Predictive Eye Movements Are Driven by Goals, Not by the Mirror Neuron System

The importance of a mirror neuron system (MNS) as a mechanism for understanding the actions of others has been established (e.g., Rizzolatti, Fadiga, Gallese, & Fogassi, 1996). Neurons in the primate premotor cortex fire both when a monkey performs an action (e.g., grasping) and when the monkey observes someone else performing the same action. It has also been proposed that the MNS is the starting point for understanding the intentions and goal-directed behavior of others (Fogassi et al., 2005; Gallese & Goldman, 1998). Consistent with this view, Falck-Ytter, Gredeback, and von Hofsten (2006) argued that the MNS is implicated in proactive goal-directed (predictive) eye movements. In a series of eye-tracking studies, participants observed a toy object moving along a trajectory toward a container. Adults and 1-year-old infants looked ahead of the toy and toward the goal container only when a hand was observed moving the toy (the human-agent condition). Proactive goaldirected eye movements were not found in two further conditions: a self-propelled condition, in which a toy with rudimentary facial features moved along the trajectory by itself, or a mechanical motion condition, in which a ball with no distinctive features moved along the trajectory. Falck-Ytter et al. interpreted these data as evidence that the MNS is necessary for proactive goal-directed eye movements. Moreover, the absence of proactive goal-directed eye movements in 6-month-old infants (who are too young to perform the actions themselves) is taken as further support for the MNS account. These claims are premature. The conditions run by FalckYtter et al. (2006) do not discriminate between predicted human motion tied to intention and predicted agent goals tied to intention. Consistent with the mirror neuron hypothesis, movement of the hand may necessarily involve the simulation of motion via the MNS, and proactive goal-directed eye movements may therefore only occur when a hand is shown to move the object. Alternatively, a hand moving an object involves the intention of an agent to place the object in the goal container, and the expectation that the agent intends for the object to end up in a goal location may cause the proactive eye movements (consistent with teleological stance theory; Gergely & Csibra, 2003). To arbitrate between these accounts, it is necessary to run a human-agent condition without human movement. Therefore, we set out to test the claims of Falck-Ytter, et al. (2006), but with the addition of a new critical condition missing in the original study. We ran three movement conditions: the human-agent condition, in which a human agent was shown moving a toy frog toward a goal container (i.e., [1human agent, 1human motion]); the self-propelled condition, in which no human agent was shown moving the frog (i.e., [ human agent, human motion]); and the new condition, in which a human agent was shown with hand behind the starting point of the frog, flicking it so as to propel it along a trajectory (as in the game ‘‘Tiddlywinks’’; i.e., [1human agent, human motion]; see Fig. 1a). In the latter condition, the human-agent intention is matched to that of the human-agent condition, but human motion is not shown along the trajectory. This allows a clean test of the MNS versus goal-intention explanations for the proactive eye-movement data. We also ran each condition in two ways. In the original humanagent condition run by Falck-Ytter et al. (2006), when the toy object reached the bucket, a sound was played and a smiley face on the bucket was animated. This could serve to heighten the desirability of the goal state, encouraging proactive eye movements in a manner consistent with teleological stance theory. Therefore, we ran each of the three conditions with and without end effects. For the conditions with end effects, when the toy The order of authorship is arbitrary; all authors contributed equally to the work reported. Address correspondence to Kenny R. Coventry, Cognition and Communication Research Centre, Northumbria University, Northumberland Building, Northumberland Rd., Newcastle Upon Tyne NE1 8ST, United Kingdom, e-mail: kenny.coventry@ northumbria.ac.uk. PSYCHOLOGICAL SCIENCE