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The mirror crack'd?
A new brain-imaging study claims to provide robust evidence inconsistent with the idea that the human brain contains 'mirror neurons' - postulated cells thought to respond both when an action is executed and when it is observed (PNAS).
Such neurons have been identified via single-cell recording studies in monkeys, and their possible existence in humans has been linked with a number of cognitive functions including empathy and language development. Indeed, the seriousness of the new claim that human mirror neurons might not exist is perhaps best captured by a reminder of the hope that was pinned on them in 2000 by celebrated neuroscientist V.S. Ramachandran. Writing for Edge Magazine he predicted 'that mirror neurons will do for psychology what DNA did for biology: they will provide a unifying framework and help explain a host of mental abilities that have hitherto remained mysterious and inaccessible to experiments.'
The new study by Alfonso Caramazza and colleagues at the Universities of Harvard and Trento depended on cellular adaptation to detect the presence of mirror neurons. Adaptation is the tendency for the response of cells to gradually reduce with repeated stimulation. There were two key tests of adaptation in Caramazza's experiment: during the observation of hand movements followed by the execution of those same movements, and secondly, during the execution of the movements followed by their observation. Unlike previous adaptation tests of mirror neurons, the current study used the observation and execution of meaningless actions not involving objects, to ensure that the results weren't contaminated by brain activation involved in anticipation or object perception.
Scans of 12 participants' brains using fMRI revealed areas of adaptation when the same hand movements were first observed and then executed, but crucially not the other way around. According to Caramazza and colleagues, this 'failure to find cross-modal adaptation for executed and observed motor acts is not compatible with the core assumption of mirror neuron theory, which holds that action recognition and understanding are based on motor simulation.'
However, leading mirror neuron researcher Marco Iacoboni (University of California) was not convinced by the new paper. 'In life science in general and in brain imaging in particular a negative result can't possibly prove the non existence of a phenomenon. It is surprising that Caramazza's paper makes those claims and that a high profile journal like PNAS publishes it,' he told us.
Beyond this general point, Iacoboni told us there were multiple, fatal flaws with Caramazza's study. For a start, he said the technique of detecting cellular adaptation using fMRI had been shown to be profoundly unreliable. 'Some initial results were promising but careful studies looking at the neural correlates of fMRI adaptation paradigms have revealed that the results of fMRI adaptation studies are uninterpretable with regard to the activity of specific neuronal populations (the mirror neurons, in this case)' (see for a review).
Iacoboni added that there isn't any reason to think mirror neurons should exhibit adaptation in the same way that sensory neurons do. 'Although nobody has done a thorough study on adaptation in mirror neurons, inspection of the single unit recordings in neurophysiological studies does not suggest that mirror neurons adapt.' What's more, he said that our understanding of most neuron types is heavily dependent on monkey work, so it doesn't make sense to doubt human mirror neurons on this basis. 'To question the existence in humans of any kind of neurons recorded only in monkeys is tantamount to questioning the evolutionary framework itself and even the ethical grounds for doing neurophysiological studies in monkeys', Iacoboni said.
But Alfonso Caramazza was staunch in defence of his findings. Providing some background, he said there was every reason to doubt the existence of mirror neurons in humans - people with apraxia, for example, who are impaired in using objects, nonetheless can have normal recognition of the use of those objects. 'There is no meaningful evidence for such a mirror neuron system in humans,' Caramazza told us. 'That is, there is no evidence that there is a motor-based system that plays a causal role - as opposed to just an associative role - in the recognition of motor acts.'
Caramazza also denied the suggestion that his paper used a negative finding to disprove a phenomenon. 'We did not report negative results,' he said, 'we reported clear and robust interactions showing that one can get reliable fMRI repetition adaptation in predicted areas but not for the theoretically critical condition.'
In further remarks, Caramazza defended the use of fMRI to study cellular adaptation, and he argued there was every reason to think postulated mirror neurons would show adaptation, given that he and others have previously shown that the relevant brain areas exhibit repetition adaptation. 'It is not theoretically impossible that mirror neurons have radically different properties from other neurons,' he said, 'but such a claim is plainly ad hoc and would constitute a remarkable discovery if true.'
Caramazza also questioned the way the monkey results have been interpreted. 'The mere fact that some premotor neurons respond both in executing an action and seeing the action does not imply that those neurons play a causal role in action recognition,' he said. 'It is an elementary error to confuse correlation with cause. So-called mirror neurons may be responding as a consequence of, and not as the basis for, action categorisation. The monkey data is better explained as the result of learning to associate a motor pattern to a perceptual event .'
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