In three new independent studies, researchers have deepened our understanding of the remarkable ability of some specialized areas of the brain to activate both in response to one's own actions and in response to sensory cues (such as sight) of the same actions perpetrated by another individual. This ability is thought to be based in the activity of so-called mirror neurons, which have been hypothesized to contribute to skills such as empathy, socialized behavior, and language acquisition. The new findings contribute to our understanding of how conceptually related instances of language and action, and sound and action, are linked in the brain, and how the brain distinguishes actions perpetrated by "self" and by "other." The studies are reported by three independent research groups: Lisa Aziz-Zadeh (now at USC) and colleagues at the University of Parma, Italy, UCLA; Christian Keysers and colleagues at the University of Groningen, The Netherlands and UC Berkeley; and Simone Schütz-Bosbach and colleagues at University College London, the Max Planck Institute for Human Cognitive and Brain Sciences in Germany, and the University of Rome. The papers appear in the September 19th issue of Current Biology, published by Cell Press.
Mirror neurons were first identified in the cortex of macaque monkeys: A particular subset of these neurons fire when, for example, a monkey picks up a banana, and when the monkey observes a human picking up a banana in a similar way. Mirror-neuron activity appears to be highly specific, such that a somewhat different set of mirror neurons would fire if a banana were poked, for example, rather than picked up. There is also evidence that mirror neurons link actions not only with visual stimuli, but also with other types of sensory cues. Technical limitations have impeded identification of individual mirror neurons in humans, but brain-imaging studies support the existence of these neurons.
In the new work from Lisa Aziz-Zadeh and colleagues, researchers used a brain-imaging technique to investigate how literal phrases describing actions performed by the mouth, hand, or foot influenced cortical neurons that are activated by the sight of actions being performed by mouth, hand, or foot.
The researchers found a significant concordance between activation of certain cortical areas in response to linguistic descriptions and observed actions relating to the different body parts carrying out the actions. For example, when individuals read literal phrases such as "biting the peach" or "biting the banana," some brain areas activated that were also stimulated by videos of fruit being bitten. Similar findings were obtained for hand actions (for example, grasping a pen) and foot actions (for example, pressing a piano pedal). Together, the findings suggest that mirror neurons play a key role in the mental "re-enactment" of actions when linguistic descriptions of those actions are conceptually processed.
In the study reported by Christian Keysers, Valeria Gazzola, and colleagues, researchers investigated a different question: how mirror neurons might contribute to our understanding of auditory cues. Past work had shown that in monkeys, so-called auditory mirror neurons activate when monkeys perform certain actions and when they hear the same actions being performed. In the new work, the researchers report new evidence for an auditory mirror system existing in humans as well.
When subjects were presented with sounds corresponding to mouth actions (such as crunching candy, kissing, or emptying a soda can with a straw) and hand actions (such as ripping a sheet of paper or opening a zipper), brain areas are activated that overlap with areas activated by the execution of those actions by the subjects themselves. Within this area, a subregion was preferentially activated when mouth actions were either heard or performed, and another subregion was preferentially activated when hand actions were heard or performed. In addition, the researchers found that most of this mirror system was also responsive to the sight of these actions being performed, suggesting that a particular area of the brain can respond similarly to execution of an action and its representation in different types of sensory cues.
Intriguingly, the researchers also found that of the subjects taking part in the experiment, those that scored higher on tests for empathy activated the system more strongly than those who scored lower on the empathy evaluation. While the relationship between motor mirror systems and empathy skills is far from clear, these findings are consistent with the existence of a link between the two.
Though mirror neurons appear to relate--and, potentially, equate--the actions of oneself with those of another, we are in fact highly adept at distinguishing our own actions from those of someone else. The basis for this distinction is explored in the study reported by Simone Schütz-Bosbach, Patrick Haggard, and colleagues, who used an established method--the so-called rubber-hand illusion--for experimentally manipulating the sense of body ownership. This approach was useful because without such illusion, it is difficult to identify meaningful differences in how the brain responds to actions performed by oneself or others--the two scenarios involve significant differences in, for example, visual viewpoint and familiarity, and other sensory inputs.
Past work had used the rubber-hand illusion to show that when a rubber hand is seen being stroked at the same time that the viewer's own (unseen) hand is synchronously stroked, the viewer feels that the rubber hand becomes part of his or her body.
