Datensatz

Zusammenfassung

Optic flow is generated by observer motion relative to stationary objects, by movement of objects relative to a stationary observer, and by combinations of those situations. To determine
the relative contributions of object and self motion to the observed optic flow, the nervous system
can use vestibular signals. An object’s speed relative to earth is given by the difference
between its speed relative to the head and the head’s speed relative to the earth. The variance
of the difference is the sum of the component variances. In contrast, if observers estimate
self-motion from optic flow and vestibular signals, and assume a stationary visual scene, visual
and vestibular estimates may be combined in a weighted average to yield more precise selfmotion
estimates. So depending on whether the subject reports object motion or self-motion,
the two-modality variance is predicted to be respectively higher or lower than the component
variances. To test these predictions and the influence of the visual scene upon them, we measured
speed-discrimination thresholds for fore-aft translations. There were two single-modality
conditions, Visual and Vestibular, and two multi-modality conditions, Self-motion and Object
motion. In the Visual, Vestibular, and Self-motion conditions, observers indicated if the movement
was faster or slower than a standard. In the Object-motion condition, observers indicated
if the object appeared to move with or against the self-motion. The experiment was run for
two visual scenes, random-dots and ground plane with columns. In both scenes, multi-modal
object-motion thresholds were, as predicted, higher than single-modality thresholds, and multimodal
self-motion thresholds were, as predicted, generally lower than single- modality thresholds.
For the ground plane scene, the self-motion thresholds could be consistently predicted
by the weighted average of the single modalities, which was not the case for the random-dots
condition.