Attention itself is ultimately a motor phenomenon. Thus: the sensory aspect of attention is vividness, and vividness is explained physiologically as a brain-state of readiness for motor discharge;<1> in the case of a visual stimulus, for instance, a state of readiness to carry out movements of adjustment to the object; in short, the motor path is open. Now attention, or vividness, is found to fluctuate periodically, so that in a series of objectively equal stimuli, certain ones, regularly recurring, would be more vividly sensed. This is exemplified in the well-known facts of the fluctuation of the threshold of sensation, of the so-called retinal rivalry, and of the subjective rhythmizing of auditory stimuli, already mentioned. There is a natural rhythm of vividness. Here, therefore, in the very conditions of consciousness itself, we have the conditions of rhythm too. The case of subjective motor rhythm would be still clearer, since vividness is only the psychical side of readiness for motor discharge; in other words, increased readiness for motor discharge occurs periodically, giving motor rhythm. <1> Munsterberg, _Grundzuge d. Psychologie_, 1902,. P. 525. It has been said<1> that this periodicity of the brain-wave cannot furnish the necessary condition for rhythm, inasmuch as it is itself a constant, and could at most be applied to a series which was adapted to its own time. But this objection does not fit the facts. The "brain-wave," or "vividness," or attention period, is not a constant, but attaches itself to the contents of consciousness. In other words, it does not function without material. It is itself conditioned by its occasion. In the case of a regularly repeated stimulus, it is simply adjusted to what is there, and out of the series chooses, as it were, one at regular periods.<2> <1> J.B. Miner, "Motor, Visual, and Applied Rhythms," _Psychol. Rev., Mon. Suppl._, No. 21. <2> Facts, too technical for reproduction here, quoted by R.H. Stetson (_Harvard Psychol. Studies_, vol. i, 1902) from Cleghorn's and Hofbauer's experiments seem to be in harmony with this view.