Abstract
The present study aimed at determining how actions executed by two conspecifics can be coordinated with each other, or more specifically, how the observation of different phases of a reaching-grasping action is temporary related to the execution of a movement of the observer. Participants observed postures of initial finger opening, maximal finger aperture, and final finger closing of grasp after observation of an initial hand posture. Then, they opened or closed their right thumb and index finger (experiments 1, 2 and 3). Response times decreased, whereas acceleration and velocity of actual finger movements increased when observing the two late phases of grasp. In addition, the results ruled out the possibility that this effect was due to salience of the visual stimulus when the hand was close to the target and confirmed an effect of even hand postures in addition to hand apparent motion due to the succession of initial hand posture and grasp phase. In experiments 4 and 5, the observation of grasp phases modulated even foot movements and pronunciation of syllables. Finally, in experiment 6, transcranial magnetic stimulation applied to primary motor cortex 300 ms post-stimulus induced an increase in hand motor evoked potentials of opponens pollicis muscle when observing the two late phases of grasp. These data suggest that the observation of grasp phases induced simulation which was stronger during observation of finger closing. This produced shorter response times, greater acceleration and velocity of the successive movement. In general, our data suggest best concatenation between two movements (one observed and the other executed) when the observed (and simulated) movement was to be accomplished. The mechanism joining the observation of a conspecific’s action with our own movement may be precursor of social functions. It may be at the basis for interactions between conspecifics, and related to communication between individuals.
Highlights
When executing in succession different actions which are functionally interrelated, the execution of the previous action influences the one [1,2,3,4]
On a PC monitor (19 inches) placed on the table, 60 cm distant from the participant’s chest, the participant was presented with one picture of grasp posture corresponding to one of the following grasp phases: initial finger opening (IFO), maximal finger aperture (MFA), final finger closing (FFC) on an object
Response Times (RTs) was significantly slower in IFO than in MFA and FFC phases (factor grasp phases: F(2, 52)=3.7 p=0.03, MFA versus IFO, p=0.04, FFC versus IFO, p=0.048, MFA versus FFC, p=0.62, Figure 2)
Summary
When executing in succession different actions which are functionally interrelated, the execution of the previous action influences the one [1,2,3,4]. Interactions of biological distal effectors (hand, mouth and foot) rather than nonbiological stimuli with differently sized objects influenced the kinematics of successive observer’s interactions with the same objects using the same or different effectors [7,8,9] This suggested that the action was automatically simulated by the observer and mnemonic traces of the simulation influenced the actual action. Voice parameters of syllables increased when observing the grasp of a large rather than small object They interpreted these results as due to automatic simulation of the observed action during which the motor commands to the hand were sent to the speaking mouth.
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