Page:Mind (New Series) Volume 12.djvu/497

 PHYSIOLOGICAL FACTORS OF THE ATTENTION-PROCESS. 483 that of the other eye ; (2) that the excitement of the tract of either eye is directly and powerfully re-enforced during activity of the intrinsic muscles of the eye through impulses carried up to the brain by afferent nerves from those muscles. The effects of contractions of the eye-muscles in deter- mining the modes of ambiguous figures are well known. Prof. Titchener points out l that in the case of the staircase- figure reproduced in figure 7, a movement of the point of fixation of the eyes from b to a favours predominance of the step-mode, while movement from a to b favours the broken- wall mode of perception. I find that it is possible after a little practice to associate the reverse change with either of these movements, nevertheless it remains true that the FIG. 8. changes occur most readily in association with movements of the directions pointed out by Titchener. Prof. Leob 2 has pointed out the influence of movements of accommodation in determining the mode of perception of such figures as Necker's cube (fig. 8). If one of the central angles, a or b, is fixated with one eye only, and then accom- modation be slightly increased, that angle tends to appear convex and as the nearest point of the obliquely lying cube ; and if then accommodation be a little relaxed, the angle fixated suddenly recedes and appears as a concave solid angle and the farthest point of the cube. These movements of accommodation render these changes of the mode of per- ception easy and are, as it were, natural to them, but I shall have occasion in a later section to point out that they are by 1 Experimental Psychology, vol. i., pt. ii., p. 312.
 * Pfluger's Arch., Bd. xl.