2007 Understanding animate agents: distinct roles for the social network and mirror system

ABSTRACT—How people understand the actions of animate agents has been vigorously debated. This debate has cen-tered on two hypotheses focused on anatomically distinct neural substrates: The mirror-system hypothesis proposes that the understanding of others is achieved via action simulation, and the social-network hypothesis proposes that such understanding is achieved via the integration of critical biological properties (e.g., faces, affect). In this study, we assessed the areas of the brain that were engaged when people interpreted and imagined moving shapes as animate or inanimate. Although observing and imagining the moving shapes engaged the mirror system, only acti-vation of the social network was modulated by animacy. The ability to detect and understand the actions of animate others is critical for survival. Although such detection is often aided by sensory cues (e.g., self-propelled motion, faces), the ability to impute animacy accurately in circumstances when such cues are absent or misleading (e.g., sleeping animals: Barrett & Behne, 2005; robots: Poulin-Dubois, Lepage, & Fer-land, 1996) points to a conceptual representation of animacy that goes beyond sensory input. Recently, two hypotheses have emerged to explain the neural substrate underlying the under-standing of animacy. One, the mirror-system hypothesis, posits that people understand the actions of an animate other by sim-ulating the other’s actions with their own motor programs (Gallese, Keysers, & Rizzolatti, 2004). Mirror neurons, the proposed neural substrate for such a mechanism (see Fig. 1), activate when a monkey initiates an action and when a monkey observes a similar action performed by others (Rizzolatti, Fo-gassi, & Gallese, 2001). A similar pattern of hemodynamic ac-tivity in human inferior parietal lobe and inferior frontal gyrus/ ventral premotor cortices has led researchers to regard these two areas as the homologous mirror system in humans (Rizzolatti & Craighero, 2004). Most recently, the human mirror system was found to be associated with imitation and with understanding others ’ intentions and emotions (Carr, Iacoboni, Dubeau, Maz