A method was worked out whereby the patellar tendon of a subject could be stimulated at regular intervals and with uniform intensity by a hammer-device, and the intensity of the reflex measured by the height of the resulting kick. At the same time that the subject was being given regular blows on the knee he was engaged in casual conversation with the experimenter, or engrossed in the solving of some difficult problem, or was as passive and inattentive as possible. The intention was to discover what the relation is between muscle-tonus as measured by the knee-jerk and what is commonly called attention. The records show clearly that there is an extremely close relation: the height of the kick was greatest during problem-solving, not nearly so great during conversation, and barely present at all during inattention. The writer states that there are three ways in which this fact may be interpreted, namely, that muscle-tonus is (a) one of the factors in the attentive process, (b) a function of the attentive process, or (c) the process of attention itself. The writer favors the last interpretation. From Psych Bulletin 22:12:00847. (PsycINFO Database Record (c) 2006 APA, all rights reserved)