Mirror Neurons

The  term  mirror  neurons  refers  to  neurons  that show  spike  activity  in  response  to  the  performance  of  an  action  and  to  the  perception  of  the same type of action in an observed subject. These neurons, first found in the brain of macaque monkeys,  are  regarded  as  first  evidence  for  a  functional  overlap  between  action  and  perception  on a neuronal level.

Mirror  neurons  were  first  described  in  the1990s  by  Giacomo  Rizzolatti’s  research  group  at the  University  of  Parma  in  Italy.  Anecdotal  evidence  given  by  members  of  the  group  indicates that  the  discovery  of  mirror  neurons  was  due  to chance.  Neurophysiologists  were  investigating neurons  expected  to  respond  to  hand  and  mouth actions,  such  as  manipulating  objects  or  picking up  food  items  for  eating.  Neuronal  activity  was measured  from  electrodes  placed  in  the  ventral premotor cortex of macaque monkeys. When the experimenter  took  a  bite  from  his  sandwich  during  the  experiment,  neurons  from  the  macaque responded unexpectedly, indicating activity owing to the mere observation of the investigated action, this time performed by the human.

The  first  mirror  neurons  were  identified  in the  F5  area  of  the  inferior  frontal  gyrus  of  the macaque brain; later studies also identified mirror neurons in the inferior parietal lobule. Following their  findings,  a  multitude  of  research  groups engaged in the study of mirror neurons. Neurons with  mirror  properties  were  found  in  the  premotor  cortex,  classically  ascribed  to  the  motor system, and in parietal areas involved in the processing  of  perceived  information.  Following  the observation of neurons responding to the perception and performance of manual actions, neurons responding to actions involving the mouth, face, and body were investigated. Today, various types of  mirror  neurons  have  been  described,  including  neurons  that  respond  selectively  to  actions observed  in  extra or  intrapersonal  space,  neurons  that  respond  not  to  visual  but  to  acoustic stimuli,  and  neurons  that  link  actions  to  related acts  of  communication.  Mirror  neurons  vary  in their level of congruency; highly specialized neurons respond only to one type of action, whereas others generalize over actions that resemble each other, have the same goal, or follow each other in typical sequences.

Results  from  studies  on  mirror  neurons  provided  strong  support  for  theoretical  perspectives that  had  so  far  lacked  neuronal  evidence.  The overlap between action and perception processing in  the  brain  had  been  addressed  over  decades  by Wolfgang  Prinz  and  colleagues  through  the  common  coding  principle  and  the  ideomotor  theory; the  concepts  of  embodiment  and  situatedness  of cognitive processes are related to this work. These perspectives, together with the discovery of mirror neurons  in  monkeys,  provided  a  strong  motivation for the quest for neural structures with similar properties in the human brain. Studies investigating  human  brain  areas  with  mirror like  properties  chiefly  used  neuroimaging  techniques  (i.e., functional  magnetic  resonance  imaging  [fMRI], positron-emission  tomography  [PET]),  electroencephalography (EEG), transcranial magnetic stimulation  (TMS),  as  well  as  behavioral  paradigms. Areas  with  mirror  properties  were  found  in  the ventral and dorsal premotor cortex and in parts of the  parietal  cortex,  including  the  inferior  parietal lobe,  the  superior  parietal  lobe,  and  the  superior parietal  sulcus.  Numerous  studies  have  corroborated that these brain regions are involved in and modulated  by  a  large  number  of  cognitive  tasks, as well as social functions. Their activity has been claimed to underlie the planning and understanding of actions, imitation, language comprehension, empathy,  social  learning,  and  cultural  transmission.  Malfunctioning  in  these  brain  regions  has been  claimed  to  play  a  role  in  the  emergence  of disorders  like  autism.  Because  of  their  broad functional  involvement,  it  has  been  argued  that the term mirror system was misleading, and other terms have been suggested instead, such as action observation network (AON).

