The Mystery of the Sensual Qualities

The principal organ of the ear is the cochlea, a coiled bony tube which resembles the shell of a sea-snail.

It has a tiny winding staircase that gets narrower and narrower as it ‘ascends’.

In place of the steps (to continue our simile), across the winding staircase elastic fibres are stretched, forming a membrane, the width of the membrane (or the length of the individual fibre) diminishing from the ‘bottom’ to the ’top’.

Thus, like the strings of a harp or a piano, the fibres of different length respond mechanically to oscillations of different frequency.

To a definite frequency a definite small area of the membrane - not just one fibre - responds, to a higher frequency another area, where the fibres are shorter.

A mechanical vibration of definite frequency must set up, in each of that group of nerve fibres, the well-known nerve impulses that are propagated to certain regions of the cerebral cortex.

We have the general knowledge that the process of conduction is very much the same in all nerves and changes only with the intensity of excitation; the latter affects the frequency of the pulses, which, of course, must not be confused with the frequency of sound in our case (the two have nothing to do with each other).

The picture is not as simple as we might wish it to be. Had a physicist constructed the ear, with a view to procuring for its owner the incredibly fine discrimination of pitch and timbre that he actually possesses, the physicist would have constructed it differently. But perhaps he would have come back to it.

It would be simpler and nicer if we could say that every single ‘string’ across the cochlea answers only to one sharply defined frequency of the incoming vibration.

But why is it not so?

Because the vibrations of these ‘strings’ are strongly damped. This, of necessity, broadens their range of resonance. Our physicist might have constructed them with as little damping as he could manage.

But this would have the terrible consequence that the perception of a sound would not cease almost immediately when the producing wave ceases; it would last for some time, until the poorly damped resonator in the cochlea died down. The discrimination of pitch would be obtained by sacrificing the discrimination in time between subsequent sounds.

The actual mechanism manages to reconcile both in a most consummate fashion puzzlingly.

I have gone into some detail here, in order to make you feel that neither the physicist’s description, nor that of the physi- ologist, contains any trait of the sensation of sound.

Any description of this kind is bound to end with a sentence like: those nerve impulses are conducted to a certain portion of the brain, where they are registered as a sequence of sounds.

We can follow the pressure changes in the air as they produce vibrations of the ear-drum.

We can see how its motion is transferred by a chain of tiny bones to another membrane and eventually to parts of the membrane inside the cochlea, composed of fibres of varying length" described above.

We may reach an understanding of how such a vibrating fibre sets up an electrical and chemical process of conduction in the nervous fibre with which it is in touch.

We may follow this conduction to the cerebral cortex and we may even obtain some objective knowledge of some of the things that happen there.

But nowhere shall we hit on this ‘registering as sound’, which simply is not contained in our scientific picture, but is only in the mind of the person whose ear and brain we are speaking of.

We could discuss in similar manner the sensations of touch, of hot and cold, of smell and of taste. The latter two, the chemical senses as they are sometimes called (smell affording an examination of gaseous stuffs, taste that of fluids), have this in common with the visual sensation, that to an infinite number of possible stimuli they respond with a restricted manifold of sensate qualities, in the case of taste: bitter, sweet, sour and salty and their peculiar mixtures. Smell is, I believe, more various than taste, and particularly in certain animals it is much more refined than in man.

What objective features of a physical or chemical stimulus modify the sensation noticeably seems to vary greatly in the animal kingdom. Bees, for instance, have a colour vision reaching well into the ultraviolet; they are true trichromates (not dichromates, as they seemed in earlier experiments which paid no attention to the ultra-violet).

Von Frisch in Munich found out not long ago that bees are sensitive to traces of polarization of light.

This aids their orientation with respect to the sun in a puzzlingly elaborate way.

To a human, even completely polarized light is indistinguishable from ordinary, non-polarized light.

Bats have been discovered to be sensible to extremely high frequency vibrations (‘ultra-sound’) far beyond the upper limit of human audition.

They produce it themselves as a ‘radar’ to avoid obstacles.

The human sense of hot or cold exhibits the queer feature of’les extremes se touchent’.

If we inadvertently touch a very cold object, we may for a moment believe that it is hot and has burnt our fingers.

Some 20-30 years ago US chemists discovered a curious compound, of which I have forgotten the chemical name, a white powder, that is tasteless to some persons, but intensely bitter to others.

This fact has aroused keen interest and has been widely investigated since. The quality of being a ’taster’ (for this particular substance) is inherent in the individual, irrespective of any other conditions.

Moreover, it is inherited according to the Mendel laws in a way familiar from the inheritance of blood group characteristics.

Just as with the latter, there appears to be no conceivable advantage or disadvantage implied by your being a ’taster’ or a ’non-taster’.

One of the two ‘alleles’ is dominant in heterozygotes, I believe it is that of the taster. I t seems to me very improbable that this substance, discovered haphaz- ardly, should be unique. Very probably ’tastes differ’ in quite a general way, and in a very real sense!