PHYSIOLOGICAL FACTORS OP THE ATTENTION-PROCESS. 339 brilliant white ground such as a white cloud, leaves a positive after-image in which each square persists as a bright area. On attending to these as vertical rows the squares of each row run together, forming uninterrupted vertical bars of light ; and on attending to them in horizontal grouping they run together to form similar horizontal bars. The two kinds of bars- can then be voluntarily produced alternately by attending to the two groupings in turn. It is noteworthy that when in this way the squares run together to form continuous bars, the bars appear less bright than the squares which brighten again as the bars split up, a fact which suggests that a limited quantity of energy becomes distri- buted through a larger number of sensory elements during the appearance of the bars. In these and a number of other experiments in which this curious effect was observed the essential condition of its pro- duction seemed to be that the spatially separated sensations peripherally excited should strongly suggest some whole figure ; the idea thus suggested then calls into existence the sensations needed to fill the gaps left between the peripher- ally excited sensations. The cerebrally induced light seems in fact to be a simple form of hallucinatory sensation experimentally produced. As in the pathological and the hypnotic hallucina- tion, the idea moulds and modifies the field of sensations, calling into existence such sensations as are needed to complete the perception of the object represented. The influence of the activity of the higher-level neural systems upon the processes of the sensory level, illustrated by the experiments above described, may be briefly summarised as follows. In reproductive imagination the higher-level system in predominant activity at any moment plays down upon the sensory level discharging itself through the efferent limbs of some group of sensori-motor arcs, so exciting the images of sensation which constitute the sensory content of the idea. In normal sense-perception the excitation of some group of sensori-motor arcs of the sensory level strikes into some upper-level system of paths and traversing it, issues in part to the efferent limbs of the same group of sensori-motor arcs thus supporting, re-enforcing and slightly intensifying the group of sensations due to their excitement, in part to the efferent limbs of other arcs of the sensory level in the same and in other sensory areas of the cortex, thus giving rise to images of sensation which enter with the sensations into the psychical synthesis which is the percept. In hallucination the upper-level system excites so strongly the arcs of the sensory level into which it discharges itself that,
