Cognitive allocentric representations of visual space shape pointing errors
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Subjects reached in three-dimensional space to a set of remembered targets whose position was varied randomly from trial to trial, but always fell along a "virtual" line (line condition). Targets were presented briefly, one-by-one and in an empty visual field. After a short delay, subjects were required to point to the remembered target location. Under these conditions, the target was presented in the complete absence of allocentric visual cues as to its position in space. However, because the subjects were informed prior to the experiment that all targets would fall on a straight line, they could conceivably imagine each point target as belonging to a single rigid object with a particular geometry and orientation in space, although this virtual object was never explicitly shown to the subjects. We compared the responses to repeated measurements of each target with those measured for targets presented in a directionally neutral configuration (sphere condition), and used the variable errors to infer the putative reference frames underlying the corresponding sensorimotor transformation. Performance in the different tasks was compared under two different lighting conditions (dim light or total darkness) and two memory delays (0.5 or 5 s). The pattern of variable errors differed significantly between the sphere condition and the line condition. In the former case, the errors were always accounted for by egocentric reference frames. By contrast the errors in the line condition revealed both egocentric and allocentric components, consistent with the hypothesis that target information can be defined concurrently in both egocentric and allocentric frames of reference, resulting in two independent coexisting representations.
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