i6i6 



IIWDIIlli IK OK I'HYSIO] ' M.'i 



Ml k( il'U\sloI.OGY III 



pig 12. ( Sentral scotoma, sur- 

 rounded by spared peripheral 

 vision, following penetrating mis- 

 sile wound of tlie occipitopolar 

 region of the human brain. I he 

 course of the missile suggested 

 involvement of upper and lower 

 lips of both calcarine fissures. 

 This patient was capable ol 

 distinguishing large forms i tri- 

 angle, circles, squares) presented 

 so that they surrounded the 

 central area of blindness. [From 

 Tcuber tl al. 1469).] 



arc other changes of a more subtle nature which do 

 not conform to any simple point-for-point correlation 

 of structure and function. If one tests those areas of 

 the visual field which lie outside the scotoma, one 

 finds that shape perception is disproportionate!) 

 handicapped by tachistoscopic presentation (33, 386). 

 In the same regions of the field, there are other asso- 

 ciated disturbances: contours may fade more rapidly; 

 fusion thresholds for flickering light are reduced; and 

 there is impairment of dark adaptation, and of per- 

 ception of real and apparent motion. These changes 

 involve the visual field as a whole (23, 294, 470-473); 

 in this respect, these changes are less specific than the 

 scotoma since they appear even in those parts of a 

 visual field which seem intact on routine perimetric 

 testing. However, the delects are restricted to cases 

 with circumscribed visual field delects, a fad suggest- 

 ing that occipital lesions produce twofold effects: 

 those (hat are focal (scotoma), and those that are less 

 focal, involving visual functions over the entire field 

 and main aspects of visual performance. 



COMPLETION V\|) EXTINCTION OK PATTERNS. These 



subtle but persisteni alterations can be manifested l>> 

 additional changes in the mode of functioning of the 

 injured si 1 list raie. Under a v arieiv of conditions, figure 

 processes .in- 'completed 1 across scotomatous regions 

 so that there i- no gap in perception although part of 

 the stimulus figure falls into areas of blindness 1 30, ; ;, 

 70, 157, 386, 469). A characteristic instance ol this 



1 completion or tilling-in process is shown in figure 13. 

 Sin li .1 filling-in is well known for the area ol the 

 normal blind spot i 501 1, bul in thai case, completion 

 is usii.iiu attributed to the fad that there is discon- 

 tinuity in the retina bul urn in the cortex (20). I In 

 argument 1 .1 1 be advanced lor completion of pat- 



terns across scotomata acquired after cerebral lesions. 

 Moreover, even in the transient scotomata of mi- 

 graine (303) or of 'visual fits' (469), completion is 

 regularly observed. The phenomenon is thus a basic 

 feature of perception; it is difficult to reconcile with 

 most perceptual theories which invoke a scanning of 

 the cortical projection areas [e.g. Pitts & McCulloch 

 (381 )] or with those versions of 'field tfieorv' which 

 assume a) that all of the perceptual process takes place 

 in the primary projection field and b) that the percep- 

 tion as such is based on an 'isomorphic process' which 

 takes place within the projection field (271, 281). 

 Either a scanning or a field theory would have to be 

 modified in view of these completion phenomena. 



As the result of completion, defective visual fields 

 may be functionally more extensive than the dis- 

 tribution of plotted scotomata would indicate. A 

 converse process, however, restricts perception in 

 seemingly functioning regions (29). This is the 

 phenomenon of extinction of patterns in certain areas 

 of a defective field. 'Extinction' occurs in mildly 

 impaired areas of the visual field when other rela- 

 tively intact areas are stimulated simultaneously, 

 Thus, a pattern may be seen on exposure to an 

 amblyopic half of a field, as long as this exposure 

 takes place against a relatively homogeneous back- 

 ground. However, as soon as a second pattern of any 

 soil is simultaneously exposed in the other (less 

 impaired) half of the field, the first pattern either 

 becomes less distinct ('obscuration') or vanishes 

 altogether ('extinction') (29). As soon as the pattern 

 in the less impaired pari of the field is removed, 

 the one in the affected field becomes visible. 



I In- phenomenon his man) variants, described by 

 Bender (28). It can occur within a lateral half of a 

 visual field, eg. between its upper and lower quad- 



