The directing ear
Alfred Tomatis’s second text in the Bulletin du Centre d’Études et de Recherches Médicales de la S.F.E.C.M.A.S., dated July 1953 (eight pages, paginated 68-75). Tomatis here makes explicit for the first time the notion of the directing ear, in analogy with the dominant eye of the ophthalmologists: a given individual possesses an ear which, as a rule, directs vocal emission — “the right ear in the right-hander, the left in the case of a left-hander”.
The article describes the monaural listening apparatus that reveals the directing ear; it relates stammering to the trans-cerebral delay introduced when the non-directing ear takes command (reference to the “delayed feedback” of Bernard S. Lee at the Signal Corps Engineering Laboratories of New Jersey); it finally opens onto the re-education of phonatory disorders by means of soliciting auditory laterality.
BULLETIN OF THE CENTRE D’ÉTUDES ET DE RECHERCHES MÉDICALES OF THE SFECMAS
July 1953
THE DIRECTING EAR
by Dr Tomatis
Deputy Director of the SFECMAS research laboratory
Following various observations whose exposition would lie outside the scope of the study undertaken here, the ophthalmologists were led to observe that the respective roles of the two eyes of an individual were not absolutely identical in the accomplishment of the normal process of visual perception. One of the two eyes plays, in every case, a preponderant role, which is to direct, as it were, the individual’s visual operations and their incidence upon the nervous system.
The eye thus defined accordingly received the name “dominant eye”.
It is very easily observed by asking an individual to aim, with their index finger, at a point situated a few metres away. To aim, the individual will instinctively close the non-dominant eye, and if they sight with both eyes open the result will be the same: the index finger will remain in line with the targeted point and with the dominant eye.
The question then arose whether this asymmetry observed in the domain of vision did not have its equivalent in the auditory domain.
The studies we have undertaken on various problems of phonation considered in relation to their bearing on hearing (stammering, vocal disorders, etc.) have led us to bring to light an analogous arrangement of the respective roles of each of the two ears and to establish the following rule: “every individual possesses a directing ear, the right ear in the right-hander, the left in the case of a left-hander”.
As we shall see in what follows, this finding, established on indisputable facts, is of very great interest, both from the point of view of medical research and from the therapeutic point of view.
We shall approach our study by a rapid description of the apparatus — very simple, indeed — which is required.
It is composed essentially of a microphone connected to the input of a low-frequency amplifier with linear response curve which feeds two earphones.
This arrangement allows an individual speaking or singing into the microphone to hear themselves instantaneously by means of the two earphones, at a sound level determined by the degree of amplification.
Let us examine the reactions of a singer’s voice as a function of the hearing possibilities we impose upon them.
The gain of the amplifier is set so that hearing through the earphones corresponds to normal hearing (without earphones).
Under these conditions, if the singer places:
-
Both earphones over their ears: they sing normally and no modification is observed in the quality of their singing.
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One earphone on the right ear, the left ear being plugged: we obtain once again the same result, that is to say, a perfectly normal emission of the sung voice.
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One earphone on the left ear, the right ear being plugged: there is then a sudden alteration of the voice, which becomes flat, monotone, lacking musicality. The individual begins to sing out of tune and at the same time a slowing of the melodic rhythm is noted.
The experiment described concerns the case of a right-hander and proves how fundamental is the role played by their right ear.
If the test is repeated with a left-hander, we observe that the voice is altered only when the subject hears with the right ear alone — that is to say, when the directing ear is eliminated.
We may thus deduce from this experiment that the directing ear alone permits the control of expression, accuracy and “tempo” during sung emission. It is the directing ear that directs the voice. It enables the singer to regulate the quality of emission at every instant. If any disturbance whatever comes to hinder its functioning, the loss of control immediately brings about a modification in the emission of the voice.
We then carried out another experiment, this time on the spoken voice. This experiment likewise gives a very clear idea of the predominance of one ear in each of us.
The procedure is as follows: the subject speaks into a microphone and hears themselves back by means of two earphones. We then bring about a sort of auditory dazzling by sending into their directing ear a pure tone of 1,000 to 2,000 c/s at an intensity of 100 dB for a duration of about 30 seconds, depending on the individual’s resistance to auditory fatigue.
