Vergence and accommodative responses in viewing near objects in the real world are behaviorally coupled to maintain clarity and singularity for the object of regard. However virtual stereoscopic stimuli, such as in 3D displays, create non-normal coupling that may cause improper vergence and accommodative responses, possibly resulting in visual discomfort. The present study examined whether the dynamic aspect of current 3D displays is the underlying cause of visual and physical discomfort. To this end, subjects' vergence and accommodative responses were measured while they tracked an approaching 2D or 3D target, and while watching a 2D or 3D movie. The tracking target either moved in steps or continuously, and it was either clear or intentionally blurred. Results show that convergence insufficiency and improper accommodation were greater when a 3D target was moving continuously toward the near position compared to a 2D target and a 3D stimulus moving in steps. Clear targets also resulted in greater vergence and accommodative responses than blurred targets. Viewing 3D movie resulted in greater vergence and accommodation, as well as more severe vision- and motion-related discomfort than 2D movie. These findings suggest that with 3D displays, disparity-induced vision difficulty and internal conflicts cause perceived visual and motion-related discomfort. Characteristics of 3D stimuli, such as the frequency and amplitude of target motion, likely critically affect the severity of reported discomfort symptoms.© (2011) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.