Neuro-Robotics by Panagiotis Artemiadis

Author:Panagiotis Artemiadis
Language: eng
Format: epub
Publisher: Springer Netherlands, Dordrecht

A somewhat different story emerges when subjects are required to make reaching movements to remembered target locations with little or no visual feedback of the arm. Here, patterns of constant and variable errors support a coordinate system for planning that is at least partially body-centered [42, 45, 49]. Interestingly, similar trends are observed when subjects make movements without delay to visible targets (not remembered ones) but without online visual feedback of the hand [53]. Results such as these suggest that the coordinate frame(s) used to plan human reaching movements are context-dependent, being determined in part by the extent and reliability of relevant visual feedback, particularly regarding the arm [49, 54–59]. Note that this conclusion is also in agreement with a recent study of the social Simon effect, which pointed to context-dependent frames for planning joint actions [31].

The above considerations for representations and frames of reference involved with pointing or reaching are likely to apply to RTG, where proximal limb motion is coupled with hand shaping to grasp an object, i.e., preshaping. Hand preshaping could be considered a process through which object geometry and grasp affordances, expressed in a world-centered frame of reference, drive a sensorimotor transformation leading to the alignment of hand and object frames of reference, culminating in a close match between the opposition axis (i.e., the line connecting the tips of the thumb and index finger [60]) and the graspable axis of an object, or between hand shape and object shape. Rather than addressing this process directly, behavioral studies have mostly focused on the sensory mechanisms underlying the time course of hand shape modulation to object shape. One of the main questions addressed by these studies is the extent to which continuous visual feedback of the hand and/or the object is required to coordinate finger movements leading to a stable grasp at the end of the reach. One possible scenario consists of computing an ‘error’ signal arising from visually-perceived object shape versus hand shape, until the hand can conform to the object at contact. An alternative scenario might involve vision, but not necessarily on a continuous basis. Specifically, object shape visually perceived at reach onset might be sufficient to drive the coordination of finger movement throughout the reach. Behavioral evidence supports the latter scenario, indicating that whole-hand shaping occurs in a similar fashion regardless of whether visual feedback of the object is present or absent during the reach [61]. Interestingly, removing tactile feedback at the end of the reach when reaching to grasp a visible virtual object also leads to a similar hand shaping observed when reaching to grasp a physical object [61]. A follow-up study confirmed these observations by showing that hand preshaping occurs in a similar fashion regardless of whether vision of the hand and object is allowed [62]. Although these observations do not rule out a role for online visual feedback for hand preshaping, they demonstrate that vision of the object prior to initiating a reach is sufficient to drive the spatial and temporal coordination of finger movements in preparation for a grasp.

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