This test produces in the ear a trauma sufficient to alter the audiometric curve for a time varying from one minute to a quarter of an hour, depending on the subject’s powers of recovery. This trauma is enough to eliminate the directing ear and one immediately obtains a very marked slowing of speech.
Moreover, when the subject attempts to fight against this slowing — of which they are nevertheless aware — they begin to stammer in a characteristic way.
By contrast, if the experiment is repeated by dazzling the other ear, none of the phenomena just described occurs, and spoken emission remains entirely normal.
We have thus been able to demonstrate the existence of a directing ear and the very important role it plays in the cochleo-phonatory circuit.
Let us now try to determine the elements that characterise this directing ear.
Audiometric curves generally show a relative hyperacusis of the directing ear in comparison with the other ear, chiefly in the band of so-called “conversational” frequencies. Let us now suppose that it is not so. From the examinations we have carried out on numerous subjects, we have been able to conclude that this condition was necessary but not sufficient to bring about the appearance of certain phonatory disorders such as bradylalia or stammering.
Indeed, among the individuals we examined who presented such disorders, all the right-handers (right ear directing) presented a left-sided hyperacusis, and all the left-handers a right-sided hyperacusis.
This observation enabled us to focus, in a much more precise way, our research on the directing ear.
Indeed, proceeding from the principle that the same effects have the same causes, we took as the basis of our research the results obtained in the field of artificial stammering.
Dr DUPON-JERSEN presented to us some time ago the experiments of “delayed feedback” or “retarded voice” using the apparatus of Bernard S. LEE of the Signal Corps Engineering Laboratories of New Jersey, which under certain conditions makes it possible to induce an artificial stammer.
The apparatus consists of a magnetic recorder, a reading head, a microphone and listening headphones.
The principle of the experiment is as follows: an individual is made to hear their own voice, which is transmitted into their ears with a certain delay relative to the emission. They speak into the microphone with their ears fitted with the headphones, and hear themselves back through the headphones.
To capture this result, the voice is recorded on a magnetic medium; the reading head is movable along a horizontal bar, and the distance separating it from the recording head can be varied.
Knowing the speed of travel of the magnetic medium and the distance between the two heads, one immediately obtains the value of the time-shift induced between the emission and the reception of speech.
The results obtained may be summarised as follows.
Delays that are too short — below 0.1 second, for example — produce only slight modifications, as do delays that are too long, of the order of 0.4 to 0.5 second. The only disturbance observed in these cases is a slight bradylalia and a slowing of the rhythm of emission.
By contrast, if the induced delay lies between 0.10 and 0.20 second, the disturbances become very considerable. They reach a maximum at a value of 0.15 second, and what struck us is that when the test is carried out with such a delay, the subject first experiences the same disturbances as in the previous case.
If the test continues, the elocution slows more and more; a first hesitation appears, then a second, and finally stammering sets in, against which the subject’s will is powerless.
The stammering thus induced often retains an after-effect lasting several hours (we have experienced this personally).
The reaction to this test is not identical in all subjects. Indeed, only 20% are brought to stammer, while the others show only a more or less normal slowing of rhythm. Among these artificial stammerers we note a strong majority of women and children. Women in fact resist the test better, which we shall justify subsequently.
In drawing a comparison between the characteristic elements of the two forms of stammering just seen (natural and artificial), the following hypothesis may be formulated:
The relative hypoacusis of an individual’s directing ear seems to bring about a certain delay between the moment at which the individual emits a sound and the moment at which they are able to monitor that sound — that is to say, between the emission and the triggering of the reflexes consequent upon hearing.
This hypoacusis, however slight, seems particularly to eliminate the directing ear from the normal cochleo-phonatory circuit.
But the question then arose of knowing in what way this elimination of the directing ear could provoke the appearance of a delay in the control of vocal emission.
Let us reproduce schematically the normal cochleo-phonatory circuit.
