Objects representations in Motor Cortex: neurophysiological and behavioral evidence

Abstract

The present series of experimental studies had the main aim of investigating if, and in such case how, graspable objects are represented within the Motor System of human observers. The first study had four different goals: to verify if object sight induces an automatic excitability increase within the Cortico-Spinal pathway of human participants, as it is known to occur in macaque monkeys; to verify to what extent this excitability modulation, if present, is effector-specific (i.e. occurring exclusively in those Cortico-Spinal neurons targeting muscles involved in reach-to-grasp action toward the observed object); to verify to what extent this excitability modulation, if present, is time-specific (i.e. occurring within a discrete time-window locked to object presentation); finally, differently from previous studies published on this issue, as the objects used as visual stimuli were always in the same central position relative to the observer, any spatial compatibility arising between participant’s hand and presented objects was ruled out as major determinant (as well as confounding factor) of any excitability modulation. To this aim, Transcranial Magnetic Stimulation was adopted as the most suitable technique device to address these topics. Indeed, by using this approach it is possible to verify in vivo the occurrence of subtle changes in excitability in the Primary Motor cortex, with an extremely high time-resolution (in the millisecond order). Furthermore, differently from animal studies, in the present one participants were not trained, nor required to grasp any of the objects, ruling out any priming effect due to the training period that necessarily precedes any experimental work performed in monkeys. The second study aimed at shedding further light on the same topic. Specifically, it primarily aimed at verifying if “graspability” was the sole and sufficient object-feature required to induce excitability changes within the observer’s Motor System. For this purpose, the object adopted was a small-sized cylinder, always presented in the same central position, and never grasped throughout the study. By so doing, differently from previous studies, any possible confounding factors related to familiarity with the object and its usage were contextually excluded. Participants were asked to perform a choice-reaction time moving the right or left index finger by contracting the right or left first dorsal interosseous muscles as fast and as accurately as possible, as soon as they recognized the cylinder or its absence (the control condition), respectively. The first two experiments were performed in two different groups of right-handed participants, but in the second one a transparent barrier interposed between them and visual stimuli prevented any hand-object interaction. Finally, to verify the influence of the observer’s motor repertoire on the recorded effects, the last experiment was performed in a group of left-handed participants.