l^(')2 



HANDBOOK OF PHYSIOLOGY ^ NEUROPHYSK It OGY III 



i ps for which localization is less accurate than for 

 either lower or higher frequencies. For complex 

 sounds, time, intensity and quality differences are all 

 probably utilized. How time and intensity differences 

 at the periphery are coded in neural centers is not as 

 yet completely clear. A number of investigators 

 (hiring the past 30 years have suggested that these 

 differences become place differences in the central 

 nervous system, i.e. that auditory space is in some 

 manner topographically represented in the brain 

 (18, 19, 25, 102, 1681. There is no experimental 

 evidence to support this view, but it has not been 

 carefullv explored. In a scries of experiments Rosen- 

 zweig and his co-workers (182-186) have recorded 

 electrical responses at the cochlea, cortex, and inferior 

 colliculus of the cat when activity in the auditory 

 system is set off by clicks which are varied in intensity 

 and in time of arrival at the two ears. They have 

 found that the pattern of response recorded by a gross 

 electrode at the cortex or the inferior colliculus varies 

 systematically as a function of the interval separating 

 two clicks successively presented, one to each of the 

 ears. The cortical response also varies when the two 

 clicks arc presented simultaneously but one of less 

 intensity than the other. 



Because of methodological difficulties encountered 

 in systematically exploring different regions of the 

 body and in restricting stimulation to the appropriate 

 receptors, there have been few attempts to discover 

 the organization of the pathways from kinesthetic 

 receptors to cortex. The electrophysiological experi- 

 ments of Mountcastle et <d. (148) have shown that 

 stimulation of nerve fibers of muscle afferents or 

 stimulation of receptors in the region of joints and 

 tendons will evoke responses from somatic areas I 

 and II of the cat cortex. Furthermore, although 

 mapping was confined to the fore- and hind limbs, 

 the topographic projection of these regions was 



approximately that of the (lit. menus afferents. As 



Mountcastle and his collaborators noted, however, 

 the receptors and nerve libers stimulated in the 

 experiment may have been those which mediate the 



sense of deep pressure or pain rather than muscle 

 movement or position. (Rose & Mountc.isllc have 



reviewed this topic in Chapter XVII of this Handbook. 1 

 Evidence for the central projection of the subdivi- 

 sions of the vestibular end organs is still more meager 

 ih. in for the kinesthetic receptors. In an) case, the 

 vestibular end organ is not a space receptor in the 



s.ime sense ,is the Other s\sieins described above. It 



il direction ol movement (lineai and angular 



acceleration) and position of the head. It has always 

 been a point of argument as to whether or not any 

 direct sensory experience was aroused by excitation 

 of the vestibular end organs and the consequent flow 

 of nerve impulses to higher centers. From introspective 

 analysis it seemed a likely possibility that the sensation 

 aroused upon vestibular stimulation was not a direct 

 one but only the awareness of eve movements, muscu- 

 lar contractions and relaxations in skeletal muscles, 

 responses of stomach muscles, and other reflex actions 

 known to be elicited by vestibular stimulation. Al- 

 though it did not necessarily follow, this position was 

 usually accompanied by the view that the vestibular 

 system had no cortical representation. It now appears 

 that different parts of the vestibular end organ may 

 be projected to different but adjacent and overlapping 

 areas of the cortex of the temporal lobe (12, 72, 108, 

 145, 187, 195, 196, 216). There is no obvious parallel 

 here, however, with the visual and tactual systems in 

 that for both of the latter, outer space is represented 

 topographically on the cortex. 



A number of behavioral studies have been made in 

 which capacity for spatial discrimination has been 

 examined before and after central nervous system 

 lesions. After complete bilateral ablation of the visual 

 cortex, rats (1 18), cats (194), dogs (137, 223 225) and 

 monkeys (113, 114, 1 16) can make correct choices 

 between two stimuli differing in luminous flux. The 

 test procedures used in the experiments performed 

 were designed to measure intensity discriminations, 

 but they may be interpreted as indicating some degree 

 of visual space localization in that the animals had 

 to make a choice of two stimuli which were spatially 

 separated, the positions of the two stimuli being re- 

 versed in a random order on successive trials. Since 

 contour discrimination is absent in animals lacking a 

 visual cortex, accurate visual localization of objects 

 separated by different angular distances is obviously 

 impossible. Ability to discriminate distance has not 

 been carefullv measured after visual cortex ablation 

 but, again as would be expected in the absence of 

 ability to discriminate contours, it appears to be lost; 



animals trained to approach a lighted door proceed 



until the) make contact with their vihrissac or skin. 

 < )ne would infer that animals such as the rat, cat and 

 inoiikev, .liter ablation of the visual cortex, perceive 

 a two-dimensional world, a world without depth, 

 without contours, differing only in brightness gradi- 

 ents "i perhaps of uniform brightness at an) given 

 instant bul changing when eve and head movements 



lead to increase or decrease in luminous flux 



