In jumpy flies and fiery mice, scientists see the roots of human emotions

Scientists need to look at the brain states that produce certain behaviors to study emotions in animals.

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We humans say that a growling dog is angry or a purring cat is happy.

David Anderson is a biology professor at Caltech who studies the brain circuits involved in emotional behaviors.

Animals cannot tell us how they feel. Anderson thinks there is a connection between animal and human emotions.

He says that emotions evolved over time by natural selection.

The Nature of the Beast: How Emotions Guide Us is the latest book by Anderson. It is central to a growing scientific effort to find new treatments for disorders like PTSD by manipulating emotion-related brain circuits in animals.

In his book, Anderson states that his research shows the brain circuits underlying human emotions have a lot in common with circuits found in mice and fruit flies.

Feelings versus emotions

Anderson says scientists need to set aside their own feelings of anger, fear, sadness or joy in order to study emotions in animals.

They need to look beyond human feelings.

Anderson says that the feeling part is just the tip of the ocean of consciousness.

Brain states that produce certain behaviors are what lies beneath feelings. Scientists can study that part of emotion. Anderson's lab has investigated fruit flies that become more active when they see a shadow like the one cast by a flying predator.

The more times we deliver the shadow, the more jumpy the flies become.

The flies are hopping after the shadow is gone.

Anderson would act the same way if he saw a snake.

He says he would jump in the air after a snake had slithered away.

The behavior is typical of a brain state called defensive arousal. Anderson believes that studying fear in an insect or a mouse can reveal a lot about human emotions.

Anderson says they can try to figure out how the brain is generating that state and what makes it last so long.

The answer appears to be specialized brain cells that become active when a mouse sees a threat, and that return to normal after the threat has passed. Anderson thinks people have the same group of cells that make them afraid.

Aggression across species

Anger is a human feeling that is probably related to animal emotion.

Dayu Lin says there is no way to know if animals have angry feelings. Aggressive behavior associated with human anger can be found in fish, reptiles, birds and mammals.

Lin is studying the brain areas involved in aggression. She has found one that is critical.

She says that it is a tiny, tiny region deep in the brain.

The bottom of the hypothalamus is where this region is found in people. Studies show that this clump of brain cells is part of a core aggression circuit.

Lin says that we can evoke aggression by using this area in the rodents.

A mouse will attack if it is switched on. An animal's natural aggression disappears if it is switched off. There is evidence that this can happen in people. Doctors sometimes use deep brain stimulation to treat violent patients.

Lin says that the aggression is usually uncontrollable.

Trauma, fear and PTSD

Post-traumatic stress disorder is one of the disorders that animal emotions are helping scientists understand.

Dr. Kerry Ressler of Harvard Medical School believes that the fear response has gone too far in the case of post-traumatic stress disorder.

A stress and fear response that lasts for hours can be experienced by a person with post-traumatic stress disorder. There is a parallel in animals.

A mouse will freeze if it hears a tone associated with an electric shock. The animal learns to ignore the tone if the stop comes.

The learning curve is changed by trauma.

If the animal has had trauma, it will take them longer to learn that the tone is safe.

The amygdala and the prefrontal cortex are involved in trauma in both people and mice. It is possible to regulate that circuit in rodents.

We now understand specific parts of the circuit that increase fear and other parts of the circuit that decrease fear, or at least the animal version of that emotion, Ressler says.

The next step is to figure out how to modify that circuit to make people less afraid.