VI.

The experimental conditions were next complicated by the introduction of abnormal positions of the eyes, head and whole body. The results of tipping the chin sharply upward or downward and keeping it so fixed during the process of location are given in the following table, which is complete for only three observers:

TABLE VIII.

Observer. Upward Rotation. Downward Rotation. C.E. A.D. M.V. C.E. A.D. M.V. _L_ (50) +43.98 43.98 5.62 +28.32 28.32 5.02 _K_ (50) -33.72 33.72 71.33 +19.49 19.49 55.22 _L_ (20) -39.10 45.90 33.60 -68.65 69.25 25.20 Average: - 9.61 41.20 36.85 -19.94 39.02 28.48 Normal: -64.14 67.08 33.51

The results of rotating the whole body backward through forty-five and ninety degrees are given in the following table:

TABLE IX.

Observer. Rotation of 45°. Rotation of 90°. C.E. A.D. M.V. C.E. A.D. M.V. _B_ (30) + 4.10 24.57 18.56 _D_ (30) +291.03 291.03 61.86 _G_ (50) +266.78 266.78 22.83 +200.16 200.16 11.00 _F_ (60) +116.45 116.45 17.14 - 36.06 36.30 6.29 _J_ (20) +174.30 174.63 30.94 Average: +170.53 174.69 30.66

The errors which appear in these tables are not consistently of the type presented in the well-known rotation of visual planes subjectively determined under conditions of abnormal relations of the head or body in space. When the head is rotated upward on its lateral horizontal axis the average location of the subjective horizon, though still depressed below the true objective, is higher than when rotation takes place in the opposite direction. When the whole body is rotated backward through 45° a positive displacement of large amount takes place in the case of all observers. When the rotation extends to 90°, the body now reclining horizontally but with the head supported in a raised position to allow of free vision, an upward displacement occurs in the case of one of the two observers, and in that of the other a displacement in the opposite direction. When change of position takes place in the head only, the mean variation is decidedly greater if the rotation be upward than if it be downward, its value in the former case being above, in the latter below that of the normal. When the whole body is rotated backward through 45° the mean variation is but slightly greater than under normal conditions; when the rotation is through 90° it is much less. A part of this reduction is probably due to training. In general, it may be said that the disturbance of the normal body relations affects the location of the subjective horizon, but the specific nature and extent of this influence is left obscure by these experiments. The ordinary movements of eyes and head are largely independent of one another, and even when closed the movements of the eyes do not always symmetrically follow those of the head. The variations in the two processes have been measured by Münsterberg and Campbell[1] in reference to a single condition, namely, the relation of attention to and interest in the objects observed to the direction of sight in the closed eyes after movement of the head. But apart from the influence of such secondary elements of ideational origin, there is reason to believe that the mere movement of the head from its normal position on the shoulders up or down, to one side or the other, is accompanied by compensatory motion of the eyes in an opposite direction, which tends to keep the axis of vision nearer to the primary position. When the chin is elevated or depressed, this negative reflex adjustment is more pronounced and constant than when the movement is from side to side. In the majority of cases the retrograde movement of the eyes does not equal the head movement in extent, especially if the latter be extreme.

[1] Münsterberg, H., and Campbell, W.W.: PSYCHOLOGICAL REVIEW, I., 1894, p. 441.

The origin of such compensatory reactions is connected with the permanent relations of the whole bodily organism to the important objects which surround it. The relations of the body to the landscape are fairly fixed. The objects which it is important to watch lie in a belt which is roughly on a horizontal plane with the observing eye. They move or are moved about over the surface of the ground and do not undergo any large vertical displacement. It is of high importance, therefore, that the eye should be capable of continuous observation of such objects through facile response to the stimulus of their visual appearance and movements, in independence of the orientation of the head. There are no such determinate spatial relations between body position and the world of important visual objects in the case of those animals which are immersed in a free medium; and in the organization of the fish and the bird, therefore, one should not expect the development of such free sensory reflexes of the eye in independence of head movements as we know to be characteristic of the higher land vertebrates. In both of the former types the eye is fixed in its socket, movements of the whole head or body becoming the mechanism of adjustment to new objects of observation. In the adjustment of the human eye the reflex determination through sensory stimuli is so facile as to counteract all ordinary movements of the head, the gaze remaining fixed upon the object through a series of minute and rapidly repeated sensory reflexes. When the eyes are closed and no such visual stimuli are presented, similar reflexes take place in response to the movements of the head, mediated possibly by sensations connected with changes in position of the planes of the semicircular canals.