Dynamic adaptation of the blind pointing characteristic to stepwise lateral tilts of body, head, and trunk

Blind pointing (i.e. pointing to visual targets without seeing the pointing arm) was investigated in normal subjects in response to stepwise lateral tilts (20 degrees) of the body, head, and trunk. Blind pointing positions of the right index finger on the outer surface of a hemispherical screen were measured relative to the positions of visual targets that were presented along a horizontal line (+/- 30 degrees in head coordinates) on the inner screen surface, thus yielding a blind pointing characteristic (BPC). (1) BPC is highly reproducible and can be subdivided into separate branches for the ipsi- and contralateral hemifields. These branches are rotated relative to the target line by individually different BPC angles pi i and pi c. (2) pi i exhibits characteristic time courses (measured within 10 min following a stepwise tilt) for each paradigm. (3) Body tilt (left ear down) causes a step-like increase in pi i of up to 14 degrees; body tilt (right ear down) causes a step-like decrease in pi i to about zero. (4) Trunk tilt (right shoulder down) produces a gradual decrease in pi i of up to 6 degrees (average time constant tau T = 5 min); trunk tilt (left shoulder down) produces a gradual increase in pi i of up to 4 degrees. (5) Head tilt (left ear down) causes an increase in pi i of up to 9 degrees followed by a gradual decrease (average time constant tau H = 6 min); head tilt (right ear down) causes a step-like decrease in pi i with unsignificant further changes. These findings are discussed in terms of a neural sensorimotor coordinate transformation process receiving separate, dynamic otolith and neck afferent influences.