GENERAL CONDITIONS OF BRAIN-ACTIVITY. 91 



tract of the nervous system is, at the moment, in this hair- 

 trigger condition. The consequence is that one sometimes 

 responds to a ivrong signal, especially if it be an impression 

 of the same kind with the signal we expect.* But if by 

 chance we are tired, or the signal is unexjDectedly weak, 

 and we do not react instantly, but only after an express 

 perception that the signal has come, and an express voli- 

 tion, the time becomes quite disproportionately long (a 

 second or more, according to Exner f), and we feel that the 

 process is in nature altogether different. 



In fact, the reaction-time experiments are a case to 

 which we can immediately apply what we have just learned 

 about the summation of stimuli. ' Expectant attention ' is 

 but the subjective name for what objectively is a partial 

 stimulation of a certain pathway, the pathway from the 

 ' centre ' for the signal to that for the discharge. In Chapter 

 XI we shall see that all attention involves excitement from 

 within of the tract concerned in feeling the objects to which 

 attention is given. The tract here is the excito-motor arc 

 about to be traversed. The signal is but the spark from 

 without which touches off a train already laid. The per- 

 formance, under these conditions, exactly resembles any 

 reflex action. The only difference is that whilst, in the 

 ordinarily so-called reflex acts, the reflex arc is a permanent 

 result of organic growth, it is here a transient result of 

 previous cerebral conditions. :{: 



very discrepant figures. . . . This coucentration of the attention is in the 

 highest degree exhausting. After some experiments in which I was con- 

 cerned to get results as uniform as possible, I was covered with perspiration 

 and excessively fatigued although I had sat quietly in my chair all the 

 while." (Exner, loc. cit. vii. 618.) 



* Wundt, Phy.siol. Psych., ii. 226. 



f Pflliger's Archiv, vii. 616. 



X In short, what M. Delboeuf calls an ' organe adventice.' The reaction- 

 time, moreover, is quite compatible with the reaction itself being of a reflex 

 order. Some reflexes (sneezing, e.g.) are very slow\ The only time- 

 measurement of a reflex act in the human subject with which I am 

 acquainted is Exner's measurement of winking (in Pflliger's Archiv f. 

 d. gesammt. Physiol., Bd. viii. p. 526, 1874). He found that when the 

 stimulus was a flash of light it took the wink 0.2168 sec. to occur. A strong 

 electric shock to the cornea shortened the time to 0.0578 sec. The ordinary 

 ' reaction-time ' is mid waj^ between these values. Exner ' reduces ' his times 

 by eliminating the physiological process of conduction. His ' reduced 



