Subjective detection of vertical acceleration: a velocity-dependent response?

Human subjective thresholds and directional sensitivity were investigated as a function of vertical linear acceleration with head erect. A hyperbolic (r=0.94) relation emerged between threshold latency and acceleration magnitude (range 0.005 to 0.06 g). This implies that detection was determined by attainment of a given velocity (21.6+/-2.65 cm/sec) rather than the acceleration magnitude per se. Re-analysis of previous data from horizontal accelerations conducted with head erect and supine revealed similar hyperbolic relations (r=0.98 in both cases) with velocity constants of 22.6+/-1.28 and 32.4+1.96 cm/sec respectively. From these findings it is inferred that with head erect (i.e. normal attitude re gravity) the thresholds to predominantly utricular (horizontal accel.) and saccular (vert. accel.) stimulation were similar (P greater than 0.7). However, with head "supine" the saccular threshold was increased to approx. 1.5Xnormal (P less than 0.001). The results also confirmed a previously reported difficulty in the subjective detection of the direction of vertical movement.