1624 HANDBOOK OF PHYSIOLOGY ^> NEUROPHYSIOLOGY III 



I^H ; I 



no. 20. Test photographs presented to chicks reared in light coming from below their cage. 

 Such birds perk predominantly at the right section of the photograph. Control chicks reared in light 

 coming from above their cage peck at the left section of the photograph. The two sections were pre- 

 pared from the same negative. [From Hess (an).] 



surface") and that he introduced numerous analytic 

 experiments into the study of depth perception. 



It is in fact possible to isolate individual cues or to 

 provide conflicting ones, but it is equally clear that 

 cues are not additive in any simple fashion, but 

 interact as parameters of depth. Moreover, the 

 evidence already presented (in the discussion of 

 pattern vision) indicates that a positive depth im- 

 pression (without definable distance) is obtained 

 in a diffusely illuminated sphere (without visible 

 microstructure — the Ganzfeld), and while considerable 

 practice is needed in man for adequate estimates of 

 distances, depth as such might well be an immedi- 

 ately given aspect of a visual scene, just as it is char- 

 acteristic of auditory and tactile-kinesthetic space. 



Thus, chicks show visually guided pecking responses 

 to grain which have considerable accuracy immedi- 

 ately after hatching [see Hess (21 1 )]. The response can 

 be molded, during the first month of their lives, by 

 providing unusual cues e.g. I>\ illuminating their 



cage from below (sec fig. 20), ('hicks so reared will 



peck .11 photographs of grain in which the shadows 



ibove rathei than below the image of the grain. 



Still, tin- confusion of photographs of grain with real 



grain attests to the compelling nature of the pictorial 

 representation. In man too, compelling depth can be 

 obtained by reproducing photographs of a scene on a 

 large screen (diminishing thereby the opportunity for 

 comparison with Teal' tridimensional objects), or by 

 looking at a small photograph under a high-power 

 wide-angle lens (413). 



The depth effects obtained from paintings and 

 photographs, in the absence of binocular disparity, 

 are particularly important because they show that 

 two other traditional 'cues' of depth — those of accom- 

 modation and convergence- are far from indispensa- 

 ble. Attempts at defining the role of these monocular 

 cues [clearly invoked ahead) l>\ Berkeley I |8) in 

 1709], can furnish particularly striking examples for 

 the artificiality of univariate experimentation: the 

 effort .11 isolating a single set of 'proprioceptive cues' 

 introduces ambiguity into perceptual situations which, 

 under normal conditions, are always multidetermined 

 [see Woodworth & Schlosberg (549, pp. 475-480) for 

 review and critique]. Undoubtedly, both accommoda- 

 tion and con\ergenc<' may furnish information about 

 relative distance (53), but this information serves 

 only over a rather short range of distances (probably 



