Abstract
To date, both in monkeys and humans, very few studies have addressed the issue of the lateralization of the cortical parietal and premotor areas involved in the organization of voluntary movements and in-action understanding. In this review, we will first analyze studies in the monkey, describing the functional properties of neurons of the parieto-frontal circuits, involved in the organization of reaching-grasping actions, in terms of unilateral or bilateral control. We will concentrate, in particular, on the properties of the mirror neuron system (MNS). Then, we will consider the evidence about the mirror neuron mechanism in humans, describing studies in which action perception, as well as action execution, produces unilateral or bilateral brain activation. Finally, we will report some investigations demonstrating plastic changes of the MNS following specific unilateral brain damage, discussing how this plasticity can be related to the rehabilitation outcome
Highlights
It has been maintained for a long time that functional lateralization is a prerogative of human beings [1,2]
Gazzola and Keysers [65] used single-subject unsmoothed fMRI data to investigate the contribution of other areas involved during the observation and execution of grasping actions performed with the hand, mouth and foot, showing that voxels shared between both conditions were localized, in addition to the classical mirror areas, in the dorsal premotor cortex (PMd), middle cingulate cortex (MCC), superior parietal lobule (SPL) and middle temporal cortex and cerebellum
The lateralization index was negatively related to performance on clinical scales. These data suggest that (1) the mirror neuron system (MNS) can be activated in unilateral cerebral palsy (UCP) patients, to TD children; (2) the type of reorganization and, as a consequence, the different involvement of the two hemispheres during the process of recovery, is the main factor explaining the degree of functional outcome
Summary
It has been maintained for a long time that functional lateralization is a prerogative of human beings [1,2]. It seems that in evolution, the achievement of a clear neuroanatomical and functional lateralization appeared in the primate lineage, starting with some example of not complete dominance in apes, and becoming consolidated only in the human species This does not mean that the cortical areas of non-human primates perform only a contralateral control. The primary motor cortex (MI) controls discrete contralateral movements, because only in this way individuals are able to perform skilled motor acts In these areas, neurons related to both body sides would be inappropriate, because the probability of a conflict between symmetric body parts would be very high. The parieto-premotor circuits, based on the properties mentioned above, constitute the anatomical scaffolds for the organization of actions performed with different effectors, such as reaching, grasping and oculomotion; 7 From these concepts it can be concluded that frontal agranular and posterior parietal cortex form together the cortical motor system. Thanks to the strong callosal connections of both posterior parietal and premotor areas, this organization is not limited to the control of the contralateral hemibody/hemispace, as it will be described below
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