Binocular rivalry is the alternating percept that can result when the two eyes see different scenes. Recent psychophysical evidence supports an account for one component of binocular rivalry similar to that for other bistable percepts. We test the hypothesisl9, 16, 18 that alternation can be generated by competition between top(cid:173) down cortical explanations for the inputs, rather than by direct competition between the inputs. Recent neurophysiological ev(cid:173) idence shows that some binocular neurons are modulated with the changing percept; others are not, even if they are selective be(cid:173) tween the stimuli presented to the eyes. We extend our model to a hierarchy to address these effects.