Authored by Denis Larrivee
The concept of the neural representation of the self, evoked when the body is dynamically engaged in intentional action, is traced to the notion of the motor image. Insights drawn from the motor image posit that bodily representation is a key feature stabilizing individual motions of the plan as teleologically situated, that is, linking an agent to an objective terminus. Current work is beginning to disentangle representational content that is globally attuned to the whole individual from that for discrete movements. These studies suggest that representations of body posture are likely to be an important proxy for global self-representation in dynamic actions and may be directly modified by goal specific content. Candidate influences for goal related modulation are likely to include egocentric coding in the posterior parietal and premotor cortices that shape self representations to yield goal directed motor movement.
The modern concept of the neural representation of the self, evoked in circumstances where the body is dynamically engaged in intentional actions, has evolved from several experimental legacies traced to the notion of the motor image [1-3]. This image is so designated to indicate a covert action undertaken only mentally and as a simulation of a non-executed action. In its current understanding the motor image represents the feature elements of an intended motor trajectory, that is, the projected series of motions that will be executed in a motor plan. Accordingly, it contains the signal features needed for plan execution [4].
Insights from the motor image posit that bodily representation is a key feature that stabilizes individual motions of the plan as teleologically situated; that is, one where inscribed actions are bound to an agent having an objective destination. Afferent input used to generate the body representation, termed the body schema, has been shown to originate from somatotopic and interoceptive input. Accordingly, the motor image includes not only the discrete movements of bodily limbs but also their association with the representation of the whole body; that is, discrete movements are contextualized with respect to the whole individual, who is neurally represented by his body. Neural representations contained in the motor image thus include those of the body schema, specific limb movements, and the target, understood to be the intended goal of actions that are undertaken [5]. Actions are therefore planned in the context of the whole individual with respect to their intended outcome [6,7], how the motor image is structured and implemented, accordingly, has substantial bearing on the optimization of motor performance.
Studies of the image reveal that its feature elements include distinct and reciprocal contributions from central and peripheral origins; accordingly, they underscore the essential unity of dynamic performance even in its covert formulation. Central influences have been classically demonstrated by the presence of movements undertaken in the absence of sensorial input. Lashley notably observed that humans and animals were capable of motor actions despite the loss of afferent input. Later experiments in monkeys also confirmed a central origin by showing that deafferentation of spinal dorsal motor roots [8] did not prevent pointing movements. When the original relationship between a movement and its spatiotemporal origin had been altered, however, limb movements failed to correspond to their expected trajectories, involving instead a mis reaching followed by subsequent compensatory movements. Taken together, these results indicated that initial movements responded to memorized, centrally evoked motor commands that were sent to the sensory cortex, termed efference copy or corollary discharges, but subsequent movements required sensory feedback to correctly maintain their targeting.
In continuous motions, feedback from sensory cues is therefore essential to motor execution, where it is coupled with central commands in a mutually reciprocal and sustained process designated a forward projection model [9]. This coupling is necessary since as the body undergoes motion, its spatiotemporal position is continually changing; so also, the sensory cues that reference it. In consequence, peripheral contributions provide an ongoing stream of sensory updating that adjusts motor execution, a phenomenon that has been likened to a perception action loop [10]. Its significance has been demonstrated in many studies, including investigations of the body’s dynamical interaction with the environment in developing infants, acquired motor abilities, biodynamic studies of movement and proprioception related to perception, and in the motoric encoding of actions such as reaching.
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