Saturday, September 20, 2008

Natural History - Mental Mirrors

Mirror neurons are all the rage in the neuroscience field. This article from Natural History offers a good introduction to the science.

Mental Mirrors

Mirror Neurons

Red area shows location of mirror neurons in the ventral premotor cortex, the part of the brain responsible for coding object-oriented movements such as grasping, holding, and manipulating. This region lies adjacent to the primary motor cortex, which sends electric signals to the muscles.

Marco Iacoboni

WHAT DO PEOPLE REALLY DO all day, every day? We “read” the world. And much of the world consists of other people. When a tennis player raises his racquet, for example, you know instantly whether he’s going to take a practice swing or throw his racket across the court in anger. We all make dozens—hundreds—of such distinctions every day. It is, quite literally, what we do, usually without a second thought. It all seems so ordinary.

In fact, it’s extraordinary—and even more extraordinary that it feels ordinary! We achieve our very subtle understanding of other people thanks to certain collections of special cells in the brain called mirror neurons. They are at the core of how we navigate through our lives. They bind us with each other, mentally and emotionally.

Mirror neurons are incredibly powerful; “vicarious” would not be a strong enough word to describe their effects. When we watch movie stars kiss onscreen, some of the cells firing in our brains are the same ones that fire when we kiss our lovers. And when we see someone else suffering or experiencing pain, mirror neurons help us to read her or his facial expression and make us viscerally feel the suffering or the pain of the other person. Those moments, I will argue, are the foundation of empathy (and possibly of morality). Research on mirror neurons gives anyone interested in how we understand one another some remarkable food for thought.

CONSIDER THE TEACUP EXPERIMENT I published an account of in 2005 [see illustration below]. Test subjects are shown three video clips involving the same simple action: a hand grasping a teacup. In one clip, there is no context for the action, just the hand and the cup. In another, the subjects see a messy table, complete with cookie crumbs and dirty napkins—the aftermath of a tea party, clearly. The third video shows a neatly set table, in apparent readiness for the tea party. In all three video clips, a hand reaches in to pick up the cup. Nothing else happens, and the grasping action observed by the subjects in all three versions of the experiment never changes. Besides the difference in context, there is only one other variation: in the “neat” scenario the cup is full, whereas in the messy one the viewer cannot tell if the cup contains liquid.

Do mirror neurons in the brains of the subjects notice the differences in context and in the contents of the cup? Most definitely. When a subject observes the grasping scene with no context at all, mirror neurons are the least active. The neurons are more active when the subject watches the after-tea-party scene, but they are most active during the neat, full-cup scene. Why? Because drinking is a much more fundamental intention for us than cleaning up. The teacup experiment—now well known in the field of neuroscience—belongs to a wealth of recent empirical evidence suggesting that our brains are capable of mirroring the deepest aspects of the minds of others at the fine-grained level of a single brain cell. Reading the intention of others is only one example of the kinds of distinctions that can be made with a remarkable lack of effort. We do not have to draw complex inferences or run complicated algorithms. Instead, we use mirror neurons.

Mirror neurons were first discovered in the brains of monkeys, where they are concentrated in two linked areas, called the ventral premotor cortex and the inferior parietal lobule, that are important for selecting appropriate motor behavior. Mirror neurons make up approximately 20 percent of the neurons in those regions, which lie close to the primary motor cortex, the area of the brain that sends electric signals to the muscles. In humans, however, mirror neurons may be located in many more regions of the brain, in varying amounts. (I hope to publish new findings about their location soon.)

Neuron Activity

Three video clips involving the same simple action of grasping a cup were shown to test subjects: in the first, the action occurs against no background (left); in the second, the background is a messy table complete with cookie crumbs and dirty napkins, implying the aftermath of a tea party (middle); in the third, the context is a neat tabletop, in apparent preparation for a party (right). The neat scenario suggests that the intent behind grasping the full cup is to drink, whereas the messy scenario suggests an intent to clean up. The blue-green bars under the images represent the relative amount of activity of the observer’s mirror neurons. Based on context, mirror neurons can distinguish intention. The activity of the observer’s mirror neurons is greatest for the neat scenario—almost double the amount in the messy one—because drinking is a more fundamental intention than cleaning up.

Melisa Beveridge

Mirror neurons seem to have nothing in common with deliberate, effortful, and cognitive attempts to imagine being in somebody else’s shoes. So how do they actually predict the action that will follow an observed scene? How do they let us understand the intention associated with such an action?My hypothesis is this: we activate a chain of mirror neurons when we watch an action. This chain of neurons can anticipate a whole sequence—say, reaching for the cup, grasping it, bringing it to the mouth—and so can simulate the intention of the human we are watching.

Mirror neurons in such a chain may be of different types. One kind—so-called strictly congruent mirror neurons—respond to identical actions, either performed or observed. For instance, a strictly congruent mirror neuron fires both when a monkey grasps an object with two fingers, in a “precision” grip, and when that same monkey sees another primate grasping with a precision grip. A different mirror neuron, also strictly congruent, fires when the monkey grasps with its whole hand as well as when the monkey sees somebody else grasping in the same fashion.

Other mirror neurons, however, show a less strict correspondence between performed and observed actions. Those are known as broadly congruent mirror neurons. They fire at the sight of actions that may not be identical, but that achieve similar goals. For instance, a broadly congruent mirror neuron may fire when the monkey is grasping food with its hand, and also when the monkey sees somebody else bringing food to the mouth.

An important subset of the broadly congruent type of mirror neurons fire in anticipation of logically related actions. These logically related mirror neurons, as they are logically called, are probably the neuronal elements needed to understand intentions associated with observed actions. I see you grasping a cup with a certain kind of grip, and my grip mirror neurons fire, the strictly congruent ones. So far I am only simulating a grasping action. However, given that the context suggests drinking, my logically related mirror neurons, the ones that code for the action of bringing the cup to the mouth, fire even before the cup is brought to the mouth. By activating this chain of mirror neurons, my brain is able to simulate the intentions of others.

Why do some cells fire for actions that are logically related? No one knows for sure, but it’s likely that mirror neurons “learn” from experience—such as when babies watch or interact with their caregiver. Suppose a baby sees a caregiver’s hand put food on the table, and then the baby grasps the food to eat. Grasping food and seeing food placed become associated in the baby’s brain. It’s not that the mirror neurons know there is a logical connection between the two actions; rather, the baby-caregiver interaction links the two as part of a sequence.

Read the whole article.


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