PERCEPTION 



1617 



FIG. 13. 'Completion' of forms projected partly into the 

 blind half of the visual field in a case of left homonymous 

 hemianopia. A-E are patterns presented on a translucent 

 screen (at 100 msec.) by a tachistoscope, A, fixation point. 

 Patterns A—C subtended 10° to either side of the fixation point 

 along the horizontal meridian. Patterns I) and E, 7° and 8°, 

 respectively. The patient reported what he saw I I / Inst 

 verbally, then by drawings, then by selection from a series "I 

 comparison patterns. [From Bender & Tcubcr (30); see also 

 Teuber el al. (469).] 



rants. It occurs in the tactile and kinesthetic modality 

 Thus, a patient with a right parietal lesion may be 

 able to report a contact on his left hand and on his 

 right hand, as long as each hand is stimulated singly. 

 On 'double simultaneous stimulation' of both hands 

 he may report only the contact on his right (the less 

 affected side). One and the same patient may exhibit 

 extinction for touch and vision, or for one or the 

 other modality alone. Unfortunately, it is still not 

 clear why extinction occurs when it does, and why 

 it fails to occur when it does not (295). Right parietal 

 or parietooccipital lesions are frequently found in 

 cases of extinction, but they are not obligatory. 13 It 



13 In fact, the earliest demonstration of one-sided 'neglect' 

 on bilateral stimulation was made in experiments on dogs with 



is conceivable that the phenomenon of extinction 

 on multiple simultaneous stimulation represents a 

 special and exaggerated form of normal lateral 

 interactions between contour processes in a sensory 

 field [see Fry & Bartley (135)]. The famous syndrome 

 of Balint (17), in which the patient can perceive 

 whatever he fixates at any time but nothing else, 

 may be another form of extinction, and similar 

 disturbances may play a role in those forms of so- 

 called agnosia in which a patient cannot organize a 

 large array with multiple patterns (121, 547; see 

 also 331). 



It is quite obscure where and how these enhanced 

 lateral interactions within a sensory sphere take place. 

 They need not occur within the primary projection 

 field itself; it is tempting to assign some role in these 

 abnormal processes to nonspecific projection systems 

 (324). A possible analogue in normal states (beyond 

 the short-ran^e interaction of contours) is the inter- 

 ference effect on pain l>v countcrirritation, i.e. an 

 intercurrent painful process set off elsewhere in the 

 both. In the auditory sphere, a possible normal 

 analogue would he the inability to attend simul- 

 taneously to dichotic messages (two sets of information 

 presented each to one ear) (63, 87, 352; Chapter 

 I. Will by Zangwill in this volume). 



COEXISTENCE OF SPECIFIC VND GENERAL PERCEPTUAL 



changes. The review of perceptual alterations after 

 subtotal lesions of the visual projection system in 

 in. in lias revealed an obligatory association of specific 

 symptoms (scotoma) with less specific symptoms 

 found elsewhere in the defective field. In addition, 

 one can construct perceptual tasks which are affected 

 in an entirely nonspecific way, i.e. irrespective of 

 whether there are scotomata or not and irrespective 

 of the site of the cerebral lesion. These tasks employ 

 "hidden' figures m which line drawings are concealed 

 by embedding them in interlacing contours (see above 

 and fig. 14). Cerebral lesions which result in visual 

 field defects produce disproportionate difficulties in 



cortical removals from either the occipital or the frontal lobes. 

 Jacques Loeb (328) showed that such dogs would invariably 

 turn toward meat on their operated side and neglect another 

 piece exhibited simultaneously to the other side of the fixation 

 point. Loeb's method of double stimulation was applied 

 after that in neurologic patients to demonstrate minimal 

 sensory or visual impairment (72, 369, 386). The curious 

 1 transient) disregard of stimuli opposite an acute unilateral 

 frontal lesion in dog and monkey has been investigated further 

 by Bianchi 143), Kennard & Ectors (254) and, quite recentlv, 

 by Welch & Stuteville 1532). 



