The mirror mechanism: recent findings and perspectives

Abstract

Mirror neurons are a specific type of visuomotor neuron that discharge both when a monkey executes a motor act and when it observes a similar motor act performed by another individual. In this article, we review first the basic properties of these neurons. We then describe visual features recently investigated which indicate that, besides encoding the goal of motor acts, mirror neurons are modulated by location in space of the observed motor acts, by the perspective from which the others' motor acts are seen, and by the value associated with the object on which others' motor acts are performed. In the last part of this article, we discuss the role of the mirror mechanism in planning actions and in understanding the intention underlying the others' motor acts. We also review some human studies suggesting that motor intention in humans may rely, as in the monkey, on the mirror mechanism.

Keywords: motor act; motor intention understanding; space-selective mirror neurons; subjective value; view-selective mirror neurons.

Figure 1.

Lateral view of the monkey brain showing the subdivisions of the agranular frontal and posterior parietal cortices. The intraparietal and arcuate sulci have been opened to show the areas buried inside them. Agranular frontal areas have been labelled according to Matelli et al. [22,23]. Note that area F5 is formed by three further subdivisions: F5c, F5p and F5a [24]. Posterior parietal areas are defined according to Pandya and Seltzer and Gregoriou et al. [25,26]. The areas buried inside the intraparietal sulcus are defined according to functional criteria (for references, see [27]). AI, inferior arcuate sulcus; AIP, anterior intraparietal area; AS, superior arcuate sulcus; C, central sulcus; DLPF, dorsolateral prefrontal cortex; FEF, frontal eye field; IO, inferior occipital sulcus; L, lateral fissure; LIP, lateral intraparietal area; Lu, lunate sulcus; MIP, medial intraparietal area; P, principal sulcus; ST, superior temporal sulcus; VIP, ventral intraparietal area; VLPF, ventrolateral prefrontal cortex.

Figure 2.

(a) Overview of MR brain activations (recorded with 3T fMRI) during the observation of grasping acts. Leftmost image: lateral view of reconstructed left hemisphere indicating the six different antero-posterior levels at which coronal slices shown on the right have been taken. Right: statistical parametric maps activation for the contrast: hand action versus static control. The data are from a single monkey, overlaid onto its coronal anatomical sections. Numbers on each slice indicate y-coordinate (antero-posterior from interaural plane). as, arcuate sulcus; ips, intraparietal sulcus; sts, superior temporal sulcus. (b) Temporo-parieto-premotor grasping observation pathways in the monkey brain. Flattened representation of STS, IPS/IPL and IAS with ROIs indicated. Arrows and areas coloured in red and blue indicate, respectively, the STPm–PFG–F5c pathway and the LB2–AIP–F5a/p pathway. For the definition of areas PF, PFG, PG and Opt, see figure 1. Areas 45a and 45b are defined according to Gerbella et al. [32]. AIP, anterior intraparietal area; FEF, frontal eye fields; F5c, F5 convexity; F5p, F5 (bank) posterior; F5a, F5 (bank) anterior; FST, fundus of the STS; IAS, inferior arcuate sulcus; LIPa, anterior part of the lateral intraparietal area; MT/V5, middle temporal area; MSTd, middle superior temporal area, dorsal part; MSTv, middle superior temporal area, ventral part.

Figure 3.

Space-selective mirror neurons. (a) Schematic view of the visual conditions of the experimental paradigm: the monkey observes an experimenter executing a grasping act in the peripersonal (left) and extrapersonal (right) space of the monkey. (b) Examples of the responses of three mirror neurons during observation of grasping acts executed in the monkey's peri- and extrapersonal space and during monkey execution. Each panel shows the raster plot (top) and the cumulative histogram (bottom) of the neuron responses. Raster plots and histograms are aligned with the time of contact of the experimenter's or monkey hand with the object. Neuron 1 responds more strongly when the observed grasping is performed in the extrapersonal space, while neuron 2 presents a stronger discharge during observation of grasping performed in the peripersonal space. Neuron 3 does not show any space-selective visual response. All neurons discharge during grasping execution. (c) Operational encoding of the monkey peri- and extrapersonal space. The top part shows the experimental conditions: the experimenter grasps an object in the extrapersonal (left) or in the peripersonal space without (centre) or with (right) a frontal panel impairing the monkey's reach into its peripersonal space. Note that in this latter condition, the object (and the act performed by the experimenter) is metrically in the monkey's peripersonal space, but operationally outside it. The lower panels show the visual responses of a mirror neuron in the three conditions. The vertical lines mark the time of contact between the experimenter's hand and the object. Before closure with the frontal panel, the neuron was activated only during observation of grasping in the monkey's extrapersonal space. However, after closure, the neuron also responded to observation of grasping performed close to the monkey's body.

Figure 4.

Responses of mirror neurons to grasping observed from three visual perspectives. (a) Experimental conditions (subjective view: 0°; side view: 90°; frontal view: 180°). (b) Examples of the responses of four mirror neurons during observation of grasping from the three perspectives. Rasters and histograms are temporally aligned (vertical grey line) with the moment at which the observed hand touches the object. Neuron 1 is selective for the subjective view, neuron 2 for the frontal view, neuron 3 for the side view. The activity of neuron 4 discharged equally well for all points of view.

Figure 5.

(a) Motor task. The monkey, starting with its hand from a fixed position (left), reaches and grasps a piece of food (or an object), then it brings the food to the mouth and eats it (grasp-to-eat condition I) or places it (or the object) into a container (grasp-to-place condition) located near the mouth (II) or near the target (III). Visual task. The experimenter, starting with his hand from a fixed position (left), reaches and grasps a piece of food or an object (right), then he brings the food to the mouth and eats it (grasp-to-eat condition I) or places it (or the object) into a container located near the target (grasp-to-place condition II). (b) Examples of the discharge of three IPL neurons during the motor task. Neuron 67 is selective for grasping to eat, neuron 161 shows the opposite behaviour, while the response of neuron 158 is not affected by the action goal. (c) Examples of the discharge of three IPL neurons during the visual task. Neuron 87 discharges stronger during observation of grasping to eat, neuron 39, on the contrary, during observation of grasping to place, while neuron 80 discharges equally well in both conditions. In both (b) and (c), rasters and histograms are aligned (vertical bar) with the moment when the monkey or the experimenter, respectively, touched the food/object. (d) Congruence between the visual and the motor response of mirror neurons encoding action goal in area PFG. (i) Example of a neuron discharging stronger during grasping for eating than during grasping for placing, both when the action is executed and when it is observed. Conventions as in (b) and (c). (ii) Population-averaged responses, showing the same pattern of differential activity between the preferred (white bar) and not preferred (black bar) action during motor and visual tasks. Depending on the neuron, the preferred action could be grasp-to-eat or grasp-to-place.