SECTION J.— PSYCHOLOGY. 



EYE AND BRAIN AS FACTORS 

 IN VISUAL PERCEPTION 



ADDRESS BY 



R. H. THOULESS, M.A., Ph.D., 



PRESIDENT OF THE SECTION. 



The title that I have chosen for my address describes a larger field than 

 it is possible to deal with adequately in a single hour. My aim to-day is 

 to consider only some aspects of this question, illustrating my remarks 

 by reference to those problems of visual perception with which I happen 

 to have been most closely concerned. 



That we see with our eyes is known to everyone and has been known 

 for a long time. That we see also with our brains is less generally realised, 

 and the implications of this fact are relatively recent importations into 

 the theory of vision. The full statement of the physiological mechanism 

 of vision would include not only the sensitive retinal surface and the 

 visual areas of the cortex but the whole system which includes retina, 

 optic nerve, visual area of the cerebral cortex, and other sensory areas of 

 the brain as well. 



I. The Transmission Theory of Vision. 



It is possible, of course, to study vision in such a way that everything 

 except the activity of the retina is neglected altogether or relegated to a 

 secondary position, and it was in this way that the scientific study of 

 vision began. This is the point of view which we find in the work of 

 Helmholtz and in much of the experimental research into vision which 

 has followed his deservedly great authority. The basic assumption is 

 that the essential process of vision is the formation of an optical image 

 on the retina and its transmission to the visual centres of the brain by 

 means of the optic nerve. Differences between the sensations transmitted 

 to the brain and the finished perception which appears in experience 

 were attributed to the action of the higher processes of judgment and the 

 influence of past experience. 



This theory of vision, which we may call the ' transmission theory,' 

 has behind it not only the weight of the authority of the great originators 

 of the experimental study of vision. It has also the advantage of being 

 the view of the man in the street. Its truth seems to many to be so 

 axiomatic that its denial may have the appearance of wilful paradox. 



It is, nevertheless, now clear that the transmission theory is wrong, 