Neurocognitive research, often applying sports and dance-related paradigms, has shown that the relevant brain regions are more strongly activated during the observation of biological and especially human  agents  compared  to  artificial  ones  (e.g., robots) and that the degree of activation is modulated  by  the  observer’s  capability  and  experience of performing the observed actions. Furthermore, there  is  evidence  that  individual  regions  within the network are specialized on different aspects of action perception and performance. Beatriz Calvo Merino  and  colleagues  studied  brain  responses to  movement  observation  in  capoeira  and  ballet experts.  They  found  that  the  activation  of  relevant  brain  areas  was  more  distinctive  while  the experts  watched  movements  from  their  own  discipline,  and  specifically  movements  belonging  to their  own  active  movement  repertoire,  compared to  movements  they  had  frequently  watched  but not  performed  themselves.  Salvatore  Aglioti  and colleagues  showed  that  expert  basketball  players performed  better  than  experienced  observers  in predicting the success of a free throw during very early stages of the movement, and that their motor system and hand muscles were activated in a movement-specific  way.  Using  a  dance-training  paradigm, Emily Cross and colleagues showed that the superior temporal cortex preferentially responded to  the  presence  of  a  human  model,  whereas  the ventral  premotor  cortex  responded  specifically  to motor familiarity of the observed movement.

In   recent   years,   critical   views   have   been expressed  regarding  the  interpretation  of  mirror neuron studies, specifically the existence and function of mirror neurons in the human brain. It has been  argued  that  neurons  with  mirror  properties might  not  represent  a  distinct  class  of  cells  but rather neurons in the motor system that have developed mirror properties as an artifact, additional to their  original  and  more  relevant  functions.  Based on  empirical  findings,  Cecilia  Heyes  proposed that human mirror neurons might emerge as a byproduct of associative learning and social interaction. Gregory Hickok strongly argued against the claim that mirror neurons were crucial for action understanding,  emphasizing  the  lack  of  evidence of this function in monkeys in which the neurons had been described. Arthur Glenberg and Patricia Churchland both pointed out that most of the phenomena the human mirror neuron system (MNS)has been associated with (including understanding intentions  and  actions,  language  comprehension, imitation,  and  disorders  like  autism)  are  not  yet fully understood and that more research is needed to substantiate such claims.

References:

  1. Calvo-Merino, B., Grèzes, J., Glaser, D. E., Passingham, R. E., & Haggard, P. (2006). Seeing or doing? Influence of visual and motor familiarity in action observation. Current Biology, 16(19), 1905–1910.
  2. Cross, E. S., Kraemer, D. J., Hamilton, A. F., Kelly, W. M.,& Grafton, S. T. (2009). Sensitivity of the action observation network to physical and observational learning. Cerebral Cortex, 19(3), 315–326.
  3. Gallese, V., Fadiga, L., Fogassi, L., & Rizzolatti, G. (1996). Action recognition in the premotor cortex. Brain, 119, 593–609.
  4. Gallese, V., Gernsbacher, M. A., Heyes, C., Hickok, G.,& Iacoboni, M. (2011). Mirror Neuron Forum.Perspectives on Psychological Science, 6(4), 369–407. Glenberg, A. M. (2011). Introduction to the MirrorNeuron Forum. Perspectives on Psychological Science,6(4), 363–368.
  5. Heyes, C. M. (2010). Where do mirror neurons come from? Neuroscience and Biobehavioural Reviews, 34,575–583.
  6. Rizzolatti, G., & Arbib, M. (1998) Language within our grasp. Trends in Neuroscience, 21, 188–194.
  7. Rizzolatti, G., & Craighero, L. (2004). The mirror-neuron system. Annual Review of Neuroscience, 27, 169–192.
  8. Rizzolatti, G., Sinigaglia, C., & Anderson, F. (2008).Mirrors in the brain: How our minds share actions and emotions: How our minds share actions, emotions, and experience. Oxford, UK: Oxford University Press.

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