Estimates of rotational self-displacement and self-velocity have been used interchangeably in vestibular psychophysics to characterize vestibular ego-motion perception. However, the assumption underlying this indiscriminate use has never been tested. The assumption holds that the two estimates are equivalent, with the displacement estimates reflecting the time integral of the signal underlying the velocity estimate. We tested this hypothesis by directly comparing displacement and velocity estimates. Two groups of healthy young subjects (2 x n = 15) were presented with the same vestibular stimuli (horizontal whole body rotations in the dark in the form of velocity steps of 5, 10, 20, and 40 degrees/s with 1, 2, 4, 8, and 16 s duration, yielding position ramps of 5, 10, 20, 40, 80, 160, and 320 degrees total displacement). The first subject group estimated peak velocity, and the second group estimated total displacement, both groups using a comparable psychophysical procedure (Stevens' magnitude estimation). The experimentally obtained velocity estimates were used to predict the displacement estimates. To this end, the velocity signal was assumed to decay exponentially from the reported peak value (reflecting the dynamics of peripheral and early central vestibular mechanisms) and was mathematically integrated. Predicted and measured displacement estimates were similar when a time constant of 20 s was assumed, which is in good agreement with earlier studies. We conclude that vestibular displacement estimates can, indeed, be considered equivalent to vestibular velocity estimates, at least for the stimulus parameters used.
