Page:Mind (New Series) Volume 15.djvu/344

330 w. MCDOUGALL : intensifying and detaining sensations in the service of the ' cerebro-ideational ' process." Fechner noted that during the struggle of two differently coloured fields presented to the right and left eyes respec- tively it was possible by an effort of will to favour the pre- dominance of either colour. In explanation of this effect he assumed that the intrinsic muscles of the eye that is stimu- lated by the colour on which attention is concentrated, are innervated more strongly than those of the other eye, and that the predominance of that colour is due to the reinforce- ment of it by afferent impulses from those muscles. 1 In the preceding section I have quoted examples of experiments which prove that the activity of the intrinsic muscles of one eye does actually reinforce and maintain in consciousness the sensation of the colour presented to that eye. In those ex- periments the intrinsic muscles of one eye were paralysed by means of atropine, so that any effort of accommodation brought into action the muscles of one eye only. But it seems improbable that in the normal state of the eyes, the intrinsic muscles of one eye can be more strongly inner- vated than those of the other, as Fechner supposed. How- ever that may be the experiments described in part iii. of this paper 2 seem to prove that the voluntary favouring of one colour is not effected, or at least not wholly effected, in this indirect manner ; for they show that a voluntary effort of attention may favour the predominance of the colour sensation excited through the eye whose intrinsic muscles are completely paralysed. The cerebro-ideational activity involved in a voluntary concentration of attention upon a sensation does, then, in some way, directly support the sensation and tend to maintain it in consciousness. In a similar way voluntary effort may be shown to favour directly any one of the various modes in which an ambiguous figure (such as figure 2, p. 335 3 and figures 7 and 8, p. 483 4 ) may be perceived. In these cases also it has been maintained that voluntary effort produces these effects indirectly only, that it directly effects only the adjustment of the sense-organ arid that it is this adjustment of the sense-organ alone which favours the predominance of one or other mode of perception. Thus it has been said of the staircase figure (fig. 7, p. 482) that, when we voluntarily cause it to appear as a staircase, it is by moving the eye from b to a, and when we cause it to appear as an overhanging broken wall it is by moving the 1 Abhandl. d. sacfis. Ges. d. Wissensch., Bd. v. 2 MIND, N.S., vol. xii., p. 481. 3 Ibid., No. 43, vol. xi. 4 Ibid., No. 48, vol. xii.