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WIREs Cogn Sci
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Mirror systems

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Mirror neurons are a class of visuomotor neurons, discovered in the monkey premotor cortex and in an anatomically connected area of the inferior parietal lobule, that activate both during action execution and action observation. They constitute a circuit dedicated to match actions made by others with the internal motor representations of the observer. It has been proposed that this matching system enables individuals to understand others' behavior and motor intentions. Here we will describe the main features of mirror neurons in monkeys. Then we will present evidence of the presence of a mirror system in humans and of its involvement in several social–cognitive functions, such as imitation, intention, and emotion understanding. This system may have several implications at a cognitive level and could be linked to specific social deficits in humans such as autism. Recent investigations addressed the issue of the plasticity of the mirror neuron system in both monkeys and humans, suggesting also their possible use in rehabilitation. WIREs Cogn Sci 2011 2 22–38 DOI: 10.1002/wcs.89

Figure 1.

(a) Lateral view of the macaque cerebral cortex. The shaded area corresponds to ventral premotor area F5. (b–d) Rastergrams (neuron activity recorded in a series of trials) and histograms (summed activity across the trials) showing the responses of an F5 motor neuron when the monkey grasps food with the mouth (b), the right or the left hand (c and d) (as, arcuate sulcus; cs, central sulcus). (Reprinted with permission from Ref 2 Copyright 2004 Massachusetts Institute of Technology).

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Figure 2.

(a) Schematic illustration of the experimental paradigm and tools used. Note that in order to grasp the object with normal pliers, the monkey has to close its hand, whereas with the reverse pliers, the monkey has to open its hand. The arrows indicate the direction of the motion of the pliers tips. (b) Activity of two neurons recorded in area F5. Rasters (10 trials) and histograms illustrate the neurons discharge recorded during grasping with normal pliers (Upper panels) and reverse pliers (Lower panels). Both rasters and histograms are aligned with the end of the grasping closure phase (asterisks). The traces below each histogram indicate the instantaneous hand position. Trace down indicates that the hand closes, and the distance between handles decreases, whereas trace up indicates that the hand opens, and the distance between handles increases. With normal pliers, Unit 210 began to fire during hand closure (trace down), reaching the maximum at approximately the moment in which the food was grasped; with the reverse pliers, the same unit started to fire with the hand opening (trace up), also reaching its maximum when the food was grasped. Unit 199 started to fire in normal pliers condition during hand opening (trace up), reaching its maximum at the beginning of the hand closure. With reverse pliers, the neuron started to fire during the hand closure (trace down), reaching its maximum during hand opening (Reprinted with permission from Ref. 3 Copyright 2008 National Academy of Sciences, USA).

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Figure 3.

(a) Lateral view of the monkey brain. Area F5 and PFG correspond to the shaded area. Nomenclature as in Figure 1 (ips, intraparietal sulcus). (b) Example of a mirror neuron responding during observation and execution of a grasping motor act. On the top is depicted the experimental condition, on the bottom the neuron discharge. The experimenter grasps the food in front of the observing monkey (left). The monkey grasps the food (right). Six trials are shown for each condition. Arrows indicate the grasping onset. (c) Rasters and histograms (10 trials) illustrating a mirror neuron discharging most effectively when the monkey observes a specific goal‐directed motor act (digging out—left) compared to a condition (right) in which the movement is identical but the target is not present (mimiced motor act). Ordinates, spikes/bin, bin width:20 ms (modified with permission from Ref. 5 Copyright 1992 Springer and Ref. 6 Copyright 1996 Oxford University Press).

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Figure 4.

