PERCEPTION 



1623 



reduction of flicker-fusion, in such amblyopic eyes. 

 These results show again that subtle alterations in 

 perceptual function may be elicited when multiple 

 or recurrent stimulation (permitting stimulus inter- 

 action) is employed (465, 469 ). 17 



Another defect in strabismic amblyopia which 

 suggests suppression had been discovered much 

 earlier by Harms (181). The pupillomotor effect of 

 light cast suddenly on the fovea of such an amblyopic 

 eye is distinctly less than that in the normal eye; 

 more curiously, the diminished pupillary constriction 

 occurs alternately in patients with alternating squint, 

 the pupil of the momentarily deviated eye responds 

 less. Extending these observations to normal eyes, 

 Barany & Hallden (18) were able to show that the 

 same suppression of pupillary response occurs during 

 retinal rivalry (induced by presenting conflicting 

 contours to the two eyes of normal observers). These 

 results again suggest that retinal suppression takes it^ 

 effect all the way 'downstream' in the visual system 



c) A final difficulty in the interpretation of depriva- 

 tion studies is the possible general (nonspecific) 

 retardation induced by such drastic changes in an 

 animal's early environment. Hebb himself (188) has 

 argued forcefully for the role of impoverished en- 

 vironments during infancy in producing generalized 

 emotional and intellectual deficits in later life. A 

 number of studies stimulated by him have tended to 

 confirm this view [e.g. those of Forgays & Forgays 

 (129), Forgus (130), Hymovitch (233) and Thompson 

 & Heron (483)]. In line with these experiments are 

 the studies by Axelrod (i(>) which proved that 

 blindness of early onset in children produced subtle 

 but consistent deficits on nonvisual (auditory and 

 tactile) tasks; there also were abnormal difficulties of 

 intermodal transfer (from tactile tasks to their auditor) 

 analogues, and conversely). These deficits were less 

 marked or absent in children who had lost their sight 

 later in life. Some care had been taken in this study to 

 provide perceptual tasks which were not spatial in 

 any obvious way. Earlier work on congenitally blind 

 children had stressed a possible impairment on tasks 



17 Wald & Burian 1514) invoke the results of occipital 

 lobectomy in monkeys as an evolutionary analogue for their 

 separation of lower from higher visual functions, since such 

 monkeys "lose virtually all capacity for pattern vision while 

 retaining sensitivity to light, brightness perception, and visual 

 space localization." However, Kluver (261) has demonstrated 

 that residual light perception in the monkey consists of a 

 response to total luminous flux. This mode of response has 

 never been observed in the foveal regions of an intact animal; 

 it is to be distinguished from brightness discrimination in the 

 usual sense (see below, pp.1652-1653). 



invoking the manipulation of objects in space 

 (109). 



The discussion of deprivation experiments, and of 

 the related results of recombination studies (e.g. 

 perception with inverted visual fields), will be resumed 

 after reviewing spatial organization, movement 

 perception and perceptual constancies. 



DEPTH, DISTANCE AND OTHER ASPECTS 

 OF SPATIAL LOCALIZATION 



As we shall see, the problem of depth perception is 

 intimately connected with that of motion perception 

 and with the perceiver's ability to apprehend a rigid 

 environment during self-induced movement. Never- 

 theless, the traditional approach to depth perception 

 reflects what has been called the 'geometric fallacv' 

 (154). It is assumed that the observer be motionless, 

 that his exes receive passively the images of objects 

 located at various distances, and that the question 

 should be how a two-dimensional arrav (the retinal 

 'image') could carrv enough information to con- 

 stitute a three-dimensional scene. 



The Traditional Approach {Depth from Clues) and 

 //v Alternative (Depth from Gradients) 



The traditional answer has been that the two- 

 dimensional retinal image is somehow reinterpreted 

 b) means of clues or cues for depth. Here the word 

 'clue' bears a frankly intellectualist connotation, 

 while •cues of depth' denote individual mechanisms 

 called onto the stage on various occasions, often 

 unbeknown to the perccivcr. These clues or cues for 

 depth are usually divided into those available to a 

 one-eyed observer (monocular cues) and those 

 dependent on binocular parallax. 



\k i\. >< 1 1 \k 1 i 1 is Monoculai determinants include 

 relative size of 'familiar' objects (the farther, the 

 smaller), relative clearness, shading [see, for example, 

 von Fieandt (497)], and interposition in which one 

 contour is partly obliterated by another in front of it 

 [see Ratoosh (395) for a mathematical formulation]. 

 All these cues are clearly derived from the art of 

 perspective painting, an invention of the Renaissance, 

 where a stationary two-dimensional picture is so 

 arranged as to give the illusion of three-dimensionality. 

 It is thus not without reason that Leonardo da Vinci 

 in his Paragone, Trat. 35, cited by Richter (316), 

 insisted on calling painting a science ("which shows 

 . . . wide landscapes with distant horizons on a flat 



