Path perception and Filehne illusion compared: model and data.

Pursuit eye movements introduce retinal motion that complicates the recovery of self-motion from retinal flow. An extra-retinal, eye-velocity signal could be used to aid estimation of the observer's path, perhaps by converting retino-centric into head-centric motion. This conversion is apparently not precise because we often misperceive head-centric object velocity: in the Filehne illusion, for example, a stationary object appears to move in the opposite direction to the eye movement. Similar errors should be expected when extra-retinal, eye-velocity signals are used in self-motion tasks. However, most self-motion studies conclude that path direction is recovered quite accurately. Path perception and the Filehne illusion were therefore compared directly in order to examine the apparent discrepancy. A nulling technique determined the velocity of simulated eye rotation that cancelled the perceived curvature of the path or, in a Filehne condition, the perceived rotation of the ground-plane stimulus. In either case, observers typically set the simulated eye rotation to be a fixed proportion of the actual eye pursuit made. No differences were found between path perception and Filehne illusion. The apparent inaccuracy of path perception during a real eye movement was confirmed in a second experiment, using a standard 'mouse-pointing' technique. The experiments provide support for a model of head-centric motion perception based on extra-retinal and retinal signals that are linearly related to pursuit and retinal speed, respectively.