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by John Michael Williams, 1999. Placed in public domain.
In the present study, as in Pulfrich Effect studies, a comparison target was adjusted to match the distance of an oscillating one. To resolve certain puzzling results in the literature concerning Pulfrich effect localization error (constant error), two observers were used to obtain data at three photopic levels of illuminance and at ten target speeds from about 1.5 to 39 degrees of visual angle per second.
Although no Pulfrich filter was used, the error in each setting was treated as though entirely because of a stereoscopic Pulfrich Effect visual latency: Settings erring too close to the observer were treated as though because the target appeared near, and settings erring too far correspondingly as though appearing far. This treatment permitted a direct comparison with existing Pulfrich Effect data. The standard Pulfrich-inferred visual latency (delta-tN or delta-tF) thus was computed for settings in which no stereoscopic latency difference would be expected.
The result was a measure of the constant error in Pulfrich-Effect settings.
The variance in the settings, a measure of stereoacuity, was not of primary
importance in this experiment.
The obtained constant error decreased in absolute value more or less consistently with decreased speed; but, the direction of change with speed was opposite for the two observers. However, as illuminance was increased, the constant error decreased consistently in the direction toward the observer.
This study therefore has confirmed previous results on the effect of illuminance; however, the personal equation for stereoscopic equidistance settings as a function of target speed still seems to depend on factors requiring further study.
The Pulfrich Effect, SIU-C. Last updated 2000-07-23