Science / Medicine : Emotional Impact : Once-Underestimated Brain Functions Are Revealing Their Role in Shaping Perception and Memory
“Don’t be so emotional,” is a phrase often heard in our society. Our emotions are believed to cloud the much more valued cerebral functions such as thinking and perceiving. But recent studies reveal that emotions are major players in these “higher” cognitive brain functions.
Researchers mapping the anatomical ties between the cognitive and emotional centers in the brain are suggesting that emotions not only shape what we perceive but sift out what we remember.
Research on animals is also generating some intriguing explanations of why certain phobias cannot be erased, why some individuals tend to be more emotional than others, and why emotionally laden events that occur before age 3 can strongly influence a person’s life.
“It’s a very exciting time for emotions research,” said neuroscientist Mortimer Mishkin of the National Institute of Mental Health. “We’re finding that emotions at some level enter into most of what happens to us during the day.”
Much of the recent research focuses on the amygdala, a small, almond-shaped structure lodged in the lower part of the brain. Neuroanatomy studies reveal that the amygdala acts as a gateway to emotions and a filter for memory by attaching significance to the information that the senses send the brain.
When a person hears a bear growl and gets a glimpse of a bear’s face peeking through the limbs of a tree, such bits of sound and sight information are routed to the amygdala and the outer layer of the brain. This region, known as the cortex, generates a more complete picture of what is being perceived--it determines that the growl heard comes from a bear. The cortex also puts things in context--it assumes that the bear is in the woods and not in a zoo.
The amygdala, in contrast, scrutinizes the information for its emotional weight. (It notes that the growl may spell danger.) If an emotional response is warranted, the amygdala sends signals along the numerous nerve cell pathways that connect it to other brain structures, which can generate a gamut of reactions that include fear, anxiety, affection and joy.
The vast impact that emotions have on behavior, which had been previously underestimated, became dramatically apparent when researchers studied animals whose amygdalas were damaged or missing. Monkeys whose amygdalas have been surgically removed have abnormal sexual and feeding behavior, are socially withdrawn, and are strikingly unresponsive to their environment. When they are pinched by investigators, the monkeys flinch but do not try to escape or mount a counterattack. Studies of people whose amygdalas have been damaged suggest that they, too, act inappropriately.
“Emotions really influence what you get out of the world,” Mishkin said. Although most people are unaware of it, emotions guide our behavior by drawing us toward something favorable and propelling us from something harmful, said New York University neuroscientist Joseph LeDoux.
“We don’t usually just act randomly, but purposely to obtain something good or avoid something bad,” he said. The amygdala enables us to make those distinctions, he said.
Overly emotional reactions, LeDoux’s studies suggest, may stem from the amygdala responding too strongly to crude information sent by the senses before receiving “the big picture” from the cortex. Some people may react so strongly to a growl that by the time they realize the growl comes from a bear in the zoo, the alarm signals set off by their amygdalas cannot be controlled.
LeDoux’s rat studies, which were published in the Journal of Comparative Anatomy in 1985, suggest that an anatomical pathway allows information gathered from the senses to travel directly to the amygdala before the same information reaches the cortex. This raw data can prompt the amygdala to trigger an emotional response before it receives a more accurate picture from the cortex.
Such fast action by the amygdala may have survival value. “By the time your amygdala receives information from the cortex that tells it the loud noise is a growl from a bear in the woods, the bear may already have you,” LeDoux said. “Your amygdala’s response may allow you to escape.”
The anatomical pathway in rats also is found in primates and most likely exists in humans, LeDoux said. “Although the human brain may have expanded anatomical features, whatever is there in the rat brain is not likely to be lost in the human brain,” he said.
In most individuals and animals, if an initial emotional response proves unnecessary (the bear is tied to a tree, for example), it is quickly overridden by signals the cortex sends to the amygdala. Some people may be too emotional because their amygdala’s response is stronger than their cortex’s ability to control it with more rational and accurate information, LeDoux said.
Because the basic features that triggered the strong emotional response remain locked in the amygdala, “these people would be at a loss to explain why they’re so emotional,” LeDoux said.
In babies, the parts of the cortex that can counter an emotional reaction do not operate fully until between 18 and 36 months--several months after the amygdala and other emotional centers in the brain become active. That may explain why babies are subject to such frequent and uncontrollable emotional outbursts, LeDoux said.
The delayed maturation of the cortex may have more profound and longer-lasting implications because the amygdala seems to make a permanent record of every emotional response. “From a very early age, we build emotional records that subsequently will influence everything we do,” LeDoux said.
