We removed frames at the very start of the movement (where hand displacement is minimal and does not affect peak velocity and acceleration) to normalize movement duration at 600 ms for the final displays, ensuring equivalence of duration and peak grasp timing. For pantomimes, movement duration was 660 msec, peak grasp was 99 mm, and maximum X plane deviation was 29 mm. For goal-directed actions, the actor’s movement duration was 625 ms, peak grasp was 99 mm, and maximum deviation of trajectory from a hypothetical straight line in the X plane was 24 mm. The actor practiced the actions with a metronome to promote equivalence of duration. We exported motion capture recordings of these markers to video to construct the PD display of the same action (and see Campanella, Sandini, & Morrone, in press). We placed 12 reflective markers-one on each fingernail, two on the wrist-on the styloid process of the radius, head of the ulna, ulna and radius proximal to the elbow, and bicep. Only the actor’s torso and right hand and arm were visible against a black background. We recorded the actor simultaneously with a digital camcorder and three Qualisys ProReflex infrared motion capture cameras. All were right-handed with normal or corrected-to-normal vision. Thirty volunteers from the School of Psychology (University of Queensland 12 females age range from 18 to 34 years, M = 25.05, SD = 6.20) provided informed verbal consent. Thus, when asking whether motion or appearance of the actor has an impact on the actions of observers, we also need to specify whether they are goal directed or pantomimed. She could not pantomime to novel objects, however, because she did not have stored representations and could not construct a visual one (Goodale et al., 1994). Conversely, the pantomimed reaches of the visual agnosic patient DF to familiar objects were intact because she had a premorbid stored representation of these. Thus, ideomotor apraxics have difficulty pantomiming meaningful actions, which is ameliorated when they are able to use the object (Buxbaum, Sirigu, Schwartz, & Klatzky, 2003).
( 1994) postulated that goal-directed movements are computed online, whereas pantomimes are based on stored representations of objects. Importantly, pantomimes have longer movement duration and smaller maximum grasp than do goal-directed actions (Goodale, Jakobson, & Keilor, 1994).
Most studies showing an advantage in action execution when watching human actors have used nongoal-directed actions (e.g., Brass et al., 2000 Kilner et al., 2003). argued that this “motor contagion” is specific to watching another human, and that the human visuomotor system processes human and robot actions differently. Participants’ trajectories were more variable during incongruent than congruent actions, but only when watching the human actor.
( 2003) had participants make horizontal or vertical movements with their arms while watching either a human or a robot make congruent or incongruent actions. Moreover, watching humans alters the spatial components of observers’ actions. Response times (RTs) are faster to fingers than to symbols (Brass, Bekkering, Wohlschläger, & Prinz, 2000 Jonas et al., 2007). In the crucial comparison, symbols (shapes or numbers) determine responses. Participants typically view videos of a hand lifting the middle or index finger, and respond by making the same (congruent) or opposite (incongruent) lift.
In ideomotor priming paradigms, observed actions trigger corresponding actions in the observer.
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