PERCEPTION 



l6l 5 



fig. 11. Complete loss of 

 peripheral field (bilateral hemi- 

 anopia) with sparing of central 

 vision, following penetrating mis- 

 sile wound of the brain. The 

 course of the projectile is indi- 

 cated in the diagrams below the 

 visual fields in order to show 

 the similarity between this case 

 of "peephole vision' and an 

 earlier one reported by Holmes 

 (218). This patient showed no 

 obvious difficulties with form 

 perception in the remnant of 

 his visual field. [From Teuber 

 ltd. (469).] 



fields, the resiliency of pattern vision as such (which 

 can be mediated by small remnants of the visual 

 cortex) and the subtle but significant changes in 

 patterning after lesions even of moderate m/<- 



Visual patterns can be perceived in cases of very 

 extensive field defect. The two complementary exam- 

 ples shown in figures 1 1 and 12 are taken from a study 

 of visual performance after penetrating gunshot 

 wounds of the brain (469). In the first case, a tunnel 

 field, following a through-and-through bullet wound 

 of the posterior lobe substance, was compatible with 

 grossly normal pattern perception in the centrally 

 located remnant of the field. This resiliency is par- 

 ticularly remarkable since the anatomical arrange- 

 ments in the optic radiations and cortex make it likely 

 that the lesion producing this tunnel field implicated 

 vascular supply to both occipital lobes sparing only 

 the pole. Return of normal pattern vision in the 

 isolated remnant is not easily reconciled with the 

 theory that patterns are recognized by some cortico- 

 cortical interaction between a primary projection field 

 (e.g. here, the visual cortex), and the surrounding 

 'associative' region (e.g. the prestriate cortex). 



Equally remarkable is the capacity, in the second 

 case, illustrated in figure 12, to discriminate large 

 visual patterns (triangles, squares, circles) that are 

 circumscribed around the central island of blindness. 



Such a capacity is specifically denied by certain 

 theories <>l shape perception, such as that of Dculscli 



(103). 



The resiliency oi shape perception in remnants of 

 defective visual fields should not detract from the ob- 

 servation that there are lasting deficits which merit 

 experimental studs. These deficits can be grouped 

 according to their relative specificity (467), i.e. the 

 degree to which they depend on focal lesions in the 

 primary projection system. Thus the scotoma repre- 

 sents a specific and circumscribed loss of shape per- 

 ception, a gap in the substrate corresponding to a gap 

 in the visual field. After the first year following injury, 

 these areas of acquired blindness remain unchanged 

 in outline, position and density (469). The defects are 

 homonymous and similar in the two monocular fields. 

 In spite of their similarity in the two eyes, they turn 

 out to be incongruent on careful plotting, suggesting 

 that the cell populations which represent each retina 

 beyond the thalamus overlap without being identical. 

 This lack of congruence aside, the scotomata reflect, 

 in shape and position, the specific anatomical arrange- 

 ments within the visual pathways. It is difficult to con- 

 ceive of any other consequence of cerebral lesions 

 which would be as static, localizable and cir- 

 cumscribed. 



In the presence of these scotomata, however, there 



