Visual search for motion-in-depth: stereomotion does not 'pop out' from disparity noise

In a visual search task, targets defined by motion or binocular disparity stand out effortlessly from stationary distractors ('pop-out'), suggesting that target and distractors are processed by different neural mechanisms. We used pop-out to explore whether motion directly toward or away from the observer (z-motion) is detected using binocular motion cues. A target moving laterally (x-motion) popped out amid stationary distractors with binocular disparity, but z-motion did not pop out. However, a small x-motion added to the target's z-motion caused it to pop out. We therefore suggest that the visual system may not be specifically sensitive to binocular motion differences.