Without the cortex’s more rational and conscious editing of such records, phobias acquired early in life apparently cannot be erased, LeDoux’s studies on rats suggest. The findings were published in the Journal of Cognitive Neuroscience in 1989.
When LeDoux surgically prevented the rats’ cortexes from influencing the amygdalas’ responses to what was seen, the animals never lost their conditioned fear of a flashing light that previously was connected to a foot shock. In contrast, normal animals stopped fearing the light a short time after it was flashed without the simultaneous shocks.
According to LeDoux, the fear of the flashing light in the surgically altered animals was apparently being maintained by their amygdalas as a permanent emotional memory. “The amygdala doesn’t seem to have an eraser,” LeDoux said.
Assuming a similar scenario exists in humans, a phobia acquired early in life before the cortex matures--a fear of dogs because of a dog bite at age 1--would be retained by the amygdala as an unconscious memory. Therapy that instills rational thoughts to keep the fear in check could control the phobia. Because the phobia could not be erased from the amygdala, it might spring to life during a stressful event.
“A person may have a fear of heights, which therapy has dealt with effectively until the patient’s mother dies. Then, all of the sudden, the phobia is back. That suggests emotional memories are never eliminated but merely held in check,” LeDoux said.
The amygdala also strongly influences what we consciously remember. For most people, events that have strong emotional overtones are recalled with much greater clarity and detail than mundane happenstances. Most people born before the late 1950s can recall in detail what happened to them the day that President John F. Kennedy was shot. Because that event had such a strong emotional impact either on them or the people they were close to, the details of that day are remembered more accurately than those of the day before the assassination.
Once again, the amygdala has a hand in this “flashbulb” memory phenomenon. Studies on monkeys reveal that both the amygdala and another brain structure called the hippocampus are needed for memories to be made permanent.
When Mishkin and his colleagues at the NIMH removed these two structures, monkeys could no longer perform well on a simple memory task that normal monkeys can do with ease. People who have damage to their hippocampus and amygdala cannot remember new information, although their memories of events before their brains were damaged remain intact, studies show.
The converging findings have led researchers to speculate that the amygdala and hippocampus serve as a filter for memory, allowing only significant information to be permanently stored in the cortex. Mishkin proposes the following scenario: An individual senses something with emotional significance--his baby being born. The sights and sounds of this birth travel simultaneously to the cortex and to the amygdala and hippocampus.
The emotional nature of this experience triggers the amygdala and hippocampus to release various compounds, including opiates, which travel the numerous nerve cell pathways that connect these structures to the cortex. Once they arrive in the cortex, these compounds prompt the anatomical changes necessary for long-term memories to form of the images and sounds of the birth. The stronger the emotional significance of what is being perceived, Mishkin speculates, the more compounds the amygdala and hippocampus release to strengthen memories.
A similar process might enable the amygdala to influence perception, allowing us to “filter out what we wish to ignore and enhance perception of what we wish to attend to,” Mishkin said. The numerous nerve cell pathways between the amygdala and the parts of the brain that interpret what is perceived suggest that such influence is possible. Common experience and several studies reveal that mood can strongly influence perception. An enthusiastic person will “hear” more during a lecture than a bored person listening to the same talk.
“We don’t need excessive feelings like terror or grief for emotions to play a major role in our lives,” said Mishkin. “Interest, affection, enthusiasm--these kinds of emotions are with us all the time and we don’t pay enough attention to them.”
Anatomy of Emotions
Much of the recent research on emotions centers on the amygdala, a small, almond-shaped brain structure. The amygdala acts as both a gateway to emotions and a filter for memory by attaching significance to the vast array of information that is sent to the brain by the senses.
1. Information from the senses is routed both to the amygdala and to the outer layer of the brain known as the cortex.
2. The amygdala scrutinizes information for its emotional weight. If, for example, the information is the sight and sounds of a growling bear, the amygdala sends signals to other brain structures that can generate emotional reactions such as fear or anxiety.
3. The cortex generates a more complete picture of what has been perceived. It may quickly override signals of alarm sent by the amygdala. The amygdala is also believed to have a strong influence on memory. If information has emotional significance, it is more likely to be recorded with clarity in one’s memory.
The parts of the cortex that can counter an emotional reaction do not operate fully in a human baby until sometime between 18 and 36 months-several months after the amygdala becomes active. The delayed maturation of the cortex may have profound consequences. Researches believe much emotional information is recorded permanently by the amygdala before it can be overriden by the cortex. This subconscious emotional record could exert a strong subconscious influence and explain some phobias.
