Reviews

Example.pdf

Journal Club

Editor’s Note: These short reviews of a recent paper in the Journal, written exclusively by graduate students or postdoctoral fellows, are intended to mimic the journal clubs that exist in your own departments or institutions. For more information on the format and purpose of the Journal Club, please see http://www.jneurosci.org/misc/ifa_features.shtml.

Mirror Neurons and the Lateralization of Human Language

Daniel R. Lametti and Andrew A. G. Mattar Department of Psychology, McGill University, Montréal, Québec, Canada H3A 1B1

Review of Aziz-Zadeh et al. (http://www.jneurosci.org/cgi/content/full/26/11/2964)

In the premotor cortex of the macaque there are neurons whose firing rates in- crease when the monkey makes goal di- rected movements and when the monkey observes similar movements being made by others. These cells are named “mirror neurons.” It has been suggested that by mapping observation of movements onto cells involved in the production of similar movements, mirror neurons help individ- uals understand and imitate motor goals (Gallese et al., 1996). Several studies have used functional imaging techniques to de- termine whether mirror neurons exist in humans. Neural activation in response to both movement production and observa- tion has been described in Brodmann area 44 (BA44), located in the pars opercularis of the inferior frontal gyrus (Iacoboni et al., 2005). In the left hemisphere, BA44 contains Broca’s area, a brain region that has been the focus of intense study be- cause of its role in language processing and speech production. The close ana- tomical proximity of mirror neurons to Broca’s area has led some to speculate that mirror neurons may have played a role in the lateralization of human language. If true, the human mirror neuron system might also be left lateralized.

In their Journal of Neuroscience article, Aziz-Zadeh et al. (2006) used functional magnetic resonance imaging to determine the extent to which the human mirror neuron system is lateralized. Blood oxy-

gen level-dependent (BOLD) neural ac- tivity was recorded in each of four exper- imental conditions. In each condition, subjects saw movies of left and right hands (each hand was presented independently) with the index finger extended between two targets. In the static observation con- dition, subjects observed a stationary hand. In the action observation condi- tion, subjects watched as the index finger moved toward one of the targets. In the execution condition, the observed hand was stationary, but the subjects moved their own index finger when instructed by an abstract cue. In the imitation condi- tion, the observed index finger moved to- ward one of the targets and subjects did the same [Aziz-Zadeh et al., 2006, their Fig. 1 (http://www.jneurosci.org/cgi/content/ full/26/11/2964/FIG1)]. Responses were made with the same hand as was observed. Images of left hands were presented to the left visual field so that sensory input was confined to the right hemisphere and vice versa for images of right hands. By con- trolling the hemisphere involved in visual processing and movement production, the authors were able to determine whether mirror neurons were lateralized.

The authors found that observation, execution, and imitation of finger move- ments resulted in reliable increases in BOLD activity within the pars opercularis. Neural activation increased linearly from observation to execution to imitation conditions, consistent with the properties of mirror neurons. Both left and right visual-field stimuli resulted in bilateral activation of mirror neurons [Aziz-Zadeh et al., 2006, their Fig. 2 (http://www. jneurosci.org/cgi/content/full/26/11/2964/

FIG2)]. Stronger activation was seen in mirror neurons ipsilateral to the stimuli: left visual-field stimuli led to stronger ac- tivations in the left hemisphere whereas right hemisphere mirror neurons re- sponded to right visual-field stimuli. These findings suggest that cells in the pars opercularis of both the left and right hemisphere demonstrate mirror-neuron like properties and, hence, that mirror neuron activity is not lateralized.

The carefully controlled studies of Aziz-Zadeh et al. (2006) confirm previous findings identifying mirror neurons in the pars opercularis, but also report a novel finding that mirror neuron activation is not lateralized to the left hemisphere. The authors conclude that because mirror neurons in the pars opercularis show bi- lateral activation they were not precursors to the development of lateralized language function in humans. However, functional imaging studies that shed light on the cur- rent organization of the human brain do not necessarily provide insights into the origins of that organization.

The idea that mirror neurons play a role in human language is an intriguing one that to date is based only on the pres- ence of mirror neurons in Broca’s area. The close proximity of these two systems may be indicative of functional linkage or it may be a coincidence. Broca’s area is classically known as a language area but it is also active during actions such as swal- lowing (Mosier et al., 1999). The func- tional role of mirror neurons in Broca’s area is, thus, unclear.

Through the action of mirror neurons, subjects can learn new movements simply by observing others (Stefan et al., 2005).

Received April 5, 2006; revised April 27, 2006; accepted April 27, 2006. Correspondence should be addressed to Andrew Mattar, McGill Univer-

sity, Department of Psychology, 1205 Dr. Penfield Avenue, Montreal, Quebec, Canada H3A 1B1. E-mail: andrew.mattar@mail.mcgill.ca.

6666 • The Journal of Neuroscience, June 21, 2006 • 26(25):6666 – 6667

This suggests that mirror neurons help hu- mans acquire novel patterns of movement control. In a study that used a robot to per- turb the path of the jaw, the nervous system corrected altered patterns of movement when the perturbations were applied during speech (Tremblay et al., 2004). Thus, speak- ing involves specific motor goals that the nervous system works to achieve. Together, these results suggest that precise control of movements during speech can be learned through observation. If true, this may indi-

cate that mirror neurons in Broca’s area aid in the acquisition of novel movement pat- terns required for speech.

References Aziz-Zadeh L, Koski L, Zaidel E, Mazziotta J, Ia-

coboni M (2006) Lateralization of the hu- man mirror neuron system. J Neurosci 26:2964 –2970.

Gallese V, Fadiga L, Fogassi L, Rizzolatti G (1996) Action recognition in the premotor cortex. Brain 119:593– 609.

Iacoboni M, Molnar-Szakacs I, Gallese V, Buccino

G, Mazziotta JC, Rizzolatti G (2005) Grasp- ing the intentions of others with one’s own mirror neuron system. PLoS Biol 3:1–7.

Mosier KM, Liu WC, Maldjian JA, Shah R, Modi B (1999) Lateralization of cortical function in swallowing: a functional MR imaging study. Am J Neuroradiol 20:1520 –1526.

Stefan K, Cohen LG, Duque J, Mazzocchio R, Celnik P, Sawaki L, Ungerleider L, Classen J (2005) Formation of a motor memory by ac- tion observation. J Neurosci 25:9339 –9346.

Tremblay S, Shiller DM, Ostry DJ (2004) The somatosensory basis of speech production. Nature 423:866 – 869.

Lametti and Mattar • Journal Club J. Neurosci., June 21, 2006 • 26(25):6666 – 6667 • 6667