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Neural Mechanisms of Hearing /6 : 3 



Another interesting case is the location of two click-like stimuli on 

 opposite sides of the finger. For long time delays between the two, 

 separate clicks are felt. If the time delay is decreased, the second click 



Forearm 



3cm 



2.5 cm 



m 



2 cm 



(a) 



Stimulus @ Sensation 

 (b) 



(c) 



Figure 5. Neural sharpening and funnelling when two com- 

 pass points are pressed on the arm. For large separation, 

 separate sensations result. Medium separation sensations 

 tend to suppress each other. Small separation sensations add 

 and are located in a "sharpened" area. 



is no longer felt. As the time interval approaches zero, a single sensation 

 is felt which approaches midway between the two stimuli and has a 

 larger apparent area. The results of an experiment of this nature are 

 shown in Figure 6. This same type of phenomenon occurs when one 

 locates a sound by the difference in the times of its arrival at the two 

 ears. This addition of more than one stimulus into a single, stronger 

 sensation is called "funnelling" by Bekesy and his co-workers. 



Mallet. 



Large Time 

 Separation 



Small Time 

 Separation 



Simultaneous 



(c) 



Figure 6. Neural funnelling when the forefinger is struck with 

 two small mallets (clicks), (a) With large time separation, 

 both "clicks" are sensed, (b) With small time separation, 

 only the first is sensed, (c) Simultaneous clicks add to com- 

 mon larger sensation half way between the two stimuli. 



These sharpening and locating effects which occur in the senses of 

 touch and sight as well as hearing are very interesting. They emphasize 

 that the nervous system does act as a complex computer with a great deal 



