NOVEMBEB 16, 1906.] 



SCIENCE. 



617 



faculty of the nervous system as it presents 

 itself to us in, so to say, the finished condi- 

 tion. We have now accumulated material 

 which contains evidence of a continuous 

 and great increase of this faculty if the 

 experimenter perseveres in subdividing and 

 varying the conditioned stimulus, and 

 thereby makes it coincide with the uncon- 

 ditioned stimulus. Here, again, is a new 

 field of enormous extent. In this material 

 relative to the conditioned stimuli there 

 are not a few cases in which an evident con- 

 nection between the effect and the intensity 

 of a stimulus can be seen. As soon as a 

 temperature of 50° C. had begun to induce 

 a flow of saliva it was found that even a 

 temperature of 30° C. had a similar effect 

 but in a much less degree. Trial was then 

 made of combinations consisting of stimuli 

 of the same kind and also of stimuli of dif- 

 ferent kinds. The simplest example is a 

 combination of different musical notes, such 

 as a harmonic chord, which consists of three 

 notes. "When this is employed as a con- 

 ditioned stimulus each two notes together 

 and each separate note of the chord produce 

 an effect, but the notes played two and two 

 together accomplish less than the whole, 

 and the notes played separately accomplish 

 less than those played in pairs. The case 

 becomes more complicated when we employ 

 as a conditioned stimulus a combination of 

 stimuli of different kinds, that is, of stimuli 

 acting upon different kinds of susceptible 

 surfaces. Only a few of such combinations 

 have been provisionally experimented with. 

 In these cases for the most part one of the 

 stimuli was a conditioned stimulus. In a 

 combination in which rubbing and cold 

 were employed the former was preponder- 

 ant as a conditioned stimulus while the ap- 

 plication of cold taken by itself produces 

 a hardly perceptible effect. But if an at- 

 tempt is made to convert the weaker stim- 

 ulus separately into a conditioned stimulus 

 it soon becomes an energetic conditioned 



stimulus. If we now apply the two stimuli 

 together we have before us an evident case 

 of them acting in combination. The fol- 

 lowing problem had for its object to ex- 

 plain what happens to an active-conditioned 

 stimulus when a new stimulus is added to 

 it. In the cases that were examined, the 

 action of the preexisting conditioned stim- 

 ulus was hindered when a new stimulus 

 of a like kind was added to it. A new odor 

 of a like kind hindered the operation of 

 another odor which was already acting as 

 a conditioned stimulus ; a new musical note 

 similarly hindered the operation of the note 

 previously employed which was a condi- 

 tioned stimulus. After a conditioned stim- 

 ulus had been applied, together with an- 

 other one which inhibited its action, the 

 action of the first one alone was greatly 

 weakened and sometimes even stopped alto- 

 gether. This is either an after-effect of the 

 inhibiting stimulus which was added or it 

 is the obliteration of the conditioned reflex, 

 because in the experiment of the added 

 stimulus the conditioned reflex is not 

 strengthened by the unconditioned reflex. 

 The inhibition of the conditioned reflex is 

 also observed in the converse case. When 

 you have a combination of stimuli acting 

 as a conditioned stimulus — in which, as has 

 been already stated, one of the stimuli by 

 itself produces almost no effect — frequent 

 repetition of the powerful stimulus by itself 

 without the other one leads to a powerful 

 inhibition of its action, even to the extent 

 of its action being almost destroyed. The 

 relative magnitudes of all these manifesta- 

 tions of stimulation and inhibition have a 

 very close connection with their dependence 

 on the conditions under which they origi- 

 nate. 



Experiments have been made in the 

 production of conditioned reflexes by 

 traces or latent remnants both of a con- 

 ditioned and of an unconditioned stim- 

 ulus. The method was that a conditioned 



