Coherent motion of random dot pattern across a stationary stereograting at 1–5.5 deg/s causes an impairment in perceiving the stereograting which is associated with optokinetic nystagmus. This study was aimed towards understanding the cause of the impairment. We tested two alternative hypotheses: (1) that the impairment is caused by efferent-afferent interactions and (2) that it is due to a temporal integration process, the effect of which is expressed at the temporal resolution limit of the stereoscopic mechanism to disparity alternation. The first hypothesis was rejected on the basis of modified displays and experimental conditions which clearly showed that in these displays stereopsis was not impaired in the presence of optokinetic nystagmus. In testing the second hypothesis, we first determined, for the original display, the threshold values of spatial frequency and angular velocity at which stereopsis ceased. We found for these values spatial frequency × angular velocity = 7.2 cycles/s, i.e., a constant limiting alternation rate for all angular velocity values tested. The averaging effect at the critical alternation rate was demonstrated by bisecting the display, each part having a different disparity value. The perceived depth levels of the two parts were different and are in accordance with an averaging process explanation. It is, therefore, argued that the cause of the impairment is a temporal integration process which averages the alternating disparity values of the moving do