Page:Mind (New Series) Volume 15.djvu/362

 348 w. MCDOUGALL : in many respects similar to, and capable of explanation on the same lines as, the fluctuations of the attention during perception of ambiguous figures ; for in some of these cases, notably in the case of the windmill-illusion, peripheral changes certainly play no part in determining the fluctua- tions of attention which, as I have attempted to show above, have as their principal condition alternating fatigue and recovery of tracts of the higher levels of the brain. If we accept the conclusion reached above as to the part played by fatigue of brain-tracts in determining the fluctua- tions of attention to an ambiguous figure through rendering impossible continued perception in any one mode, we may I think apply it generally to the explanation of the fact that normally attention always plays about its objects, passing on from feature to feature of any one object or from object to object, and cannot by any ordinary effort be arrested or fixed upon any one detail for more than a brief moment. The state of consciousness at any moment during attention to any given object has for its neural correlate the flow of nervous energy from the afferent towards the efferent or motor side of the central nervous system through some system of paths which includes paths of the higher brain-levels. Every such system forms part of some larger system or is intimately connected with other systems, and the conjunction of variable conditions, which at any moment determines the main current of energy to strike into one system of paths rather than another, must be regarded as determining in every case the realisation of one among many almost equally possible routes. Of these variable conditions the state of each path as regards degree' of resistance presented by it to the passage of the current is one of principal importance. In all cases this resistance is rapidly increased by the fatigue induced by the process of transmission of the impulse from neurone to neurone across the cell-junctions. The onset of fatigue is more rapid the less canalised the path, for the less canalised the path, the higher the resistance of its cell-junctions, the greater is the consumption of potential energy in the process of trans- mission of the impulse and therefore the more rapid the onset of fatigue. Hence in the higher-level paths of low- degree of canalisation a very brief activity induces sufficient fatigue to turn the balance of conditions in favour of some one of the alternative systems of higher-level paths. Hence the perpetual shifting of the current from one path to an- other and the rapidly succeeding changes of the state of consciousness.