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Eliana M. Klier, Dora E. Angelaki, Bernhard J. M. Hess; Gravitational signals contribute to visuospatial updating in humans. Journal of Vision 2005;5(8):757. doi: 10.1167/5.8.757.
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© ARVO (1962-2015); The Authors (2016-present)
Primates can update, and thus keep track of, the locations of objects in space. Specifically, they can accurately look to the remembered location of a flashed target even if the eyes/head are subsequently moved from their initial location. To do this, the brain requires information about the amplitude and direction of the intervening movement. Such information can be provided either by motor signals (i.e., efference copies) or by sensory signals. These sensory cues include proprioception and vestibular (i.e., canal and otolith) signals that also provide information on the body's orientation relative to gravity.
To determine if efference copy signals are necessary for spatial updating, we had subjects sit, with their heads and bodies fixed, on a three-dimensional turntable. In complete darkness, subjects were passively rotated torsionally (left ear down or right ear down) to a new orientation, briefly shown an eccentric target, returned to upright, and then asked to make a saccade to the remembered target. Using this paradigm, we tested subjects' abilities to update from 7 tilt angles to 8 target directions and 2 target amplitudes.
We found that subjects were able to update remembered target locations from all tilt angles and all amplitudes. Slopes of directional errors vs. tilt angle ranged from −0.01 to 0.15, were similar to a slope of 0 (perfect updating) and significantly different from a slope of 1 (no compensation for head torsion). Since the head and body were fixed throughout these rotations, efference copies were not required for updating.
To investigate the contribution of canal vs. gravitational signals, we repeated the above experiment with subjects supine. This time the slopes of directional errors vs. tilt angle ranged from 0.60 to 0.82, indicating poor updating performance. Thus, we conclude that information specifying the body's orientation relative to gravity is critical for maintaining spatial constancy and for distinguishing space-fixed vs. body-fixed targets.
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