DISCRIMINATIVE RESPONSES TO VISUAL STIMULI 463 



DESCRIPTION OF THE METHOD ADOPTED 



The use of the time required for visual discrimination as an 

 index of the appropriateness of a lighting condition implies that 

 the time required for this form of reaction will be shorter and 

 the dispersion of the individual reactions will be smaller under 

 the more favorable lighting conditions than under the less favor- 

 able ones. As some students of illuminating engineering may 

 not immediately see the justification for this assumption, certain 

 elementary physiological doctrines will be recalled. 



When a stimulus adequate to excite a sensory-motor response 

 is presented to an organism, certain physiological changes occur, 

 which for simplicity's sake are described as if they occurred 

 within the fictitious "reflex arc. 7 ' These changes are: 



1. An electrical or chemical process is excited in the end-organ, 

 or receptor. 



2. An afferent neural impulse is transmitted to some selective 

 ganglion. This may lie in the brain, the spinal cord, or some 

 plexus. 



3. An efferent neural impulse is transmitted to an effector, 

 i.e., a muscle or gland. 



4. The effector responds by secretion, contraction or change 

 in tonicity. 



5. An afferent neural impulse, originating in receptors con- 

 tiguous to the effector, is transmitted to some center, and thence, 

 as an efferent impulse, to the same or some other effector. 



All except possibly the last of the above series of physiological 

 changes are regarded as a necessary condition of a change in 

 "consciousness." The most satisfactory hypothesis of mind- 

 body relationship, namely, the hypothesis of "functional corre- 

 lation," assumes that these physiological changes occur simul- 

 taneously with the changes in "consciousness." 



In the special case of responses to visual stimuli, the excita- 

 bility of the receptors i.e., the rods and cones of the retina- 

 is greatly modified by the character of fixation, of accommodation 

 and of adaptation. All these factors may be influenced by the 

 distribution of brightnesses over the visual field. If the distri- 



