Page:Mind (New Series) Volume 15.djvu/354

340 rendering them paths of low resistance, it determines a redistribution of the energy coming in from the sense-organs, diverting it from its normal paths in the sensory area to these arcs, so giving rise to sensations which normally would not be excited by the objects affecting the sense-organs at the moment of hallucination and partially or wholly suppressing others which normally would be excited by them. When we voluntarily re-enforce one group of sensations and determine their predominance over others or their appearance in some significant grouping, we do this in part by bringing about suitable adjustments of the sense-organs, but principally by re-enforcing the excitement of a neural system which plays down upon the arcs of the sensory level; this re-enforcement of the excitement of such a neural system is the immediate physiological effect of voluntary concentration of attention, of all volition. We must briefly consider it later.

In an earlier paper I have tried to show that the rapid alternation in consciousness of two colours, when differently coloured fields are presented simultaneously to the two eyes, as when one looks at a white card with a red glass before the right eye and a blue glass before the left eye, is due to fatigue and inhibition. The red rays falling on the retina of the right eye excite a certain sensory tract or chain of neurones, R, in the visual area of the occipital cortex, the blue rays falling on the retina of the left eye excite a different chain of neurones, B, in the same area of the cortex. If both rays, and the corresponding excitations which they initiate, are of low intensity, the two excitation-processes may be propagated simultaneously through the tracts R and B, and the subject then sees the surface looked at as a purple surface, there is binocular fusion of the two colour-fields. If the two rays are of higher intensity the tracts R and B function alternately; B ceases to function while E is active and conversely, and the subject sees the surface alternately red and blue.

The cessation of activity of tract B while the blue rays continue to fall on the retina is clearly due in some way to the influence of the activity of tract A, and conversely; the two tracts are in a relation of reciprocal inhibition such that the inhibitory effect is only complete when the intensity of