Pointing to remembered visual targets after active one-step self-displacements within reaching space

We studied pointing movements to remembered visual targets in a completely darkened room with and without self-made step movements in order to investi gate in which coordinate system and to what extent target representations r elative to the body are updated for self-induced egomotion. A small red-lig ht-emitting diode on the fingertip provided visual feedback about fingertip position at all times. We asked subjects to make pointing movements that s tarted 2 s after disappearance of a visual target. In this interval of 2 s the subject did or did not make a step. The pointing errors without a step showed that subjects undershot faraway targets in a systematic way, whereas they sometimes overshot nearby targets. We found that the step causes larg er pointing errors both in amplitude and direction with a bias in the direc tion of the step. We explored three different versions of a descriptive mod el in which polar coordinates were used to describe the pointing movement, and in which either Cartesian or polar coordinates were used to update targ et position relative to the shoulder for the step. The results suggest that incorporation of the step displacement in the new target position relative to the subject is done in a Cartesian frame of reference. Moreover, the am plitude of the step displacement tends to be underestimated by subjects.