XIII 



SOME ASPECTS OF STEREOPSIS 



In this chapter we present some purely formal considerations of 

 the visual perception of space without providing any specific neural 

 mechanism. Such a formal analysis is, of course, a necessary pre- 

 liminary since evidently when we do not know in advance the struc- 

 ture of the neural mechanism involved we must know what is re- 

 quired of it if we are to deduce the structure. 



The structure of subjective space is developed gradually during 

 the life of the individual and is the resultant of diverse sensory cues — 

 visual, auditory, kinaesthetic, and perhaps others. The recognition 

 of two pin-pricks simultaneously applied to different parts of the 

 body, as distinct, involves discrimination of a certain primitive type 

 and requires a certain minimal separation of the points; to recog- 

 nize that one pin-prick is located at such a distance and in such a di- 

 rection from the other involves a judgment much more advanced in 

 form and requires a neural mechanism of much greater complication. 

 To account for the first judgment no assumption is required beyond 

 the distinctness of the neural pathways, and of the cortical centers 

 ultimately affected by the two pricks. But the second judgment, while 

 keeping the pricks distinct, assimilates them into a certain continuum 

 and hence relates them in a definite way. Consequently the cortical 

 centers must be connected with each other and with the motor cen- 

 ters in some definite way, perhaps in such a way as to make possible 

 the continuous movement of, say, an index finger from the location 

 of one prick to that of the other. 



Similar remarks may be made of vision. Let us confine ourselves 

 for the present to monocular vision. The judgment that two objects, 

 seen simultaneously, are distinct, and the judgment as to their rela- 

 tive positions, are quite different judgments and the first does not by 

 any means imply the second. The second judgment may be somehow 

 associated with the ocular rotations that would be necessary for the 

 fixating of first the one point and then the other (cf. Douglas, 1932), 

 in which case if the visual space has been integrated into a unified 

 space of perception as a whole, there may be associations of some sort 

 with movements of the body or of a member from the one point to 

 the other. In either case, however, that of the pin-pricks or that of 

 the visual objects, we say nothing about whether the motor and kin- 

 aesthetic connections are a part of the native endowment of the or- 



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