The habitual motor vertical of humans depends on gravicentric and egocentric cues, but only little on visual cues.

Many studies have evaluated the interplay of gravicentric, egocentric, and visual cues for our perception of the vertical, but little is known about their interplay for motor control. Thirty-five participants flipped a switch "down" in experimental conditions which systematically varied body posture (upright; tilted 45° left-ear-down), visually indicated vertical (absent; aligned with the long body axis; rotated 45° counterclockwise with respect to the long body axis), and egocentric tactile information provided by a handhold (absent; present). Since we were interested in the participants' habitual rather than an instructed motor vertical, we did not specify which cues they should rely on. Our data revealed two response categories. Type-1 responses depended moderately on switch position; they relied mainly on gravicentric and egocentric cues, but only marginally on visual cues. Type-2 responses depended strongly on switch position; they relied on egocentric, but not on gravicentric or visual cues. We interpret the dependence on switch position as evidence that egocentric cues for type-2 responses may be anchored in the participants' arm rather than in their long body axis. In conclusion, the habitual motor vertical can vary when available cues are not aligned, and this variability can compromise manual performance of human operators.