Walsh & Hoyt: Pulfrich Phenomenon

In 1922, Pulfrich reported that when a small target oscillating in a frontal plane is viewed binocularly with one eye covered with a filter to reduce light intensity, the target appears to move in an elliptic path, rather than a to-and-fro path. When the filter is placed over the right eye, the rotation appears counterclockwise; if it is placed over the left eye, the rotation appears clockwise. The explanation for this stereo phenomenon is that the covered eye is more weakly stimulated than the uncovered eye, resulting in a delay in the transmission of visual signals to the striate cortex.