Human spatial orientation in non-stationary environments: relation between self-turning perception and detection of surround motion.

We investigated the relative weighting of vestibular, optokinetic and podokinetic (foot and leg proprioceptive) cues for the perception of self-turning in an environment which was either stationary (concordant stimulation) or moving (discordant stimulation) and asked whether cue weighting changes if subjects (Ss) detect a discordance. Ss (N = 18) stood on a turntable inside an optokinetic drum and turned either passively (turntable rotating) or actively in space at constant velocities of 15, 30, or 60°/s. Sensory discordance was introduced by simultaneous rotations of the environment (drum and/or turntable) at ±{5, 10, 20, 40, 80}% of self-turning velocity. In one experiment, Ss were to detect these rotations (i.e. the sensory discordance), and in a second experiment they reported perceived angular self-displacement. Discordant optokinetic cues were better detected, and more heavily weighted for self-turning perception, than discordant podokinetic cues. Within Ss, weights did not depend on whether a discordance was detected or not. Across Ss, optokinetic weights varied over a large range and were negatively correlated with the detection scores: the more perception was influenced by discordant optokinetic cues, the poorer was the detection score; no such correlation was found among the podokinetic results. These results are interpreted in terms of a "self-referential" model that makes the following assumptions: (1) a weighted average of the available sensory cues both determines turning perception and serves as a reference to which the optokinetic cue is compared; (2) a discordance is detected if the difference between reference and optokinetic cue exceeds some threshold; (3) the threshold value corresponds to about the same multiple of sensory uncertainty in all Ss. With these assumptions the model explains the observed relation between optokinetic weight and detection score.