MOVEMENT, PERCEPTION OF. Psyehologie ally a movement is a consciousness of a con tinuous change of position. It is an idea which is intimately hound up with notions of space and time, or, to speak more strictly, it is an idea which, from the point of view of system. must he treated both as an extensive and as a temporal idea, since every movement must possess a certain extension (q.v.) and a certain rate or duration Our idea of movement is structurally com plex. It consists partly of ideas of an object in different positions, partly of eertain sensations set up in the body. These sensations, both because they are easily aroused by movements, and be eause they serve as the most effective basis for the estimation of movements. have by many writers been grouped together under the terms 'movement sensations,' kimesthetic sensations,' or. more simply, as the 'muscle sense' (q.v.). But a classification of sensations in terms of the stimuli which evoke them, or in terms of the objective processes for which they come to stand, i both inadequate :rod unsatisfactory; to speak of sensations of 'movement' is as unwar rantable as to speak of sensations of 'time. 'weight.' resistanee,"spaee,' etc. As a matter of fact, too, we are now able to isolate the components of the idea of movement psychologic ally and to assign their dependency upon definite bodily processes. There are three qualitatively distinct sensations within the 'muscle sense': (1) the ningele sensation proper, (2) the strain or tendinous sensation, (3) the joint or articular sensation. The latter alone is concerned in our knowledge or estimate of position, and change of position. or movement, of any member of the body. The actual liminal excursion has been found to he least in the case of the larger joints (0.22° to 0.60° for shoulder, hip. and elbow), greatest for the smaller joints 0.50° to 1.30° for knee, linger, and ankle).
Bat our ideas of the extent of movement are not limited to those which refer to members of our body; we can also estimate the extent of the movement of an object felt (skin) or of an object seen (eye). A stimulus moving over the skin excites end-organs of pressure which pos sess different 'local signs.' If the first local sign has not lapsed from consciousness when the last is reached, we are able to estimate the extent of the movement in purely cutaneous terms; otherwise we may make judgments in visual terms. The least noticeable extent of cutaneous movement depends upon the place stim ulated, the intensity of the pressure, and the rate and direction of the motion. On the fore head it may amount to 10 nun. A very slow
movement may pass unnoticed. Movements lengthwise of a limb are less readily noted than movements crosswise. on account of the distribution of the nerve endings in the skin. Movement is often noted before direction of movement, either because the starting point is forgotten or because the judgment 'movement' is more easily aroused than the judgment 'move ment in this direction.' The visual idea of ex tent of movement may be variously formed. (I I If the eyes remain fixed while the object 1110Vt; across the visual field, the estimation results from the stimulation of different local signs in a manner analogous to that of the purely cu taneous estimation. The least noticeable amount of movement is probably about equal to the 'minimum visible.' (See ExTENSIoN.) (2) If the fixation point of the eyes follows the moving object, the estimation of the extent of movement of the object is made in terms of the strain, pressure, and articular sensations evoked by the movements of the eyes in their sockets, of the head upon the shoulders, or of the body as a whole. Without the aid of some fixed point of reference, such as is actually used in 'eye meas urement' (see ExTExsiox) and convergence. es timations of this second type are extremely uncertain, on account of the occurrence of un noticed movements of the eyes themselves.
Turning to the temporal aspect of the per ception of movement, we can say in general that quick movements are more readily noted than slow, whether they appeal to joint, skin, or eye. On the skin a uniform rate in the stimulus is not perceived as a uniform rate in sensation. for a given movement appears more rapid Si here localization is more accurate. The slowest per veld ible visual movement is at the rate of 0.002R inni. per second. In discriminating between two different rates, the optimal speed is rather slow, because rates of movement which are at all quick are confused by the persistence of the exeitation in the form of after-images.
Finally, rate of movement or. more strictly. change in the rate of movement of the boil as a. whole, is pereeptihle, although no estimation of the extent of such a movement is possible. Upon vehicles, like elevators, boats, etc.. where there is little jar, it is easy to observe that, once the speed is uniform, it is impossible to perceive any motion whatsoever: acceleration or diminu ti,m of this rate. however. sets up certain sensa tions due to the inertia of the body, and perhaps, too, other sensations mediated by the semicir cular canals. See STATIC SENSE.