It is almost impossible to take up a discussion of the time relations of neural processes without having to read of physiological and psychological time.  The time of nerve transmission, we are told, is pure physiological time and has nothing whatever to do with psychic processes; the time occupied by the changes in brain centers is, on the contrary, psychological time.  At the very beginning of my discussion of this subject I wish to have it clearly understood that I make no such distinction.  If one phase of the neural process be called physiological time, with as good reason may all be so named.  I prefer, therefore, to speak of the time relations of the neural process.

Of the value of reaction-time studies, one may well believe that it lies chiefly in the way of approach which they open to the understanding of the biological significance of the nervous system.  Certainly they are not important as giving us knowledge of the time of perception, cognition, or association, except in so far as we discover the relations of these various processes and the conditions under which they occur most satisfactorily.  To determine how this or that factor in the environment influences the activities of the nervous system, and in what way system may be adjusted to system or part-process to whole, is the task of the reaction-time investigator.

The problems of reaction time naturally fall within three classes:  Those which deal with (1) nerve transmission rates; (2) the time relations of the spinal center activities, and (3) brain processes.  Within each of these groups there are innumerable special problems for the comparative physiologist or psychologist.  Under class 1, for instance, there is the determining of the rates of impulse transmission in the sensory and the motor nerves, (a) for a variety of stimuli, (b) for different strengths of each stimulus, (c) for different conditions of temperature, moisture, nourishment, fatigue, etc., in case of each stimulus, (d) and all this for hundreds of representative animals.  From this it is clear that lines of work are not lacking.

Closely related to these problems of rate of transmission are certain fundamental problems concerning the nature of the nerve impulse or wave.  Whether there is a nerve wave, the reaction-time worker has as favorable an opportunity to determine as anyone, and we have a right to expect him to do something along this line.  The relations of the form of the nerve impulse to the rhythm of vital action, to fatigue and to inhibition are awaiting investigation.  Some of the most important unsettled points of psychology depend upon those aspects of neural activities which we ordinarily refer to as phenomena of inhibition, and which the psychologist is helpless to explain so long as the physiological basis and conditions are not known.