Abstract

The aim of this thesis is to provide new insights about the role of the left human parietal operculum (OP) in sensory motor transformations in the context of object-directed behavior. This work is divided in two main parts: and introductive part about the theory underlying the sensory motor integration and the existing literature about the parietal operculum, and an experimental part in which the experiments realized during these three years are described. In Chapter 1, the theory underlying the sensory motor transformation in the visual modality and the possible functions of the different front-parietal circuits are described on the basis of the theory proposed in the literature. A specific paragraph is dedicated to ventral premotor cortex (PMv) and its role in visually guided grasping. The Chapter 2, is a review of the literature about the cytoarchitecture, the connectivity and the physiology of humans and non- human primates parietal operculum. In Chapter 3 the literature about the role of OP of primates and humans in sensory motor integration is reviewed together with some literature about studies on lesions. In the experimental part, four transcranial magnetic stimulation (TMS) experiments are described. The last two experiments have been grouped in a single major work and results have been discussed together. In Chapter 4 the Experiment I is described. This experiment aimed to characterize the fronto-parietal network involving the connection between left OP and ipsilateral primary motor cortex (M1). By means of double coil TMS paradigm we were able to asses on five healthy volunteers the functional interaction occurring between these two regions, defining the optimal spot located into the left OP region that was producing the highest inhibition on motor evoked potentials (MEPs) peak-to peak amplitude. We moreover characterized the timing of their interplay obtaining a 5 ms short latency interaction in support of a plausible direct cortico-cortical connection. In Chapter 5 the Experiment II and Experiment III (which is subdivided in two different experiments named Experiment IIIa and Experiment IIIb) are described. On the basis of the functional connectivity between OP and ipsilateral M1 at rest proved in Experiment I, the Experiment II aimed to asses whether and how the haptic information about objects’ geometry (size) is encoded within the left OP and whether such information is used for guiding object-directed motor actions. Again, by means of double coil TMS paradigm we provided evidences supporting that the excitability of left M1 of twelve healthy volunteers, in presence of a motor plan involving the grasping of an object, was modulated accordingly with the size information stored in ipsilateral OP of such object when such physical feature was previously acquired by haptic exploration. Such modulation didn’t occur when the object size was acquired by visual modality. As control we run the same paradigm on other twelve volunteers by conditioning the ventral premotor cortex (PMv) which is well know to play a key role in the visually guided grasping obtaining a clear double dissociation between haptic and visual modality, supporting the hypothesis that haptically guided behavior relies on a distinct network respect to visually guided one.Experiment III is divided in two different experiments (Experiments IIIa and Experiments IIIb). In both of these experiments we aimed to test the effect introduced by high frequency repetitive online TMS (rTMS) during two different tasks both involving only haptic modality. In Experiment IIIa a purely discriminative match-to-sample task, in which two cylinders (either equal or different in the diameter feature) were haptically explored in sequence with 2 seconds of delay, was implemented. RTMS was delivered during the delay period and effects on accuracy and sensitivity on discrimination were tested. In such non motor task, no significant effects were obtained on performances of the twelve healthy participants. In Experiment IIIb the effects of rTMS applied during different phases of a delayed grasping of a previously haptically explored object on the kinematics of the hand preshaping were assessed. Differently form experiment IIIa, in this motor task we obtained a significant perturbation of the hand aperture during the preshaping when stimulation was applied between the exploration of the object and the grasping, during the delay phase. Such results might suggest of the involvement of OP in sensory-motor transformations as a putative element of memory when a object-directd action plan is implemented supporting the theory of the existence also for the haptic modality, of a model dual to the one proposed for the visual modality, made of two different streams processing sensory information according with the final use (for action or for perception).