THE RELATION OF STRENGTH OF STIMULUS TO 

 RATE OF LEARNING IN THE CHICK 



By LAWRENCE W. COLE, (The University of Colorado) ] 



From the Harvard Psychological Laboratory 



ONE FIGURE 



The experiments described in this paper were undertaken in 

 order to learn under what strength of stimulus chicks most 

 rapidly learn to make, respectively, an easy, a medium, and a 

 difficult discrimination. Yerkes and Dodson discovered, in the 

 case of the dancing mouse, that when " discrimination is ex- 

 tremely difficult the rapidity of learning at first rapidly increases 

 as the strength of the stimulus is increased from the threshold, 

 but, beyond an intensity of stimulation which is soon reached, 

 it begins to decrease," while when " discrimination is easy, the 

 rapidity of learning increases as the strength of the electrical 

 stimulus is increased from the threshold of stimulation to the 

 point of harmful intensity."' In other words, there appears to 

 be an optimal strength of stimulus for each degree of difficulty 

 of discrimination and the intensity of this optimal stimulus is 

 lesf the more difficult the discrimination which is to be made. 



It was proposed, then, to test the chick's rate of learning to 

 discriminate by a method simiilar to that which had been em- 

 ployed with the dancing mouse. The work was done in the 

 Harvard Psychological Laboratory and my thanks are due to 

 Professor R. M. Yerkes for the plan of the investigation. The 

 method of measuring the units of electrical stimulation and of 

 calibrating the inductorium for that purpose is that of Doctor 

 E. G. Martin of the Harvard Medical School.- The values of 

 stimuli are relative, not absolute. Since the publication of the 

 paper of Yerkes and Dodson, referred to above. Doctor Martin 

 has discovered that certain corrections should be made which 

 were not made for the original calibration published in the 

 Yerkes and Dodson paper. All of the values of stimuli used in 



^ Yerkes, Robert M. and Dodson, John D. The relation of .strength of stimulus 

 to rapidity of habit formation. Jour, of Comp. Neur. and Psycli., 1908, vol. 18, 

 pp. 459-482. 



^ Martin, E. G. A quantitative study of faradic stimulation. I. The variable 

 factors involved. Anier. Jour, of Physiol., vol. 22, pp. 61-74. IL The calibration 

 of the inductorium for break shocks. Ibid., pp. 116-132. 



Ill 



