We investigated whether the human brain encodes and memorizes object orientations with respect to external references, such as gravity and visual landmarks, or whether it uses egocentric representations of the task. To this end, we applied a new analysis to a previously reported experiment on a reach-to-grasp-like movement, in which we used sensory conflict to identify how the CNS encodes target and hand orientation. Whereas in the preceding study deviations of responses provoked by the conflict provided evidence for the simultaneous use of visual and kinesthetic representations of target and hand (Tagliabue and McIntyre, 2011 ), here we used an analysis of response variability in the presence of conflict to test for ego- versus exo-centric encoding within each sensory modality. Our results show an increase of response variability with the amplitude of the head rotation, indicative of errors that accumulate when updating egocentric representations during head movements. In addition, the effect of conflict on error accumulation showed that the brain selects different information about the head movement for the updating, depending on the modality of the egocentric representation (visual or kinesthetic) that is retained. In particular, the CNS appears to privilege the sensory information about head movement that can most easily be combined with each internal representation. Moreover, a combined analysis of response variability and response deviations induced by the conflict suggests the coexistence of independent ego- and exo-centered internal representations within each sensory modality.
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