When we look at a historical section of a sense organ with a large surface, we find nerve fibers going directly to the sensory ganglion, and in addition we find a large number of lateral fibers running parallel to the surface and connecting two or more end organs with one another. It was Held (1926) who showed that in the organ of Corti there are never fibers running from every hair cell more or less direclty to the modiolus of the cochlea, and also other fibers running perpendicular to these along the basilar membrane, often connecting the hair cells over nearly a half turn of the cochlea. Unfortunately these lateral connections often seem difficult to stain. (...) The lateral fibers of the cochlea are represented in Fig. 20. At present it is still uncertain to what extent these fibers are involved in the immediate process of hearing. (...) I am unable to understand how this retinal circuitry fails to produce constant oscillation. It is difficult to imagine what type of reduction of feedback is used to avoid this oscillation. Even a simples feedback system requires many precautions to prevent its going into oscillation and to cause it to return to its original equilibrium condition after a stimulus has ceased to act upon it.
Figure 20
Figure 23
For present purposes, a simple scheme is sufficient to illustrate the problem. The small dots near the bottom of Fig. 23 represent a system of receptors on the surface of the skin. The variations of a stimulus are indicated by the lowermost graph with a maximum in the middle. There are lateral connections between the receptors and nerve fibers that run upward to the first-order ganglion cells in a row a. Alongside each of these nerve fibers is a representation of its pulse rate: the number of spikes for a certain unit of time is shown. Adrian in 1928 found that the discharge rate of a sensory nerve fiber increases with the magnitude of the stimulus acting on its end organ. In the two fibers on the far left and right sides the stimulation is minimal and the discharges represent spontaneous activity. Toward the middle the stimulus intensity increases and so also does the discharge rate.
From the first layer of ganglion cells we go to a similar level above, shown at b, and from there to still higher level, c. As far as we can now discover, the result of the lateral interconnections is to reduce or "funnel" the laterally spreading stimulation to a progressively localized section of the neural pathway, as indicated by the sketch of the sensation at the top of the figure.
This simple scheme immediately brings up the question of the kind of frequency sensation that will be produced by a stimulus whose amplitude tapers away from a central maximum as shown in this figure. It is apparent that the lateral inhibition that occurs in sense organs is not the straightforward kind that we find in muscle systems. The lateral inhibition of sense organs is actually a funneling action that inhibits the smaller stimulus effects and collects the stronger effects into a common pathway.
(...)
In a crude way we can distinguish four types of inhibitory pathways, as illustrated in Fig. 24. As A is shown the simple form of lateral inhibition that has just been described. At B is shown a forward type of inhibition, at C a backward type, and at D a form of central inhibition. In human sense organs there is an interplay of all these types of interaction.
Figure 24
The question arises whether inhibition is the correct term for this whole series of phenomena. With our limited knowledge of inhibition phenomena in single neural pathways it is difficult to decide on the proper category. In addition to inhibition we might consider the term "sensory distortion", or even "disinhibition" - because it may happen that normally the neural pathways are blocked, and the effect of a stimulus is to remove the blockage. I prefer to call the effect "funneling", because even for central inhibition there is usually an increase in the magnitude of the sensation referred to certain places.
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