Motor facilitation following action observation: A behavioural study in prehensile action Martin G. Edwards, a, * Glyn W. Humphreys, b and Umberto Castiello c a School of Sport and Exercise Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK b Behavioural Brain Sciences Centre, School of Psychology, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK c Department of Psychology, Royal Holloway, The University of London, Egham, Surrey TW20 0EX, UK Accepted 7 April 2003 Abstract Previous research has shown that the observation of actions and the execution of actions activate common neural systems. More recently, we have presented data showing that action observation of prehension primes subsequent execution (Castiello, Lusher, Mari, Edwards, & Humphreys, 2002). In the current paper we examined action priming under conditions in which the size of the prime did not predict the size of the target (only 20% of trials were valid). We demonstrated reliable priming under these conditions, consistent with the effect occurring automatically. In addition, we show priming even when observers saw just the object rather than the object and a reaching action on the prime trial. We discuss the findings in relation to the role of mirror neurons and object affordances. Ó 2003 Elsevier Inc. All rights reserved. Keywords: Mirror neurons; Prehension; Action goals; Object affordance 1. Introduction Recent research has shown evidence that the obser- vation of actions and the execution of actions activate common neural systems. The first data to show this came from the neural measurement of area F5 in the ventral premotor cortex of monkeys (Di Pellegrino, Fadiga, Fogassi, Gallese, & Rizzolatti, 1992; Gallese, Fadiga, Fogassi, & Rizzolatti, 1996; Rizzolatti, Fadiga, Gallese, & Fogassi, 1996a). These studies demonstrated the existence of neurons that were active when a monkey observed a directed action to an object and also when the animal executed an action itself to the same object. Rizzolatti coined the term mirror neurons to describe their sensitivity to combined self and observed action (see Rizzolatti & Arbib, 1998). Observation of the object without the action, or the action without the object evoked no such response (Gallese et al., 1996). Later work by Gallese and colleagues has shown neurons with similar properties in the inferior parietal lobe of mon- keys (Fogassi, Gallese, Fadiga, & Rizzolatti, 1998; Gallese, Fadiga, Fogassi, & Rizzolatti, 2002). Evidence that a mirror-neuron system may operate in humans comes from studies using transcranial magnetic stimulation (TMS) and from brain imaging studies. Fadiga, Fogassi, Pavesi, and Rizzolatti (1995) stimulated the motor cortex of participants with TMS to induce motor-evoked potentials (MEPs) in the muscles of the hand. They then presented participants with four obser- vation conditions: (i) the experimenter reaching and grasping an object, (ii) the experimenter performing aimless arm movements, (iii) just an object and (iv) the observation of a dimming light. The results showed that the observation of the experimenter performing an action (reach and grasp or aimless arm movement) increased the MEP activity of the hand. They suggested that this was due to action observation being associated with action execution processes (for replications of these findings see Avikainen, Forss, & Hari, 2002; Hari et al., 1998; Strafella & Paus, 2000). Evidence from functional imaging studies also demonstrates that the observation of actions activate the premotor cortex in humans (BrocaÕs area) (Buccino et al., 2001; Chaminade, Meltzoff, & Decety, 2002; Decety * Corresponding author. Fax: +44-121-414-4121. E-mail address: m.edwards.1@bham.ac.uk (M.G. Edwards). 0278-2626/$ - see front matter Ó 2003 Elsevier Inc. All rights reserved. doi:10.1016/S0278-2626(03)00210-0 Brain and Cognition 53 (2003) 495–502 www.elsevier.com/locate/b&c