Who Needs Emotions? The Brain Meets the Robot

how do we decipher others’ minds? 159
the self or not. One network would be involved with recognizing actions as
belonging to the self, and another would correspond to attributing actions
to another person. We know from the results described above that brain areas
activated during self-produced actions (executed or not) and when observing
actions of other people partly overlap: this is the basis for the concept of
shared representations, introduced by Daprati et al. (1997) and Georgieff and
Jeannerod (1998), according to which different mental states concerning
actions (e.g., intending an action and observing it from another person) share
the same neural representations yet still have distinct patterns of neural
activity.
To clarify this concept, let us briefly describe experimental results obtained
from monkeys. A dramatic example of a shared representation is offered
by the finding of mirror neurons (Rizzolatti, Fadiga, Gallese, & Fogassi,
1995). Mirror neurons were identified in the monkey premotor cortex. They
are activated in two conditions: first, they fire when the animal is involved
in a specific motor action, like picking a piece of food with a precision grip;
second, they also fire when the immobile animal watches the same action
performed by an external agent (another monkey or an experimenter). In
other words, mirror neurons represent one particular type of action, irrespective
of the agent who performs it. At this point, it could be suspected
that the signal produced by these neurons, and exploited by other elements
downstream in the information-processing flow, would be the same for
actions performed by the self and by another agent: the two modalities of
that action (executed and observed) would thus have the same neural representation.
The problem of actor identification, however, is solved by the
fact that other premotor neurons (the canonical neurons), and presumably
many other neuron populations, fire only when the monkey performs the
action and not when it observes it from another agent. This is indeed another
critical feature of the shared representations concept: representations
overlap only partially, and the nonoverlapping part of a given representation
can be the cue for attributing the action to the self or to the other. The
same mechanism operates in humans. Brain activity during different conditions
where subjects were simulating actions (e.g., intending actions and
preparing for execution, imagining actions, or observing actions performed
by other people) was compared (Decety et al., 1994, 1997; Grafton, Arbib,
Fadiga, & Rizzolatti, 1996; Rizzolatti et al., 1996; Gérardin et al., 2000). The
outcome of these studies is twofold: first, there exists a cortical network
common to all conditions, to which the inferior parietal lobule (areas 39 and
40), the ventral premotor area (ventral area 6), and part of supplementary
motor area contribute; second, motor representations for each individual
condition are clearly specified by the activation of cortical zones which do
not overlap between conditions (Ruby & Decéty, 2001).