PER CEPTION OF A CTIVE MO VEMENT. 1017 



Amount of active movement, its extent and direction, is under 

 many circumstances erroneously judged. A subject desired to compress 

 a dynamometer half a dozen times in succession, with all his strength, is 

 surprised to find he has not exerted the same force at last as at first. 1 

 In Loeb's 2 experiments, one arm was required to perform a movement 

 of similar extent and direction to a movement simultaneously performed 

 by the other, but from an initial position asymmetrical with the latter's. 

 The discrepancy between the movement executed and that judged to 

 have been made, illustrates the shortcomings of the muscular sense 

 under certain conditions. Loeb found the discrepancy in extent and 

 direction between the willed movement and the executed movement, as 

 shown by the difference between the copying movement and the copied 

 movement, vary proportionately with the degree of shortening of the 

 muscle at the commencement of the copying act. The shorter the 

 active muscle already was at outset of the movement, the greater the 

 deficit in its extent of performance, as compared with the intended 

 movement, as measured by that of the movement copied, in other words, 

 the more its actual performance was over-valued ; and conversely. He 

 found that analogous conditions similarly distort the perception of 

 direction of movement. 



In view of observations by E. H. Hering and myself, 3 it would be preferable 

 were these results to be stated, not in terms of degrees of shortening of a muscle, 

 but in distance from limit of movement in a particular direction. Thus the 

 sensible effects of lengthening (relaxation) of muscles would be included as well 

 as those of shortening (contraction). From Loeb's research the initial position 

 of the joint appears of importance from the point of view of active (willed) move- 

 ment, though Goldscheider showed it affects little the sensible limen of passive 

 movement. An illusion studied by Loeb is this : from symmetrical points the 

 two hands are moved simultaneously, horizontally in the same direction, e.g. 

 both toward the right, with the intention that both should traverse the same 

 rectilinear distance ; the hand moving inward toward the median plane of the 

 body is then found to traverse a greater distance than the other. The high 

 delicacy of perception of movement, both passive and active, revealed by 

 Goldscheider's work, contrasts with the large errors of estimation of amount 

 and direction of movement instanced by such illusions as these. This argues 

 for existence of a sensation of movement per se as an elemental product of 

 the muscular sense, and not as a judgment based on comparison of discrete 

 sensations of position. The extent of the illusions seems partly due to 

 simultaneity being a relatively disadvantageous condition for comparison 

 between perceptions. When the willed movement is a successive, not a 

 simultaneous, imitation, the error is not so great. The error varies con- 

 siderably with the individual. Also in Loeb's illusions there is the bias given 

 to the judgment by the intention to make the movements equal. But the lack 

 of definiteness of the muscular sensations received is strikingly illustrated. 4 



A person stands, eyes shut, facing a vertical screen. A and B are two 

 points on the screen that are placed symmetrically with regard to the median 

 vertical plane of the body, and each about 25 cms. from that plane. The 

 vertical heights of the two points are the same. The person has to outline, 

 eyes shut, two similar simple figures, simultaneously with both hands. The 

 hands are to be moved simultaneously in the same direction, e.g. both to right, 

 and the figures are to be made as nearly as possible identical. The left hand 

 starts from point A, the right from B. Ascending or descending parts of the 



1 Delboeuf, Rev. phiL, Paris, 1881, tome xii. p. 526. 



2 Arch.f. d. ges. Physiol., Bonn, 1890, Bd. xli. ; 1892, Bd. xlvi. 



3 Ibid., 1897, Bd. Ixviii. 4 Ibid., 1892, Bd. xlvi. 



