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Human Emotion Processing in Individual Brain Cells

IOWA CITY, Iowa -- A region at the front of the brain's right hemisphere, the prefrontal cortex, plays a critical role in how the human brain processes emotions. Data from previous studies of brain lesions (areas of damage that alter the brain's ability to generate normal emotions) and data from functional brain imaging studies have delineated the extent of the area involved. However, a recent University of Iowa study is the first to investigate human emotion processing by the right prefrontal cortex at the level of individual brain cells.

"This kind of single-cell study is very rarely performed in humans," said Ralph Adolphs, Ph.D., assistant professor of neurology and principal investigator on the study. The findings appear in the January issue of the journal Nature Neuroscience.

A rare surgical situation allowed the UI Health Care researchers to record the activity of individual brain cells, neurons, in an awake, alert patient as he was shown images designed to elicit an emotional response.

The patient was undergoing neurosurgery to treat epilepsy, which had not responded to medication. Usually, electroencephalogram (EEG) electrodes placed on the scalp would be used to pinpoint where in the brain the epileptic seizures are localized. However, in this case that approach did not work so, for treatment purposes, the surgeon implanted depth electrodes into the patient's brain to monitor where the seizures originated.

"We used a custom-designed hybrid research-clinical depth electrode, which provided the neurosurgeon with the clinical information necessary to locate the area causing the seizures," explained Adolphs. "The electrode also had a series of special contacts on its shaft, through which we were able to isolate the activity of single brain cells. Recording the activity of the neurons posed no additional risk to the patient."

Monitoring single neurons in the right prefrontal cortex, the researchers found that these cells responded remarkably rapidly to unpleasant images, which included pictures of mutilations and scenes of war. Happy or neutral pictures did not cause the same rapid response from the neurons.

To ensure that these neurons were not reacting to pictures that were brighter or larger or had more of a particular color, the researchers were particularly careful to make sure that the only difference among these pictures was their emotional content.

"The changes in firing pattern of neurons responding to the aversive visual stimuli happened within about 0.12 seconds, which is very fast and probably prior to the patient consciously "seeing" the image," Adolphs said.

"The speed at which these cells change their firing rates is surprisingly rapid. We thought it would take much longer for these neurons to be able to extract information about an emotion category, which is really a very high level cognitive function," Adolphs added.

Although the researchers were surprised by the speed at which the neurons reacted to the aversive images, Adolphs indicated that the findings are consistent with the idea that the brain has systems that can respond extremely rapidly to potentially dangerous or threatening kinds of stimuli.

"It makes a lot of sense from an evolutionary point of view," he said.

The UI study shows that neurons in the right prefrontal cortex are able to distinguish, or categorize, emotional information from visual stimuli very rapidly. Adolphs also indicated that it seems likely that signals from these cells may serve to modulate visual information processing by other regions of the brain.

"The area of the brain that we recorded from, the prefrontal cortex, is only one component of a widely distributed neural system for encoding this information," Adolphs said. "We think that another part of that information is encoded in visual cortices. The visual cortices would respond when the stimulus is seen, then those responses would be changed by subsequent input from the prefrontal cortex."

Although the study involved only one patient who had epilepsy, the region of the brain where the recording was performed was distant from the site of the epileptic seizures. This meant that the tissue being studied was essentially normal, healthy prefrontal cortex.

---University of Iowa

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