In the new work, the researchers used this illusion--though in this case, the rubber hand was replaced by a real hand of an experimenter--to control whether a subject experienced that an experimenter's hand was the subject's own or not. This allowed the researchers to investigate whether finger movements made by the experimenter's hand were able to facilitate the subject's own finger movements--this facilitation was measured by the ability of benign stimulation of a particular brain region to promote motor signals (corresponding to those finger movements) in the subject's own hand.
The researchers found that such facilitation did occur, but, curiously, it occurred when the illusion was not effective, and subjects felt that the experimenter's hand was not their own. Observing actions interpreted as one's own tended to suppress motor facilitation. Taken together, the findings indicate that the observation of others facilitates the motor system. The authors point out that the findings also suggest that the neural mechanisms that underlie action observation are intrinsically "social"--that the neural mechanisms map the actions of others onto one's own body, rather than initially treating all observed action (whether perpetrated by one's self or by others) as essentially neutral in ownership. These findings inform our understanding of the motor system's role in social cognition, and support previous suggestions that the motor system may have strongly influenced developments in human social evolution.
For excellent commentary on mirror-neuron function and some of the research described here, be sure to see the Dispatch in this issue from Arthur Glenberg, "Naturalizing Cognition: The Integration of Cognitive Science and Biology."
Keysers et al.
The researchers include Valeria Gazzola and Christian Keysers of University Medical Center Groningen and University of Groningen in Groningen, The Netherlands; Lisa Aziz-Zadeh of University of California, Berkeley and International Computer Science Institute in Berkeley, California and University of Southern California in Los Angeles, California. The research was supported by a Nederlandse Organisatie voor Wetenschappelijk Oderzoek vidi and a Marie Curie Excellence grant to C.K.
Aziz-Zadeh et al.
This research was conducted at Università di Parma in Parma, Italy, University of California, Los Angeles, University of California, Berkeley, and the International Computer Science Institute, Berkeley, California. The researchers include Lisa Aziz-Zadeh, now at University of Southern California, Stephen M. Wilson of University of California, Los Angeles, Giacomo Rizzolatti of Università di Parma in Parma, Italy, and Marco Iacoboni or the University of California, Los Angeles.
For generous support, the researchers thank the Brain Mapping Medical Research Organization, Brain Mapping Support Foundation, Pierson-Lovelace Foundation, The Ahmanson Foundation, William M. and Linda R. Dietel Philanthropic Fund at the Northern Piedmont Community Foundation, Tamkin Foundation, Jennifer Jones-Simon Foundation, Capital Group Companies Charitable Foundation, Robson Family, and Northstar Fund. The project described was supported by MIUR, grant Cofin to G.R., and FIRB, RBNE018ET9, grants from the National Science Foundation (REC0107077), National Institute of Mental Health (MH63680), and grant numbers RR12169, RR13642, and RR00865 from the National Center for Research Resources (NCRR), a component of the National Institutes of Health (NIH); its contents are solely the responsibility of the authors and do not necessarily represent the official views of NCR or NIH.
Schütz-Bosbach et al.
The researchers include Simone Schütz-Bosbach of University College London and Max Planck Institute for Human Cognitive and Brain Sciences in London, UK; Benedetta Mancini and Salvatore M. Aglioti of University of Rome "La Sapienza" and Istituto di Ricovero e Cura a Carattere Scientifico in Rome, Italy; Patrick Haggard of University College London in London, UK. This work was supported by the Economic and Social Research Council and the British Biotechnology and Biological Sciences Research Council grants to P.H. and by a Royal Society European Science Exchange Programme grant to P.H. The fellowship from the Max Planck Society supported S.S.-B. A bursary from University of Rome "La Sapienza" supported B.M.
Gazzola et al.: "Empathy and the Somatotopic Auditory Mirror System in Humans." Publishing in Current Biology 16, 1824–1829, September 19, 2006. DOI 10.1016/j.cub.2006.07.072. www.current-biology.com
Aziz-Zadeh et al.: "Congruent Embodied Representations for Visually Presented Actions and Linguistic Phrases Describing Actions." Publishing in Current Biology 16, 1818–1823, September 19, 2006. DOI 10.1016/j.cub.2006.07.060. www.current-biology.com
Schütz-Bosbach et al.: "Self and Other in the Human Motor System." Publishing in Current Biology 16, 1830–1834, September 19, 2006. DOI 10.1016/j.cub.2006.07.048. www.current-biology.com