Page:Mind (New Series) Volume 15.djvu/363

 PHYSIOLOGICAL FACTORS OF THE ATTENTION-PROCESS. 349 THE INHIBITORY ASPECT OF THE ATTENTION-PROCESS AND THE EECIPROCAL INHIBITIONS OF NEURAL SYSTEMS. In the normal mind the coming to the focus of conscious- ness of one object banishes from the focus any other object ; as the attention is drawn to, or given to, a new object it is withdrawn from its object of the preceding moment. And in proportion as attention is keenly concentrated upon one topic, or system of related objects, it is more difficult for any other object to attract the attention to itself, so that even the most interesting objects which affect the sense-organs under the most favourable external conditions may fail to be noticed and the most violent sense-stimuli may excite only marginal sensations. These are facts of common experience and there is no need to dwell upon them. We can hardly doubt that this singleness of the focus of consciousness and this mutual exclusiveness of the objects of attention have their parallels in the neural processes of the brain-cortex, and without necessarily accepting psycho-physical parallelism or epiphenomenalism, one may be inclined to believe that this singleness and narrowness of the field of attention is due, not to the nature of consciousness or of the soul, but rather to the peculiarities of the neural mechanism by which our conscious life is so largely conditioned. But, whatever view one may be inclined to hold, it is clear that considera- tions of sound method compel us to seek to conceive the neural mechanism in such a way that these peculiarities of the attention-process shall have their neural counterparts. In terms of the scheme of the neural mechanism adopted in these papers, the corresponding physiological facts would seem to be that the activity of any one system of higher- level neurones or neural arcs is incompatible with the simul- taneous activity of any other, and that the more intense the activity of any one such system the more difficult is it for processes of the sensory level to propagate themselves into any other upper-level system ; that the coming into activity of any one such system brings to a stop the activity of any other system and that its activity tends to prevent any other coming into activity ; in other words, there obtains a rela- tion of reciprocal inhibition between any two such systems, no matter how widely separated in the brain they may be and how different their functions. Physiologists are not agreed as to the nature of the processes of inhibition in the central nervous system. In an earlier