DISCRIMINATIVE RESPONSES TO VISUAL STIMULI 465 



the sensitivity of the sense-organ would also operate to increase 

 the latency of the synapses, and thus to increase the time required 

 for reaction. 



In the special case of vision it has been shown that the retina 

 is anaesthetic during rotational eye-movement, 5 so that if fix- 

 ation is disturbed, a longer time of exposure to the stimulus may 

 be required for the accumulation of enough energy in the various 

 synapses involved to overcome the resistance of the surfaces of 

 separation. 



Increased difficulty of maintaining fixation under a lighting 

 condition which does not permit of stability of adaptation, has 

 been hypostasized by Cobb, 6 and seems to be supported by the 

 self-observation of a number of subjects, although objective 

 evidence, such as photographic records, is lacking. There is a 

 well-established tendency to frequent and excessive changes in 

 the size of the pupil, as the various parts of the retina are succes- 

 sively exposed to brightnesses to which they are not adapted. 

 Great relative instability of the pupillary size may be a source 

 of much discomfort, and it may certainly operate to reduce the 

 sensitivity of the retina. 



Disturbances of accommodation operate in the same way, by 

 tending to equalize the differences in brightness of the various 

 portions of the stimulus as imaged on the retina. 



The latent period of muscular tissue is subject to important 

 modifications from temperature-changes, toxins, etc., especially 

 the toxins resulting from "fatigue." In work such as the present 

 one, these variables can be minimized, as the muscular responses 

 involved are relatively simple, easy and not sustained. 



Inasmuch as discriminative reactions involve more numerous 

 synaptic connections than simple ones, it was assumed that the 

 time required for executing them would be the more sensitive 

 indicator of the influence of external variables. The fact that 



5 Holt, E. B. : Psychol. Rev. Monogr. Supp., iv, 1903; pp. 



8 Cobb, P. W. : Physiological points bearing on glare. Tr. Ilium. Eng. Soc., 

 vi, 1911, pp. 153 ff. ; also, Physiological aspects of illuminating engineering. (Two 

 lectures.) In: Lectures on illuminating engineering, vol. 2. Baltimore, The 

 Johns Hopkins Press, 1911, pp. 559 ff. 



PSTCHOBIOLOQT, VOL. I, NO. 6 



