Depth Perception during Diplopia is Direct

Although depth is experienced with targets at large disparities when they are seen as double or diplopic, whether that depth is as direct as with fused targets has been a matter of considerable uncertainty. Researchers have often claimed that judgments of the depth of diplopic targets during simple near/far tasks rely upon indirect associations with eye-muscle proprioception or a copy of the vergence drive signal. We designed a four-alternative task that could not be performed without a direct appreciation of depth. Observers judged the depths of each of two Gabor stereo pairs presented simultaneously. Disparities were always above each observer's measured diplopia threshold. The signs of the disparities were varied independently and observers reported the perceived depth near and far for each target. Our results demonstrate conclusively that depth during diplopia requires neither proprioception nor an efferent copy but is direct.

Efferent Copy and Voluntary Eyecup Movement in the Crab, Carcinus

1. Use is made of the fact that a freely moving blind eyecup is driven by movement perceived by the opposite seeing eye when the crab is in an optomotor drum. 2. Forced extension of the seeing eyecup causes movement of the other blinded eyecup in the opposite direction, because it is driven by the perceived relative motion. 3. Voluntary extension of the seeing eyecup causes no movement of the other blinded eyecup, although simultaneous oscillation of a striped drum round the crab is seen perfectly. 4. Preventing voluntary extension of the seeing eyecup with a mechanical stop causes no movement of the other, blinded, eyecup. 5. With both eyes blinded, or with the crab in the dark, voluntary or forced movement of one eyecup causes no movement of the other. 6. This behaviour is that expected of an animal which can retract and extend either eyecup in the light or in the dark without disturbing the other, but no simple model is compatible with all the experiments.