Page:Mind (New Series) Volume 12.djvu/488

 474 W. MCDOUGALL I aim is therefore, in the first place, to show that the physio- logical changes underlying these abrupt changes of conscious- ness are essentially similar in character in all these groups of cases, for to do this is to justify the scheme of the physiological processes underlying the states and the movements of atten- tion that was outlined in part i. of this paper. Secondly, assuming the essential similarity of the physiological processes in all these varieties of phenomena, I have studied the simpler varieties because it seems probable that, if we can gain in- sight into the physiological processes in these simpler cases, we shall at the same time gain insight into those underlying the true states and movements of Attention, and in fact into the physiology of mental activity in general. I have studied especially the effects upon these allied processes (1) of activity of the muscles of the sense-organs, (2) of fatigue, (3) of re- enforcement by the idea (including under this vague phrase the effects of voluntary preperception or expectation), and (4) the evidences of reciprocal inhibitions. As regards the influence of activity of the eye-muscles upon the complete fading of a steadily fixated image, I have not been able to devise any form of experiment that will unfailingly demonstrate the effect, because it is im- possible altogether to avoid slight waverings of fixation save for very brief periods. I can only assert with a con- fidence drawn from a considerable experience that though complete fading may occur while both intrinsic and extrinsic eye-muscles are contracted as in convergence with accom- modation, yet it occurs far more readily when all the muscles are as far as possible relaxed and at rest. In the case of after-images the disturbing effects of waver- ings of fixation are absent, but in all other respects the conditions of the after-sensation are similar to those of the sensation itself (as I have endeavoured to prove 1 ), namely a continuous excitation of nerve-endings in the retina by chemical substances. That after-images, besides undergoing changes of brightness and colour, are apt suddenly to dis- appear and reappear is a familiar fact and the causes of these changes have been discussed by many authors. It has been frequently asserted that movements of the eyeballs tend to cause disappearance of an after-image, and this is certainly true in the case of after-images projected upon surfaces that present any appreciable details of texture. Prof. Exner 2: argues that in such cases the disappearance of the after- image is due to the attention being diverted from it and. 1 MIND, vol. x. 2 Zeitschrift f. Psychologic, Bd. i., S. 47..