Page:Mind (New Series) Volume 8.djvu/47

 SUBJECTIVE COLOURS AND THE AFTER-IMAGE. 33 its positive effects are due to the fact that, whenever we ' voluntarily ' attend to any peripherally excited conscious, contents, its peripheral excitation is reinforced by a central excitation, and its intensity is thereby actually increased. This effect of increased intensity would naturally be espe- cially well-marked and unmistakable in the case of after- images, where the peripheral process is of but slight intensity. Nothing but an actual reinforcement by central excitation can account for the cutting short or lengthening at will of the colour stages in the after-image. The evidence which these experiments afford of the func- tion of central excitations in attention suggests a criticism of the reason for assuming the existence of a special ' at- tention centre ' somewhere in the frontal lobes. Every function which the attention or apperception centre is sup- posed to perform can be explained on the hypothesis that the organ of attention is the cortex as a whole ; or more definitely, the sum total of those brain centres which are connected functionally according to the laws of association and habit with the centre whose accompanying conscious process is the object attended to. Attention means central reinforcement of an excitation which may be in the first instance either central or peripheral in origin. And this central reinforcement comes not from a single supreme centre, but from associated centres of the same order aa that in which the original excitation takes place. The in- flux of nervous energy which occurs in such a process of course involves increased intensity on the part of the result- ing conscious state. It also involves diminished intensity in the rest of the field of consciousness. For the cortex as. a whole represents a certain limited amount of nervous- energy. When the currents of central excitation are de- termined to a given brain centre or group of centres, the intensity of nervous processes elsewhere is necessarily di- minished, precisely as the electric lights in a trolley car grow dimmer when the car starts. Hence comes the pheno- menon known as abstraction, which surely needs no such hypothesis as that of an inhibitory influence exerted by a special centre. Although attention is thus seen to involve increased in- tensity on the part of the mental state attended to, it does, not follow that the more intense a conscious state is, the greater the degree of attention to it. The increase in in- tensity must be of central, of associative origin. Otherwise an idea could never be attended to in the waking life, unless, it assumed the intensity of an hallucination. It is not the 3