
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
"A face is a very powerful stimulus," says neurosurgeon
Itzhak Fried of the UCLA School of Medicine in Los Angeles.
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," Fried said. "Those neurons
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.
Fried and colleagues conducted the study identifying a neuron
that fired most rapidly when viewing a woman's face showing anger.
"You might wonder what's the use of a neuron that responds
only to angry females," 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, 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 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, Koch said.
Other studies of specific neurons reveal curious aspects of learning
and memory, 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,"
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, 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, Fried said, a balance of excitation and inhibition
must be necessary for recognizing objects.
Other tests of memory suggest that the prospect of remembering
specific words can be predicted by monitoring neuron activity, Fried
said. One neuron in the hippocampus, for example, responded more
strongly when presented with words that would soon be forgotten.