Right auditory centre ─────► Left auditory centre
│
Phonatory centre
│
BRAIN
│
Control
│
Right ear Left ear
▲ ▲
└───────────┬───────────┘
Phonatory organs
(Diagram 1 — Normal cochleo-phonatory circuit: case of a right-hander. The sound emitted by the phonatory organs reaches the directing ear — the right ear; it is directed to the left brain at the auditory level adjacent to the phonatory centre, which is under its control. Once this control is complete, the nervous impulse descends to the phonatory organs and directs the emission.)
If the directing ear is not used to govern phonatory control, this control then falls under the dependence of the other ear, that is to say, the left ear.
Right auditory centre Left auditory centre
▲ │
│ BRAIN │
│ │
│ Control │
│ ▼
Right ear Left ear
▲
│
Phonatory organs
(Diagram 2 — Circuit with left directing ear, hence not used by the right-hander: the sound captured by the left ear is conducted to the right brain, at the auditory centre; the reaction, once the control is complete, must return to the left phonatory centre, whence the nervous impulse then descends to the phonatory organs as previously.)
Let us examine what happens in this case: the sound captured by the left ear is conducted to the right brain, at the auditory centre; the reaction, once the control is complete, must return to the left phonatory centre, whence the nervous impulse then descends to the phonatory organs as previously.
The essential difference between the two processes envisaged above lies in the inequality of the paths taken by the nervous impulse.
1st case: The left auditory centre and the phonatory centre are both situated in the left side of the brain. We have seen earlier that they are close neighbours. The impulse will therefore pass from one to the other almost instantaneously.
2nd case: (hypoacusis of the right ear) the right auditory centre has no relay to the left ear. But the phonatory centre is still on the left of the brain, since the subject is right-handed. Thus the impulse will have to cross the brain to pass from one to the other. This trans-cerebral transfer entails, as can be seen by examining the two diagrams, an inevitable disparity in the length of the circuit, equal to the distance “right auditory centre — left phonatory centre”. For we may admit that the paths (left ear — right auditory centre) and (right ear — left auditory centre) are equal. Moreover, the last part of the circuit, from the centre to the phonatory organs, is identical in both cases.
The hypothesis we propose, following this exposition, is therefore the following: the trans-cerebral transfer — an additional path imposed on the nervous impulse owing to the hypoacusis of the directing ear — produces a delay whose value depends on the individual, the effect of which is to derange the control of vocal emission and which is consequently liable, in certain cases, to bring about the appearance of phonatory disorders. One may also suppose that the nature of these disorders depends on the delay introduced.
Thus, we may define the directing ear in an individual as that which permits the control of vocal emission with a minimum delay — in other words, that whose transmission to the motor phonatory centre takes the most rapid path.
It is therefore possible to measure the duration of the trans-cerebral transfer by neutralising the directing ear until phonatory disorders are obtained.
Then, by means of “delayed feedback”, one seeks to obtain the same disorders.
The conclusion is that at this moment the delay introduced by the trans-cerebral transfer corresponds to the time-shift “emission — reception” due to the apparatus, this extending, of course, only to air conduction. This is what explains why women present fewer phonatory disorders than men. In cases of hypoacusis of the directing ear, they retain a possibility of cochleo-phonatory self-monitoring by bone conduction, thanks to the percentage of high harmonics that their voice carries and that spreads over a wide band exceeding 2,000 c/s.
The demonstration of the existence of the directing ear and of the importance of the role it plays in the cochleo-phonatory circuit has already allowed us to obtain telling results in the re-education of subjects presenting phonatory disorders.
We are convinced that this new idea will find many other applications, in particular in the aetiology of certain disorders such as diplacusis, which until now have not been determined with precision.
It will also be of interest to study the relative roles of the directing ear and of the other ear in the perception of “sound relief”.
This principle therefore offers new possibilities to audiology, both in the therapeutic domain and in that of research.
Source: Tomatis A., “L’oreille directrice”, Bulletin du Centre d’Études et de Recherches Médicales de la S.F.E.C.M.A.S., July 1953, pp. 68-75. Digitised document from the personal archives of Alfred Tomatis.