The link between observation and execution of biological movement : behavioural correlates and the underlying neural network

Behavioural as well as neurophysiological data support the notion that the execution of human body movements and the perception of the same movements in other individuals are closely linked. Automatic effects of observed movements on movement performance have been attributed to a common coding of movements in the perceptual and motor domain, enabling an action observation-execution matching (AOEM) mechanism that directly maps a perceived movement onto its internal motor representation. Common activation of a set of motor-related brain areas during both the observation and execution of biological movements is supposed to constitute the neural correlate of the AOEM mechanism. Specifically, inferior frontal and parietal cortical areas are reported to constitute core areas of a human "mirror (neuron) system" that serves AOEM. In a series of four two-alternative choice reaction time (RT) experiments, the present dissertation provides evidence for imitative response tendencies following observed simple intransitive finger movements. Using single-stimuli paradigms in RT Experiments 1 and 2 yielded immediate facilitatory effects of observation on execution of corresponding finger movements. Priming/cueing paradigms employed in RT Experiments 2 and 3 revealed delayed effects of observed finger movements (S1) on subsequent imitative movements (in response to a second finger movement S2). Patterns of facilitatory and inhibitory effects depended on defined stimulus onset asynchronies (SOAs). Results suggest that the observation of a biological movement leads to a transient activation of its internal motor representation. A facilitation of the corresponding response becomes manifest if the resulting response tendency can be released immediately. Otherwise, facilitation rapidly turns into an inhibitory effect. Strictly matched control cues were used in all behavioural studies, i.e. "biologically" moving objects that controlled for spatial and also kinematical stimulus characteristics. This permitted demonstration of priming effects which were specific to the observation of real human body movement, and not attributable to the unspecific inherent motion component. However, both specific and unspecific facilitatory and inhibitory effects were similarly affected by temporal expectancies. In order to investigate the neuronal correlates of the facilitatory priming effects of biological movement obtained in the single-stimulus RT experiments, an event-related functional magnetic resonance imaging (fMRI) study was conducted. This experiment followed-up on previous fMRI studies which reported a preferential activation of putative human "mirror areas" in the inferior frontal and parietal cortex during imitation of intransitive finger movements as compared to motor control tasks. Task-related changes in regional BOLD signal during the mere observation and imitation of simple intransitive finger movements were contrasted with the observation of spatially and/or kinematically matched control stimuli and execution of finger movements in response to these control cues. In accord with previous behavioural results, participants were faster at imitating a finger movement than at performing the same movement in response to a static or a moving control stimulus. However, the behavioural advantage was not paralleled by a difference in regional activity of inferior fronto-parietal areas. Moreover, during observation, BOLD signal in putative mirror areas was increased only when the finger movement was made more salient by attaching an object to it. The lack of a functional signature of inferior fronto-parietal "mirror activity" was attributed to an enhanced contribution of stimulus-response mapping based on common spatial coding in observation-execution of intransitive finger movements within the specific experimental situation. Taken together, the present behavioural and neurophysiological results suggest that AOEM or "mirroring" processes might differ qualitatively between intransitive and transitive movements. Whereas mirroring processes in motor-related brain areas induced by intransitive movements might rely primarily on spatial-dynamic properties of the movement, mirroring of transitive movements might rather be based on representations of the interaction between effectors and (objects in) the external world. Consequently, visuo-spatial and dynamic stimulus characteristics seem to have a higher impact on visuo-motor transformation processes during observation-execution of intransitive as compared to transitive movements. Moreover, a potentially higher interference between specific context factors of an observation/imitation situation and mirroring/AOEM processes which are elicited by intransitive movements might lead to a more variable engagement of inferior frontal and parietal "mirror (neuron) areas" in visuo-motor transformation of intransitive as opposed to transitive movements.