The use of optical velocities for distance discrimination and reproduction during visually simulated self motion

Successful navigation through an environment requires precise monitoring of direction and distance traveled ("path integration" or "dead reckoning"). Previous studies in blindfolded human subjects showed that velocity information arising from vestibular and somatosensory signals can be used to reproduce passive linear displacements. In these studies, visual information was excluded as sensory cue. Yet, in our everyday life, visual information is very important and usually dominates vestibular and somatosensory cues. In the present study, we investigated whether visual signals can be used to discriminate and reproduce simulated linear displacements. In a first set of experiments, subjects viewed two sequences of linear motion and were asked in a 2AFC task to judge whether the travel distance in the second sequence was larger or shorter than in the first. Displacements in either movement sequence could be forward (f) or backward (b). Subjects were very accurate in discriminating travel distances. Average error was less than 3% and did not depend on displacements being into the same (ff, bb) or opposite direction (fb, bf). In a second set of experiments, subjects had to reproduce a previously seen forward motion (passive condition), either in light or in darkness, i.e., with or without visual feedback. Passive displacements had different velocity profiles (constant, sinusoidal, complex) and speeds and were performed across a textured ground plane, a 2-D plane of dots or through a 3-D cloud of dots. With visual feedback, subjects reproduced distances accurately. Accuracy did not depend on the kind of velocity profile in the passive condition. Subjects tended to reproduce distance by replicating the velocity profile of the passive displacement. Finally, in the condition without visual feedback, subjects reproduced the shape of the velocity profile, but used much higher speeds, resulting in a substantial overshoot of travel distance. Our results show that visual, vestibular, and somatosensory signals are used for path integration, following a common strategy: the use of the velocity profile during self-motion.