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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



tionality as a function of binaural interaction; how- 

 ever, whatever may be the mechanism of such interac- 

 tion, it is our concern to discuss whether, to what 

 extent and by what means binaural interaction is 

 reflected in the anatomicophysiologic organization 

 of the central acoustic mechanisms. The evidence 

 relevant to these questions is limited but does yield 

 some useful information which we will examine 

 presently. 



A second question has to do with the extent to 

 which each ear is bilaterally represented in the pro- 

 jection pathway. This question is probably not alto- 

 gether separable from that of the central reflection 

 of binaural interaction, although in certain contexts 

 it may be. For this reason, discu.ssion of the two ques- 

 tions will overlap, especially from the anatomical 

 point of view. 



The crucial anatomical problem in any theory of 

 bilateral interaction must be the opportunity for 

 side-to-side communication. There is no dearth of 

 such opportunities in the auditory pathway. Crossing 

 occurs at the trapezoid body, the earliest opportunity 

 since this begins at the level of entrance of the cochlear 

 nerves. The trapezoid crossing seems to provide not 

 only the first but also the most essential crossing for 

 maximum representation, both quantitative and 

 qualitative, of the left ear in the contralateral hemi- 

 sphere and vice versa (5). It is also at this level 

 that the superior olivary complex, by virtue of its 

 bilateral input from cochlear nuclei, provides many 

 of the ascending fibers of the lateral lemniscus which 

 represent either ear. Thus the basically bilateral 

 projection of this system has its anatomical foundation 

 almost at the level of entrance of the nerves. 



Fibers of the lateral lemniscus cross at the somewhat 

 diffuse commissure of Probst, just below the inferior 

 colliculus. No functional significance of this commis- 

 sure has been demonstrated (5). 



A third opportunity for crossing of auditory nerve 

 fibers occurs at the commissure of the inferior collicu- 

 lus. The significance of this crossing appears to be 

 largely local, carrying fibers from colliculus to oppo- 

 site colliculus or, at most, to opposite medial genicu- 

 late (i 1 1). Its function with respect to the pathway as 

 a whole, as measured by the influence of its presence 

 or absence on cortical response to stimulation of the 

 contralateral ear, seems to be negligible (5, 90). 



The cortical auditory area, like other cortical areas, 

 communicates by strong connections with its counter- 

 part on the opposite hemisphere by way of the corpus 

 callosum (62; also Ades, H. \V.., unpublished ob- 



servations). The functional significance of this with 

 special reference to audition is not clear. 



A problem which particularly stimulated some of 

 the early modern research in audition, and on which 

 incidental observations have since been made, is 

 that of the bilateral representation of each cochlea. 

 This has been tested, more or less adequately, in 

 various ways (5, 13, 18, 45, 46, 63, 88, 108, 113). 

 Functionally, the results have generally indicated 

 some difference but usually so small as to make it 

 difficult to detect an effect on acuity of even the de- 

 struction of one ear. Most of the observations on cor- 

 tical electrical response to contralateral versus ipsi- 

 lateral and bilateral stimulation of the ears have 

 revealed some small difference in representation. 

 Similar results have been differently interpreted, 

 apparently depending upon the point of view of the 

 individual investigator more than on any other fac- 

 tor. One could sum up by saying the difference in 

 representation of the two ears at one cerebral hemi- 

 sphere is often statistically different Ijut probably 

 not practically different. 



In a more ingenious way, however, Rosenzweig 

 (89) has succeeded in demonstrating that, while 

 quantitatively the difference in the effect of the two 

 ears on the cortical area may appear nearly equal 

 when one ear is stimulated at a time, there is never- 

 theless a more significant difference when the position 

 of the stimulus is varied with respect to the two ears 

 simultaneously stimulated. He found that when a 

 sound is presented at one side, the cortical response 

 s greater at the contralateral than the ipsilateral 

 hemisphere — the farther to the side, the greater the 

 difference. When the sound is in the median plane, 

 the cortical activity is equal at the two hemispheres. 

 Here we have, then, a clear correlation between audi- 

 tory localization and differential response of right 

 and left cortical auditory areas. We know, however, 

 from the work of Neff rf al. (67) that auditory localiza- 

 tion is a function which is not abolished by destruc- 

 tion of the cortical areas. In a second group of ex- 

 periments, Rosenzweig & Wyers C90) found some 

 evidence for binaural interaction in the inferior 

 colliculi, although not of the same kind as in the cor- 

 tex. 



Other than the studies cited briefly in the foregoing, 

 the evidence on the bilaterality of auditory function 

 is relatively scanty. The system has been more often 

 than not treated without regard for, or with only 

 incidental attention to, this structural and functional 

 feature. 



