Mirrors in the Brain
- Giacomo Rizzolatti &
- Corrado Sinigaglia
When a paradigm-shattering discovery is made in science, it goes through three stages before gaining acceptance. First, people don't believe it; second, they claim it is of no interest; and third, they say that they have always known it. The discovery of mirror neurons in the early 1990s by Giacomo Rizzolatti, Vittorio Gallese, Marco Iacoboni and others, has been through all three stages. Happily, the idea seems to have emerged unscathed, judging from Mirrors in the Brain.
In their readable new book, Rizzolatti and philosopher of science Corrado Sinigaglia survey the growing field that the research has spawned, setting it in historical context. They begin with an overview of the neural circuits in the brain that are involved in simple goal-directed movements. When a monkey reaches for a fruit or puts something in its mouth, motor-command neurons in area F5 in the frontal lobes fire. Different neurons fire for different actions.
Rizzolatti discovered that some of these motor-command neurons fire even when a monkey just watches another monkey performing the same action. He called these cells mirror neurons. They allow one monkey to simulate or imagine another monkey's impending action. Mirror neurons have also been found that fire when a monkey watches another monkey being touched. Another class, canonical neurons, fire both when orchestrating the precise hand and finger movements required to grab a specific object and when one simply looks at that object. It is as if mere readiness to engage in a cylindrical grasp is synonymous — in terms of neural activity — with the perception of a cylinder.
Monitoring mirror-neuron activity might allow us to decipher the computations that lie at the elusive interface between perception and action, providing a key to understanding human cognition. If you extend the definition of action to include more abstract behavioural propensities, you could speak of the cells as representing 'meaning'. For example, an apple can conjure many ideas that neural activity may represent differently: it can be reached for and eaten, be used to tempt Eve, can keep the doctor away, go into a pie and so on.
Mirror neurons may be involved in seemingly unrelated mental abilities, such as pretend play in children, imitating skilled actions, emotional empathy and constructing a useful model of another's actions to predict his or her intentions. Because these abilities are lost in autism, it has been suggested that the condition may result partly from mirror-neuron deficiency.
If a mirror neuron fires when someone touches you and when you watch someone being touched, how do you know the difference? One possibility is that when you watch someone else being touched, tactile receptors in your skin inform the regular, non-mirror neuron cells in your brain that they are not being touched, which inhibits the output of your mirror neurons. This would explain our observation that people who have had a hand amputated experience touch sensations in their phantom hand when watching another person's intact hand being touched. The absence of signals from the missing hand removes the inhibition, causing the patient to literally experience another person's sensations, dissolving the barrier between self and others. I have dubbed the cells involved here 'Gandhi neurons'. But the inhibition of mirror-neuron output by non-mirror neurons isn't perfect even in people with no amputation, as first noted by Charles Darwin. He observed that we tend to unconsciously tense our calf muscles when watching someone getting ready to throw a javelin, for example.
We may have evolved mirror neurons or acquired them by associative learning. To explain the latter, every time the network that a neuron is part of sends a command, you see your hand moving; eventually, as a result of conditioning, the mere appearance of a moving hand (even someone else's) triggers the same neuron. This hypothesis cannot explain why regular, non-mirror, sensory neurons do not also develop such properties through associative conditioning. To explain this, one has to invoke a pre-existing, genetically specified scaffolding that imposes constraints on what is learned.
Infants imitate their mother smiling or sticking out her tongue, implying a mirror-neuron-like computation for translating the visual appearance into the sequence of muscle twitches. Learning cannot be involved because the infant has never seen its own face. It is possible that the infant's smile is just a reflex that doesn't require elaborate translation. This can be ruled out if the newborn can also mimic an asymmetrical smile or a peculiar expression, which demands a sophisticated interfacing between visual appearance and motor output.
Mirror neurons may also have clinical relevance for phantom pain and stroke rehabilitation. If a mirror is propped up vertically on a table in front of a patient with, for example, a paralysed left hand (so that one edge of the mirror is against his chest), the patient gets the illusion that the left hand is moving when he moves his right hand. We and others have found that this causes recovery from paralysis, perhaps by visually reviving dormant mirror neurons.
Mirrors in the Brain documents how this new science has been received. Some psychologists have criticized the idea of mirror neurons as being reductionist. Others think it is a mere metaphor for what psychologists have long called the 'theory of mind module' — the ability of our brains to construct internal models of other people's minds to predict their behaviour. This criticism reveals a fear that neuroscience might displace psychology ('neuron envy'), and a misunderstanding of reductionism. It is a bit like saying that the complementarity of the two strands of DNA is a metaphor for the complementarity of offspring and parent.
Psychological and neural explanations are complementary, not mutually exclusive. In their book, the authors present a long-awaited review of many empirical findings, and discuss the deeper implications of these observations for psychology, cognitive science and psycholinguistics, including language origins. Moreover, they write in a jargon-free style that should be intelligible to all.
There has been a lot of media hype surrounding mirror neurons. The real danger is that too much is explained, not too little. This is inevitable with any new discovery but does not, in itself, vitiate the discovery's intrinsic importance. Nearly a decade ago, I wrote that “mirror neurons will do for psychology what DNA did for biology”. It remains to be seen whether they will turn out to be anything as important as that, but as Sherlock Holmes said to Watson: “The game is afoot.”
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Ramachandran, V. Reflecting on the mind. Nature 452, 814–815 (2008). https://doi.org/10.1038/452814a