CENTRAL AUDITOR^- MECHANISMS 



605 



with respect to single unit responses. The response 

 areas are much narrower than in the more caudally 

 situated nuclei. The threshold is just as sharp, but 

 one is less impressed with the high frequency cutoff, 

 there being some tendency for band width to widen 

 toward the higher as well as the lower tones with in- 

 crease in intensity, though the low tone bias is still 

 prominent. The inferior colliculus has not been sys- 

 tematically explored, so we do not know to what rela- 

 tive degree each part of this complex organ may be 

 populated with frequency-selective units. We can 

 only be sure that such units can l)e found in consider- 

 able numbers. 



The medial geniculate body responsive elements 

 show, again, some similarities and some differences 

 to the situation in the medullary nuclei. While there 

 are many units which can be stimulated by pure 

 tones, there are also many which cannot, though the 

 latter group includes many units which do respond 

 to clicks, noise or both. Of those responding to pure 

 tones, it is noted the frequency ijands to which they 

 respond are broader at threshold than those of the 

 other nuclei. Furthermore, the bands widen, with 

 increasing intensity, almost equally toward higher 

 and lower tones. The available data offer us little 

 or nothing which would point to the existence of a 

 nuclear plan or map of frequency-specific areas; 

 however, this is really an open question which can 

 be .settled only by a more systematic survey of fre- 

 quency-biased (if not specific) units throughout the 

 nucleus. 



A recent study of auditory cortical single unit 

 response to pure tone (25) has demonstrated some 

 units (few relative to brain-stem nuclei) which are 

 responsive to tonal stimuli (as well as others respon- 

 sive to other auditory stimuli). Tone-sensitive units 

 are said to be usually maximally sensitive within a 

 restricted frequency band, this band widening rela- 

 tively little as intensity increases. In the terms we 

 have been using, the response area follows the pro- 

 gressive tendency for narrowing of the band width 

 overall while widening it somewhat at threshold. 

 It should be emphasized that among the units sensi- 

 tive to any kind of auditory stimulus, which alto- 

 gether constitute less than 60 per cent of all units 

 identified, those sensitive to pure tone represent 

 only a small fraction. With respect to location of 

 frequency-specific units, the findings indicate that 

 those most sensitive to low frequencies predominate 

 in the posterior A I field and high frequency units 

 predominate in anterior A I, although in neither case 



is the characteristic t\pe the exclusive frequency 

 sensitive type. 



In review of the studies cited so far, it can be said 

 that the sharply restricted frequency specificity of 

 fibers for threshold intensity in the cochlear nerve 

 persists in units of the medullary auditory nuclei but 

 in the thalamus and cortex gives way to restricted 

 though broader bands of threshold sensitivity. On 

 the other hand, the very broad frequency response 

 for tones below the high frequency cutoff, characteris- 

 tic of the cochlear nerve fibers, diminishes steadily 

 in width as we ascend the pathway and the sharp 

 high frequency cutoff is lost. Thus, a given auditory 

 nerve fiber may respond at threshold only to 2000 

 cps tone (for example) and, with increasing intensity 

 may respond to all tones lower than 2000 cps, but will 

 respond to no higher tones, no matter how intense; 

 however, a cortical unit may respond at threshold 

 to a restricted band centering at 2000 cps but re- 

 spond at considerably higher intensity to a band not 

 much wider. From the situation in the cochlear nerve 

 where virtually all fibers are sensitive to tonal stim- 

 uli, we go to that in the cortex where only a fraction 

 of the total elements are tone sensitive. Although we 

 do not have very exact information on the percentage 

 of tone-sensitive elements at all levels, the indications 

 are that there is a proportional decrea.se, but no ac- 

 tual numerical decrease, and, more probably, some 

 increase of elements whose prime preoccupation has 

 to do with stimulus frequency. 



Postponing for the moment any interpretation of 

 the microelectrode studies, let us turn to other studies 

 in which the basic questions have to do with overall 

 specificity of projection rather than that of indi\idual 

 fibers or cells of the pathway. Both types of evidence 

 will have to be incorporated into any effort at inter- 

 pretation. 



We have followed the upward progress of single 

 tone-sensitive elements of the projection pathway 

 and noted that through the several synapses and proc- 

 essing centers, a certain change in character of rela- 

 tive frequency sensitivity has occurred together with 

 a dispersion of these elements among others which 

 seem to have different concerns. It would now be 

 well to examine the overall situation to determine if 

 there is, indeed, any pattern of anatomical or ph\sio- 

 logical integration which can be discerned by correla- 

 tion of elements or groups of elements at one end of 

 the system with those of the other. Several relevant 

 studies on cat, dog and monkey at once present them- 

 selves. 



Let us consider first those primarily concerned with 



