Stereo-thresholds: simultaneity, target proximity and eye movements

Stereo-thresholds are much higher when adjacent targets are presented without temporal overlap than when they are shown simultaneously. Sequentially presented adjacent targets also evoke small involuntary eye movements toward the newly presented target. Neither of these phenomena is evident with widely separated targets; for sequential presentation of targets 10 degrees apart, stereo-thresholds are only slightly higher (a factor of about 1.5) than for simultaneous presentation; and stable fixation can be maintained. If the differing influence of simultaneity on stereoacuity for adjacent and for widely separated targets arises because adjacent alternating targets evoke eye movements, that effect is apparently not mediated exclusively by displacement of retinal images due to the measured eye movements. It could, however, be due to a general long-term instability of fixation associated with repetitive small involuntary eye movements.