PERCEPTION 



1605 



tivity and transposition, and their implications for 

 studying motion and depth perception, illusions and 

 constancies. To each of these sections, we shall add 

 the corresponding discussions of altered perception 

 following defect, isolation or recombination of parts. 



PERCEPTION OF SHAPE 



Basic Aspects of Figure Processes 



MINIMUM ARTICULATION, ASSIMILATION AND CONTRAST. 



Perhaps the best way to begin a survey of the role of 

 patterning in perception is to consider those rare 

 situations where patterning is minimal. The classical 

 demonstration for this in the visual modality is the 

 homogeneous field, or Ganzfeld [see Metzger (345 1 and 

 KofTka (284)]. The subject looks into a half-sphere 

 which has been painted a uniform gray or while. 11 

 illumination is lowered to the point where the subject 

 no longer sees the texture of the walls of this sphere 

 (its microstructure), lie docs not perceive any surface 

 bul a mist of undefined depth. Similar effects can be 

 obtained by an ingenious variation of the method 

 devised by Hochberg ei <rf. (217). The observer wears 

 eyecaps made from halved table tennis balls tilted 

 over the eyes. Under these conditions, even the view 

 of his own facial structures is excluded. Colored 

 transillumination of these eyecaps leads, in most 

 instances, to a complete fading of any experienced 

 hue, an event postulated by KofTka (284), although 

 brisk eye movements or interruptions of the illumina- 

 tion can transiently restore the impression of color. 

 These Ganzfeld demonstrations suggest that percep- 

 tion is dependent on some articulation of the stimulus 

 array; in fact, absence of such articulation leads in 

 many subjects to visual hallucinations. These observa- 

 tions should be considered in interpreting electro- 

 physiologic studies of retina or cortex where the 

 stimulating fields employed are often devoid of struc- 

 ture. 



Static articulation of the visual field, however, 

 seems insufficient, unless the texture of the field moves 

 relative to the eye. Even during fixation, fine move- 

 ments of the eye transport contours continuously 

 over the retina [see Ratliff & Riggs (394)]. While 

 these involuntary motions do not seem to play the 

 role in the maintenance of acuity that was attributed 

 to them by Marshall & Talbot (340), they neverthe- 

 less seem necessary for the maintenance of vision. 

 Stabilization of the retinal image is followed by dis- 



appearance of contours ^ and the appearance of a 

 mist of undefined depth. 



Curious effects are obtained when a Ganzfeld of the 

 original type (345) is modified by the introduction of 

 a gradient of illumination, so that one lateral half 

 receives more light than the other. If the gradient 

 remains below threshold at any one point, the per- 

 ceptual effect is that of a uniform brightness inter- 

 mediate between those of either side; the gradient as 

 such is not perceived. If, however, a shadow line is 

 cast across the field, separating the lateral halves, 

 each half is immediately seen as possessing a charac- 

 teristic brightness, one darker, the other brighter, 

 with a 'step' along the contour, although each half 

 to either side of it is still seen as uniform within itself. 

 Such a sequence of events illustrates a tendency to- 

 wards assimilation ( homogeneity 1 in the absence of 

 contours, and the opposing role of contrast where 

 contours are formed [see Krech & Crutchficld (293)]. 

 An analogous demonstration is given in figure 4. 



The lateral interactions within the visual field 

 involved in assimilation and contrast can be demon- 

 trated on main phylogenetic levels and may have 

 electrophysiological correlates in the eyes of the 

 horseshoe crab (183). Behavioral demonstrations of 

 contrast effects have been extended throughout the 



vertebrate series from chimpanzees 1 1 71 ) to fish (206), 

 and have been accomplished for such diverse in- 

 vertebrate forms as butterflies [Macroglossum stella- 

 tarum (263)] and scallops [Pectin (495)]. While as- 

 similation and contrast are potent factors in the 

 perception of shapes, they are not sufficient in them- 

 selves to produce shape or figure processes. 



iK, 1 re and GROUND. It was the Danish psychologist 

 Edgar Rubin (40!)) who proposed that a basic aspect 

 in the formation of shapes was their perception as a 

 'figure' against a "ground." He described and illus- 

 trated primitive visual figures as 'standing out' with 

 object character [and hence surface color, according 

 to Katz (249, 251 )] against their backgrounds which 

 seemed to possess film color, and were often seen as if 

 extending behind the figure itself Thus, the contour 

 dividing figure and ground is perceived as belonging 

 to the figure. Numerous attempts were made by 



8 Such stabilization has been achieved in two ways : a) by 

 Ditchburn & Ginsborg (107) who mounted the visual targets on 

 a stem rigidly connected with a contact lens and b) by Riggs 

 et at. (404) who employed an optical system including a 

 front-surface mirror mounted on a contact lens. By either of 

 these methods, image stabilization is accomplished since there 

 is no relative motion between the visual display and the eye. 



