138 Visuo-Sensortj and Visuo-Psychic Areas [CHAP. 



Next we have to analyse the literature with the view of ascertaining to what extent 

 other varieties of visual disturbance, well known to the clinical observer, can be placed on 

 an anatomical basis, and whether data exist to justify a corresponding subdivision of the 

 visual area. The list of disorders includes such conditions as psychic blindness, alexia, optic 

 aphasia, colour blindness, stereoscopic vision, etc., and of these psychic blindness first demands 

 consideration. 



Psychic Blindness. 



Psychic blindness or " Seelenblindheit " is a name for which, of course, we are indebted to 

 Munk, and which arose out of his experiments on the lower animals. In defining the con- 

 dition I cannot do better than quote the appearances presented by one of Munk's dogs, 

 "while the animal exhibited no abnormalities regarding the functions of hearing, smell, taste, 

 motion, and sensation, and was able to move about a room without colliding with any of 

 the objects therein, it yet showed pronounced impairment of the visual sense and this 

 defect was of a psychic nature. Thus, even when the dog was hungry and thirsty it would 

 pass food and water unnoticed ; it snapped at morsels of meat only when it smelt them ; 

 to everything which it saw it was indifferent ; threatening movements with a whip did not 

 disturb it, a bright light brought close to its eyes did not cause it to wink, and the 

 presence of its master and of other dogs created no impression." This condition resulted 

 when circular portions, If, cm. in diameter, were removed from the centre of Munk's visual 

 area, in both hemispheres ; it did not last more than four or five weeks, and was accompanied 

 by central blindness. 



Now, in man, numerous instances of precisely similar, psychic visual disturbance have 

 been observed, but its degree has varied : it has been represented by some increased difficulty 

 of orientation in space, by some diminution of the perceptive faculty, by a certain inability to 

 read, or, finally, by a complete failure to recognise and interpret any objects seen ; and the 

 question we have to consider is, what is the necessary situation for the lesion which will 

 produce these phenomena ? Now, in spite of the fact that over 30 cases followed by a post- 

 mortem have been reported, we cannot yet say that the cortex which dominates these 

 specialised psychic processes is definitely localised. I have mentioned already, however, that 

 a destructive process confined to the calcarine cortex is probably sufficient to induce simple 

 unmodified blindness, and it appears that for the production of the superaclded psychic element 

 it is only necessary that the destruction shall be distributed more widely. 



Von Monakow, who lias made a special study of the recorded instances of mind blind- 

 ness, insists strongly on the necessity for this widespread lesion, pointing out that the further 

 the destruction advances into the white substance of the parieto-occipital lobe, the more are 

 long tracts of association fibres involved and connections with other cortical centres inter- 

 rupted. And in support of this statement, we find in the majority of recorded cases that 

 the lesion has been deep-seated and that it has involved, not only the occipito-thalamic 

 radiations proceeding to the calcarine region, but also other association bands of fibres, to 

 which we shall have occasion to refer when we discuss some anatomical details bearing on 

 visual localisation. And unlike the lesions in cases of uncomplicated cortical blindness, we 

 find here that the destruction frequently has been asymmetrical, and a further point of 

 importance is that a deep lesion in the left occipital lobe seems more likely to bring 

 psychic defects in its train than one affecting the right. 



