Our ability to perceive other people's emotions plays an enormous role in our lives. Without this skill, social interaction would be fraught with peril. But how does the brain actually process the emotional signals that we sense in faces and tones of voice? An interdisciplinary research group in Germany is attempting to find out.
Assessing emotions is largely controlled by the limbic system, deep in the brain. This network of small structures gives incoming sensory data its emotional coloration. As numerous studies have confirmed, the amygdala is particularly important in creating these associations. Among other tasks, it sets off alarms when we see a hostile face or hear an angry voice, readying the body for fight or flight. But certainly other brain regions are involved.
For decades, neuroscientists have suspected that the brain's two hemispheres partition the work needed to evaluate emotional signals. Deficits in patients who have brain injuries support this notion. For example, people with damage to the right hemisphere, such as that which occurs after a stroke, may no longer be able to recognize the emotions underlying facial expressions. Such findings led to what is called the right hemisphere hypothesis: the right hemisphere is responsible for feelings, whereas the left deals with language. But is it really the case that the right hemisphere preferentially processes negative emotions such as sorrow, anxiety and disgust, and the left side works on the more pleasant part of our emotional lives? The German team designed several experiments to pursue an answer.
The team exposed test subjects to a variety of stimuli, such as emotion-laden images or words, while their brain activity was measured. The researchers used a combination of probes, including the common electroencephalograms (EEGs) that sense brain waves; magnetoencephalography, which measures magnetic fields produced by the brain during neural activity; and functional magnetic resonance imaging (fMRI).
As a first step, psychologist Markus Junghfer of the University of Constance showed healthy subjects photographs of neutral, disturbing and exciting scenes. He flashed images on a screen more than 200 times in rapid succession. This pattern did not give the brain enough time to "think" about what it was seeing, only to react.
The upshot was that barely 200 milliseconds after a flash, emotionally charged images triggered a strong EEG signal from the visual cortex in the back of the brain. The more charged the picture was, the stronger the signal, regardless of whether the images depicted an attacking snake or naked men and women engaged in sex.
The experiment verified what other researchers have proposed for some time--that the limbic system responds to extreme sights before the sensory information perceived by the eyes even reaches the visual cortex [see "Friend or Foe?" by Marion Sonnenmoser, on page 78]. In doing so, the amygdala instantly rivets our attention on potentially important stimuli. The team's fMRI images supported this conclusion as well. From an evolutionary standpoint, such lightning reactions of "motivated attention" provided the ability to escape danger or quickly kill for a meal.
The Right Tone
The same flurry of images also brought to light another finding: the reactive brain signals were considerably stronger in the right hemisphere than in the left. Conventional wisdom would say this imbalance occurred because the neuronal networks responsible for attention and spatial orientation are located in the right hemisphere. But what happens when the same test subjects are confronted with emotional words? In most people, the language-processing regions are in the left hemisphere. To check, the researchers at Constance read out loud a series of neutral, positive and negative adjectives to the volunteers. Once again, they found very quick brain responses, but in this trial the left side was dominant.
Thus far the team had verified by experiment what had been expected in theory. It now wanted to see if more complex stimuli followed the straightforward patterns. Neurologist Dirk Wildgruber of the University of Tbingen designed tests that relied on the intonation of language rather than on its plain content, because how a person says something often transmits more emotional information than what he says.
Wildgruber had test subjects listen to recorded sentences such as "I've been visiting Agnes every weekend." This sentence was spoken by an actor in a voice that was happy, frustrated or neutral. A computer then processed the sentences so that they differed only in terms of sound amplitude and vowel length. These traits were enough for the subjects to distinguish the intensity of the emotional expression.
The fMRI images showed that the emotional coloration inherent in tone stimulated two cortical regions--one in the frontal lobe and one in each of the parietal lobes--but it did so more markedly on the right side. Differentiating intonation was the result of numerous small contributions from both hemispheres. This conclusion gave the researchers much to ponder. Suddenly, the question of whether the right hemisphere is primarily responsible for emotions could not be answered clearly. Which hemisphere takes the leading role seems to depend not just on which sense is stimulated (vision or hearing) but also on the nature of the stimulus (tone versus words).
The team will further probe this puzzle in 2005, in part by studying unusual emotional processing by certain psychiatric patients who are emotionally unstable. Gabriele Ende of the Central Institute of Mental Health in Mannheim is already looking at people who are suffering from depression, who seem to be less able to recognize the feelings of others.
There is still much to learn about how humans process emotional inputs. But it appears that the popular notion of an "emotional" right hemisphere that contrasts sharply with a "rational" left hemisphere is like a crude pencil sketch made before a full-color painting: many nuances are waiting to be added. Emotional communication involves so many brain regions and connecting channels that the right hemisphere hypothesis has only limited validity.