Perception of the immediate extrapersonal space through proprioceptive inputs

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Abstract
In the present work we sought to examine the idea that an internalized representation of space can be formed and used in the absence of visual information. To this end human subjects were trained to locate 5 points within a horizontal plane at shoulder level, under guidance of an acoustic signal. Next, reaching accuracy was determined for both the trained and the untrained hands without external cues. This allowed comparison of hand positioning that could be based upon a memory trace of proprioceptive inputs (trained hand) to hand positioning that must be based upon a more abstract internal representation than proprioceptive memory (untrained hand). The effect of training on reach accuracy of the two hands and the effect of prolongation the time interval between guided and recall (unguided) trials were studied. Finally, subjects were asked to interpolate between already learned target positions, a procedure which tested their ability to utilize an abstract construct of space. The relative contribution of the elbow and shoulder joints to reaching errors was determined. This analysis served to evaluate whether representation of spatial locations was based on joint angle coordinates, or on a more abstract extracorporeal reference system. Our results indicate that a memorized sensory image of the limb, which corresponds to the hand being at the target position, is used whenever possible. An abstract representation of position is used whenever comparison of the actual sensory input to a memorized sensory trace is made impossible, costing an approximately 25% reduction in accuracy. The interpolation procedure indicates that the same abstraction is used when target positions are determined mentally, as when the untrained limb performs the task. Finally, it appears that this abstract spatial representation is based on extrapersonal coordinates rather than on joint angles.