6o6 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



anatomical integrity of projection. We have in the 

 report of Woolsey & Walzl (11:5) a pure example 

 of anatomical study of the neural projection of the 

 cells of the spiral ganglion to the cortical projection 

 area, by physiological means uncontaminated by the 

 mechanical characteristics of the end organ. The only 

 element of control lacking will he apparent as the 

 experiment is briefly described. The procedure was: 

 /) to expose the cochlear duct by removal of the ex- 

 ternal bony cap.sule, in the process of which the organ 

 of Corti and basilar membrane were removed to 

 expose the ends of the peripheral processes of the 

 ganglion cell in the free border of the osseous spiral 

 lamina; and 2) to stimulate by brief electric shocks 

 small groups of these fibers while exploring the sus- 

 pected cortical areas for the responses and map these. 

 The only methodological fault (which was unavoid- 

 able) is that, while the stimulus is electrically and 

 geographically speaking quite localized, not all of 

 the nerve fibers at any such local spot are. Some of 

 them innervate only the inner hair cells immediately 

 beyond the position of the stimulating electrode, 

 but the rest innervate more widespread groups of 

 outer hair cells, and the latter, to an unknown de- 

 gree, mav extend the cortical response area of a given 

 point or perhaps blur its edges. 



On the basis of these experiments, Woolsey & 

 Walzl postulated a double cortical projection area 

 (A I and A II as they have since come to be called 

 and have been referred to in this chapter). Within 

 A I, they found the basal end of the cochlea to project 

 to the anterior part of A I and the apical to posterior 

 A I, with intermediate cochlear stations projecting 

 in orderly fashion between. A II showed a similar 

 pattern except it was inverted, so that the basal 

 cochlea is represented in posterior A II, the apical 

 in anterior A II; however, it was less easy to trace 

 the intermediate loci between these two focal regions. 

 Part of this difficulty could be due to the fact the evi- 

 dence is limited to the half of each cochlear spiral 

 which can be surgically exposed, while the situation 

 on the still inaccessible obverse turns must be logi- 

 cally inferred without demonstration 



We know from the foregoing that there is a direct 

 relation of cochlear region to cortical area. Inferen- 

 tially, we can postulate further from this work that 

 high tones (basal cochlea) should e.xcite anterior A I 

 and posterior A II, while low tones should be repre- 

 sented in posterior A I and anterior A II. This has 

 been experimentally tested by several investigators. 

 The recent work of Erulkar et al. (25), using the micro- 

 electrode method, has alreadv i)een mentioned. 



With macroelectrode techniques, the cortical re- 

 sponse to sustained pure tones (or noise for that mat- 

 ter) is less conspicuous than one might have thought, 

 and it is difficult to evaluate reliably for purposes of 

 mapping areas excitable by sound. This applies to 

 the sound after it has been turned on during what 

 Rosenblith calls a quasi-stationary state, the prefix, 

 'quasi', in this case representing the overwhelming 

 burden of our ignorance of the continuously changing 

 ' backgroimd' electrical activity, and the ways it may 

 be influenced to change further by sound. The same 

 handicap does not apply to the onset response to 

 an\- kind of sound stimulus (onset can be seen as a 

 high-voltage wave response, due presumably to 

 arrival at cortex of a surge excitation), a fact which 

 has been capitalized in the use of clicks, which are 

 brief complex noises, and of tonal pips, which are 

 brief pure tones in which the frequency characteris- 

 tics are established and brought to threshold intensity 

 within a very few cycles. Response to tonal pips has 

 been used to map frequency-sensitiv'e cortical areas 

 and so has another method, the evoked strychnine 

 spike technique. The latter depends upon the fact 

 that a small part of the auditory cortical area can be 

 .sensitized with strychnine so that onset response of a 

 tone to which the area is normally sensitive evokes a 

 strychnine spike which, unlike the response of the 

 untreated cortex, is so characteristic it cannot lie 

 lost in the background activity. 



In earlier efforts at mapping the auditory cortex 

 with respect to differential frequency sensitivity, 

 both in cat and monkey, the tonal pip and tonal on- 

 set methods were used, recording from the un- 

 treated auditory cortex in anesthetized animals. 

 In the cat (53), Licklider found that rough focal areas 

 of maximal response to higher or lower frequencies 

 could be found which are in general agreement with 

 the more recent stud\' of Hind (44)- Licklider felt 

 the situation could be better described in terms of 

 gradients rather than restricted tonal foci because 

 of the extensive overlapping. 



Hind (44), using the evoked strychnine method, 

 presented more extensive data and extended the fre- 

 quency range studied. (It should be noted that the 

 technique was originalh' worked out by Tunturi 

 (103) and applied to study of the dog's auditory 

 cortex. Tunturi's work will not be described here 

 because, while in general agreement with others, 

 the comparison of data on dog and cat is troublesome 

 due to configural differences in the brain. We will, 

 therefore, to conserve space and avoid confusion, 

 confine the discussion to the cat studies which are 



