Translations are processed slower than rotations: reaction times for self-motion stimuli predicted by vestibular organ dynamics

Reaction times (RTs) to purely inertial self-motion stimuli have only infrequently been studied, and comparisons of RTs for translations and rotations, to our knowledge, are nonexistent. We recently proposed a model [1] which describes direction discrimination thresholds for rotational and translational motions based on the dynamics of the vestibular sensory organs. This model also predicts differences in RTs for different motion profiles (e.g., trapezoidal versus triangular acceleration profiles or varying profile durations). The model calculates a signal akin to the change in firing rate in response to a self-motion stimulus. In order to correctly perceive the direction of motion the intrinsic noise level of the firing rate has to be overcome. Based on previously identified model parameters from perceptual thresholds, differences in RTs between varying motion profiles can be predicted by comparing the times at which the firing rate overcomes the noise level. To assess these predictions we measured RTs in 20 participants for 8 supra-threshold motion profiles (4 translations, 4 rotations). A two-alternative forced-choice task, discriminating leftward from rightward motions, was used and 30 correct responses per condition were evaluated. The results are in agreement with predictions for RT differences between motion profiles. In order to describe absolute RT, a constant is added to the predictions representing both the discrimination process, and the time needed to press the response button. This constant is calculated as the mean difference between measurements and predictions. It is approximately 160ms shorter for rotations, thus indicating that additional processing time is required for translational motion. As this additional latency cannot be explained by our model based on the dynamics of the sensory organs, we speculate that it originates at a later stage, e.g. during tilt-translation disambiguation