Example of a neuron responding to observation of a motor act in full vision and in hidden condition but not in mimed conditions. The lower part of each panel illustrates schematically the experimenter's motor act as observed from the monkey vantage point: the experimenter's hand started from a fixed position, moved toward an object, and grasped it (a and b) or mimed grasping (c and d). The black frame depicts the metallic frame interposed between the experimenter and the monkey in all conditions. In (b) and (d), the gray square inside the black frame represents the opaque sliding screen that prevented the monkey from seeing the motor act that the experimenter performed behind it. The behavioral paradigm consisted of two basic conditions: full vision condition (a) and hidden condition (b). Two further conditions were performed: miming in full vision (c) and hidden miming (d). In each panel above the illustration of the corresponding experimenter's hand movements, histograms representing the average activity (spikes/seconds) recorded during 10 consecutive trials are shown. Histograms are aligned with the moment at which the experimenter's hand was closest to a fixed marker placed, as a reference point, in the middle top part of the metallic frame. The illustrated neuron responded to the observation of grasping and holding in full vision (a) and in the hidden condition (b) in which the interaction between the experimenter's hand and the object occurred behind the opaque screen. The neuron response was virtually absent in the two conditions in which the observed action was mimed (c and d) (modified from with permission from Ref. 9 Copyright 2001 Elsevier).

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Figure 5.

(a) Apparatus and paradigm used for the motor task (top). Below is shown the activity of three IPL neurons during grasping in the two conditions (grasp‐to‐eat and grasp‐to‐place). Rasters and histograms are synchronized with the moment when the monkey touched the object to be grasped. (b) Apparatus and paradigm used for the visual task. Visual responses of IPL mirror neurons during the observation of grasp‐to‐eat and grasp‐to‐place done by an experimenter (modified from with permission from Ref. 18 Copyright 2005 The American Association for the Advancement of Science).

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Figure 6.

(a) Experimental setting and design. In each session the motor responses of neurons during active movements of the monkey were tested (I). The visual responses of these neurons were further tested with the experimenter executing goal‐directed motor acts in the peripersonal (II) and extrapersonal (III) space of the monkey. (b) Responses of three mirror neurons during observation of motor acts performed in the monkey's extrapersonal (top row) and peripersonal (middle row) space, respectively, and during execution of motor acts (bottom row). Each panel shows rasters (top) and a spike density function (bottom) of the cells responses. Responses are aligned with the time of contact of the experimenter's or monkey hand with the object. Cells 1 and 2 exhibited a visual preference for motor acts performed in the monkey's extrapersonal and peripersonal regions, respectively. Cell 3 instead responded to the visual presentation of motor acts independent of the spatial region in which they were performed (Reprinted with permission from Ref. 21 Copyright 2009 The American Association for the Advancement of Science).

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Figure 7.

Left lateral view of the human cerebral cortex with classical cytoarchitectonic subdivision showing the areas that belong to the mirror system (colored areas) in ventral premotor cortex (lower part of the precentral gyrus, area 6, in yellow), in part of the human inferior frontal gyrus (posterior part of area 44 in yellow, and anterior part of 44 and area 45 in blue), and in the inferior parietal lobule (area 40 in red) (Reprinted with permission from Ref. 25 Copyright 2006 Raffaello Cortina Editore).

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Figure 8.

(a) Videos presented to the subjects during the functional magnetic resonance imaging (fMRI) scan. The images are organized in three columns and two rows. Each column corresponds to one of the experimental conditions. From left to right: context, action, and intention. In the context condition there were two types of clips, a ‘before tea’ context (upper row) and an ‘after tea’ context (lower row). In the action condition two types of grips were displayed an equal number of times, a whole‐hand prehension (upper row), and a precision grip (lower row) on an objectless background. In the intention condition there were two types of contexts surrounding a grasping action. The ‘before tea’ context suggested the intention of drinking (upper row), and the ‘after tea’ context suggested the intention of cleaning (lower row). (b) fMRI signal increases for intention minus action and for intention minus context conditions. The black arrow indicates the only area showing signal increase in both comparisons. The area is located in the dorsal sector of pars opercularis (part of area 44), where mirror activity has been repeatedly observed (from Iacoboni et al.47).

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Figure 9.

Example of a tool‐responding MN activated by the observation of a motor act made with a stick. In each panel, the rasters and the histogram represent the neuron response during a single experimental condition (10 trials). Rasters and histograms are aligned with the moment in which the stick or the experimenter's hand touched the food or the tray. (a) The experimenter approaches with the stick, held in his hand, a piece of food placed on a tray, and then punctures and holds it. (b) The experimenter grasps with his hand a piece of food placed on a tray and then holds it. (c) The experimenter makes the same approaching and sticking movement as in (a), but no food is present (modified with permission from Ref. 59 Copyright 2006 Massachusetts Institute of Technology).

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