MODELS FOR SPECIFIC NERVES FOR SPECIFIC FREQUENCIES 49 



thus establishing standing waves, as in an ordinary musical instrument. 

 The positions of the maximum pressures in these waves would activate 

 pressure-sensitive nerve endings in the membrane. Thus the localization 

 of points of maximum pressure would activate different nerves, depend- 

 ing on the frequency of the incoming waves. Unfortunately, this simple 

 picture is entirely out of the question. It would require the existence of 

 a cochlea with dimensions similar to those of musical instruments, and 

 this is obviously not the case. In addition, the cochlear fluid is essentially 

 incompressible, so that a more refined analysis is required. (We shall 

 return to this aspect toward the end of this chapter.) Further, because of 

 the viscosity of the fluid, it is hard to imagine that the frequencies could 

 be so sharply separated from each other in what are called standing 

 waves that there could be good frequency (pitch) discrimination. There- 

 fore we confine our attention to membrane vibrations. 



At first sight, standing waves in the membrane would seem to be ruled 

 out for the same reason as were waves in the fluid. There exists, however, 

 a phenomenon called, among other things, neural sharpening. We can 

 illustrate this with an experiment you yourself can perform. Touch the 

 tip of a finger with a reasonably sharp pencil point; it is best to do this 

 with your eyes closed. Then touch approximately the same place with 

 the tip of a ball-point pen whose point is retracted. The statement of 

 neural sharpening is that you cannot tell the difference. Schematically, 

 the statement is that your neural structure is such that you cannot dis- 

 criminate between a point (•) and an object (O) centered around the 

 point — provided the touch is not too strong. The stimulation of nerves 

 around the periphery of the circle is interpreted by the individual as a 

 stimulation of the center point itself. 



A more readily appreciable experiment (though less directly illustra- 

 tive) that exhibits the neural sharpening phenomenon involves touching 

 the skin at the same time with two reasonably sharp pencils. If the 

 points touch the skin less than about 2 cm apart, only one stimulus is 

 felt, whereas at about 3-cm spacing one is aware of the two separate 

 pencil points. 



The application to standing waves in the membrane comes in observ- 

 ing that even if a relatively broad area of the membrane is set into 

 vibration by the incoming wave, neural sharpening results in the sensa- 

 tion that only the central point of the vibrating area has been affected. 

 Thus only the particular frequency at that point will be communicated as 

 having been heard. 



According to this kind of theory, the place which is stimulated in the 

 membrane is different for each frequency and is therefore the mechanical 

 agency of sorting out the frequencies. There are other varieties of the 

 place theory which will not be discussed. 



