ASSESSMENT OF THE PERCEPTION OF VERTICALITY AND HORIZONTALITY WITH SELF-PACED SACCADES

We investigated the ability of human subjects (Ss) to make self-paced saccades in the earth-vertical and horizontal directions (space-refere nced task) and in the direction or the head-vertical and horizontal ax is (self-referenced task) during whole body tills of 0 degrees, 22,5 d egrees, 45 degrees and 90 degrees in the frontal (roll) plane. Saccade s were recorded in the dark with computerised video-oculography. Durin g space-referenced tasks, the saccade vectors did not fully counter-ro tate to compensate for larger angles of body tilt. This finding is in agreement with the 'A' effect reported for the visual vertical. The er ror was significantly larger for saccades intended to be space-horizon tal than space-vertical, This vertico-horizontal dissociation implies greater difficulty in defining horizontality than verticality with the non-visual motor task employed. In contrast, normal Ss (and an alabyr inthine subject tested) were accurate in orienting saccades to their o wn (cranio-centric) vertical and horizontal axes regardless of tilt in dicating that cranio-centric perception is robust and apparently not a ffected by gravitational influences.