204 SECTIONAL ADDRESSES 



the seen size of an object is similarly a product not only of size of retinal 

 stimulation but also of the perceived distance of the object, and that seen 

 brightness is not a product only of the brightness of the retinal image but 

 also of the perceived illumination of the object (being greater if the object 

 is seen to be shadowed and less if it is seen to be strongly illuminated). 

 Such facts as these show that a visual characteristic of an object is not 

 (as we should expect on the transmission theory) a product only of the 

 corresponding local stimulation of the retina or of a projection of this 

 local stimulation on a corresponding area of the visual cortex. We must 

 rather regard it as the product of the combined action of different activities 

 of the visual cortex which also may make their contributions to other 

 characteristics of the perceptual field. 



Thus apparent size is not the product only of size of retinal stimulation 

 but is determined also by those cortical activities which give us the 

 perception of depth (such as those aroused by binocular disparity of 

 retinal images). If we do not accept the theory of ' perceptual judg- 

 ments,' we must conclude that such factors as binocular cues to distance 

 affect seen size as directly as they affect seen distance. In the same way, 

 we must suppose that sensory cues indicating illumination and illumination 

 gradients affect the seen brightnesses of objects as directly as do the 

 brightnesses of their retinal images. 



We are led then to think of the visual cortex to some extent as acting 

 as a whole in determining the properties of parts of the visual perceptual 

 field. This whole activity is not, however, confined to the visual cortex 

 since we perceive visually characteristics of objects whose sensory origin 

 is not visual. Thus we see the surface of the table as smooth and hard, 

 that of a carpet as rough and soft, a slug as slimy, and so on. These 

 appearances certainly form part of our visual world and no effort of ours 

 can get rid of them, although they are appearances referring to properties 

 which come from other sense organs than the eye. Their appearance in 

 the visual world can be best explained by supposing that not only do 

 various parts of the visual cortex contribute to the visual appearance of a 

 particular object, but also sensory areas of the cerebral cortex other than 

 the visual areas. 



The converse fact that visual cues may contribute to other sense modali- 

 ties than the visual is shown by the size-weight illusion. If we ask a 

 subject to compare the weights of two canisters, a small and a large one, 

 both weighing fifty grams, he reports that the smaller one has the greater 

 apparent weight. The sense of resistance has, therefore, been determined 

 not only by the sensory impulses from the muscles and joints but also by 

 visual cues. Again we must remind ourselves (as in all the appearances 

 which are here being discussed) that we are dealing with the relative 

 phenomenal weights of the two canisters and not with judgments about 

 their actual weights. If the subject puts the two canisters on opposite 

 sides of a balance he will see that their weights are equal, so he will no 

 longer judge that they are unequal. They will, however, still feel unequal 

 to him. The phenomenal difference in weight contributed by the visual 

 factor does not disappear when he has correct knowledge of the actual 

 weights. This contribution from vision is a genuine determinant of 

 phenomenal weight. 



