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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.

Human subjects were vertically oscillated in the dark over a frequency range of 1-0.02 HZ at peak-to-peak acceleration amplitudes of 0.2-0.6 g, using both a vertical movement simulator and real flight. Subjective tracking of the movement was unreliable and showed no systematic dependence of phase upon frequency. In contrast, reflex oculomotor response, although weak, clearly demonstrated progressive and substantial phase lag with increasing frequency. The similarity of this characteristic to that previously obtained from brainstem neural responses in cat suggests the oculomotor response was primarily of a vestibular origin. The in-flight studies demonstrated that head movement in a changing linear accelerative field, especially at 0.1 Ha, is highly provocative of motion sickness despite subthreshold angular movement of the aircraft.

Human subjects with maintained reversal of their horizontal field of vision exhibit very substantial adaptive changes in their 'horizontal' vestibulo-ocular reflex (v.o.r.). Short durations (8 min) of vision reversal during natural head movement led to 20% v.o.r. attenuation while long periods (4 weeks) eventually led to approximate reversal of the reflex. The reversed condition is approached by a complex, but highly systematic, series of changes in gain and phase of the reflex response relative to normal. Recovery after return to normal vision exhibits a similar duration, but different pattern, to that of the original adaptation. A chronic cat preparation with long-term optical reversal of vision has now been developed and shows similar adaptive and recovery changes at low test stimulus amplitudes, but different patterns of adaptive response at high amplitudes. An adaptive neural model employing known vestibulo-ocular pathways is proposed to account for these experimentally observed plastic changes. The model is used to predict the adapted response to patterns of stimulation extending beyond the range of experimental investigation.

The vestibulo-ocular reflex (VOR) can be suppressed in darkness if a subject tries to imagine that he looks at a head fixed target. This mental suppression of VOR was used to induce adaptive changes in VOR gain during 3 h of active head oscillations in complete darkness. VOR gain changes were tested by asking the subject to look at a visual target; then passively or actively the head was turned in darkness while the subject "fixated" the same target. Corrective saccades occurring at the end of the movement when lights were turned on give an elegant measure of VOR gain. Three hours of training induced in 3 subjects a mean of 10.9% and 11.4% decrease of VOR gain for passive and active conditions, respectively. This demonstrates that reflex adaptation can be obtained without external cues, and probably with only an internal reconstruction of target and eye movement.