Page:Mind (New Series) Volume 15.djvu/356

 342 w. MCDOUGALL: the fatigued tract. (4) It has been shown l by continued stimulation of the motor cortex of animals that the cortical tracts are liable to a fatigue which very rapidly comes, on and as rapidly passes away.' 2 The aim of this section is to show that our inability to continue to perceive an object in any one aspect, or more generally, our inability to attend continuously to any object in one way for more than a very brief period of time, is due to just such fatigue of the higher-level paths as is revealed in the paths of the sensory level by the foregoing observations, or in other words, that the perpetual fluctuation of the atten- tion which is one of its most peculiar and constant features and which is but little controllable by any effort of the will, is due to the extreme liability to fatigue of the higher levels of the brain. The evidence on which I chiefly rely is afforded by observations which show that the fluctuations of attention which occur during the perception of ambiguous figures obey in almost all respects the same laws as the alternation of colours in the case of rivalry of two differently coloured fields presented simultaneously to right and left eyes respectively. (1) Firstly, in the struggle of differently coloured fields and in the fluctuations of the mode of attention to ambiguous figures the periods of alternation are of the same order of duration (in my own case from about one half to two seconds) and exhibit similar large differences in different individuals. (2) In both cases, as has been shown above, the predom- inance of one of the alternating phases may be induced and prolonged by appropriate activity of the intrinsic or extrinsic muscles of the eyes, or by a voluntary concentra- tion of attention on one of the phases without muscular activity ; in both cases the prolongation of the appearance of one phase effected in either of these ways is slight, but the influence of voluntary effort is somewhat greater in the case of the ambiguous figures. The change takes place involun- tarily and in spite of every effort of the will to prevent it. (3) Just as the period of dominance of one of two rival colour- fields, prolonged by any means beyond its usual duration, is followed by an unusually long period of dominance of the rival field, so when by any means the period of one mode of perception of an ambiguous figure is abnormally prolonged it is followed by an unusually prolonged period of dominance of the alternative mode of perception, i.e. in both cases the 1 Levi, " Fatigue of Cerebral Cortex," Journal of Physiology, vol. xxvi. cell-junctions or synapses of the cortex (Brain, 1901).
 * I have shown reasons for believing that this fatigue has its seat in the