Formant structure of the voice during the intensive acute hypoxia

The influence of ;Iltells;ve {lcllte hypoxia 011 the !requency-alllpJilllde formant vocal 0 characteristics was investigated i/l 'his slUdy. Examinees were exposed 10 'he simulated altitudes of5500 III and 6700 III ill cJimabllro chamber ami resoll'ed Lotig 's test ill the conclitiolls of /lormoxia, i.e. prot/out/ced tile cllree·digit ,lUmbers beginning from 900. bllt ill reversed order. Frequency and intensity I'Glues o/llocal 0 (Flo Fl. FJ wul F.,), extracted fr011l flie colltext of the pro1lunciation of rile word eight (osam ill Serbia,,), were measured by spectral speech signal analysis. Changes ;11 frequency wllues alld the ill tensity of the jorl1lo1lts were examined. The obtained results showed thar there lI'ere flO significa1lt dlal/ges of tI,e formant frequencies in hypoxia condition compared to /Iormoxia. Though. signijicant changes oj formam's imens;ties were found compared to nortnoxia 0/1 the cited alritudes. Tlte rise offormants intensities was jound at the altitude of5500 m. Hypoxia ar the altitude oj6 700 m caused the signijicam fall oj the imellsities if! the initial period, compared to normoxia. The prolonged hypoxia exposure caused the rise oj the forma1lf ;fIl(!lIs;ties compared to the altimde of 5500 Ill. III may be cOllcluded that, due to dijferem altitudes, hypoxia causes differelll effects Oil fi,e formants struCl/ire challges, compared to IlOrmoxia.


Introduction
Hypoxia -the lack of oxygen in the lissue.is an imponant stressor which effects are manifested at all levels of lhe tissue organization.According to the previous studies (1-7) hypoxia was confirmed as an intensive stressor which influenced person's psychophysiological status.The more intensive and longer hypoxia, the grealer and deeper the changes, and the lesscned adaptation of a person.Moreover, the changes were found on the formant frequency in the condiLions of psychological stress (8).
The most sensitive of all the tissues is nerve tissue.alt hough all the brain structures and not equally sensitive to the lack of oxyg~I1.The most sensitive tissucs are the phylogenetic youngest nerve structures and functions.According LO the phylogeneLic age, the speech is a young psychophysiological function.For thaI reason.in cases of hypoxia lhe speech signal undergoes certain changes and dcformat ions, in fact the changes in phonctics and articulation occur.The frequency and the amplitude of formant voice structure arc above all detcrmined by resonants -cavities in the vocal tract, which configuration is specific when il comes to the production of the certain voice structure.Formant frequencies are essentially central frequencies of the supralaryngeal vocal tract.acting as a complex filter Ihal lets the max\mum of sound energy trough in several bands of frequency.Vowels owe their phonetic quality to their different fornunt frequencies.The symbols Fit F 1 and F) are usually used to denote formant frequencies of speeCh sounds (9).The position of Ihe formants in the spectrum depends initially on the vowel.as well as on gender and age (10) of the person and his/her psychophysiological (psychophysical.emotional and health) state.The changes of acoustic voice paramctcrs.e.g., formant struclure, dcpend on the reached height, or the intensity of hypoxia.the duration of exposure to hypoxia, and the individual resistance or vulnerability (4-6, I!.12).The aim of this sludy was to invcstigate Ihe innuence of the intensive acule hypoxia on lhe frequency-Olmplitude formant vocal "0" characteristics at the altilude 5 500 m and 6700 m.The choice of the effects of hypoxia at 5 500 m and 6700 m in this sludy was determincd above all by the Crpalla 156 BOJHOCAHHTETCKH nPErflE):\ Gpoj 2

Results
content of syllables and phonemes of these tWO numbers.which is in larger part identical.

Hypoxia at 5 500 III altitude
In the initial exposure to hypoxia at 5 500 III altitude (f number) ~lS well as the prolonged effect of hypoxia at 5 500 altitude (II number) the intensity of thc vocal 0 formant was gre<Hcr when compared to the intensity of formants in the normoxic condition (Figure I   • C ~'S i " ~~, So' S 1

S;
>T he frequency-amplitude formant values of the vocal "0" were specified (F i , F 2 , F) and F 4 ) and the relation between the frequency and the intensity in the following environment was established: I. hypoxia at 5 500 m altitude compared to nonlloxia, 2. hypoxia at 6 700 m ahitude compared to normoxia, and 3. hypoxia at 5 500 m altitude compared to hypoxia at 6700 m altitude.Statistic data processed by variance analysis showed, that there were no significant changes of the fonnant frequencies in the hypoxic condition, compared to normoxia.However, the intensity of formant underwent certain changes in hypoxic conditions, compared to normoxia.relevancy of the changes of biological parameters.The greatest altitude on which person's habitats are formed and cxust is 5 500 In height above the sea (Ands, Tibet.Tjen-Schen, Himalaya).The impossibility of survival above this height confirmed that this height repersentcd the funhest border of the adaptive power and the final range of individual psychophysiologicnl capacity.A healthy individual not yet adapted to such height remained conscious for about 30 minutes.Hemoglobin saturation with oxygen progressively decreased with the allitude.frollllhe value od 90% at the altitude of 3000 rn to the value od 70% at the altitude of 6000 m, i.e. 40-50% at the altitude of?000 m (13).
Due to the practical reasons it was important to speci fy the intensity of the stressor which sustainabililY was on the border of person's performances, yet not causing irreversih ie damage.These conditions could be found m the altitude of 5 500 m, and were used for the estimation of the resis• tance degree to hypoxia, i.e. the l:lck of ox.ygen.While at Ihe altitude of 5 500 In :l permanent adaptalion is possible.at the altitude of 6 700 m the survival during the longer period is nor possible.Survival is possible only for a short period and the complete ability for work ("the time of useful consciousness") is preserved in rhe first ten minUles of exposing an individual to this height, and hemoglobin saturalion with oxygen is 60%.Thus, the height of 6700 m represents an intensive stressor an ambient from which, in order lO survive, an individual must be taken away.This altitude is very mportanl for the pilors, especially in the critical occasions such as the mask cancellation or depressurization of lhe p\;.II1C cabin.The discovering of the indicators of hypoxia, such as vocal indicators, might predict pilot's reaction and behaviour in the critical situation of the flight.The changes of the spectral voice characteristics recorded in the real time, during flight, in the situation of hypoxia, might be a significant sign to underto.keadequate steps to prevent an accident of catastrophe.

Methods
Twenty-two male subjects participated in this study.The examinees were the pilots in good health.28-37 years of age.The lirsl eleven were exposed to the hypobaric hypoxia in a c1imabarochamber at 5500 III altitude and the other eleven to hypoxia at 6700 III altitude.The examinees in the c1imabarochamber and "on earth" did Lotig's test of useful consciousness: they pronouced [he three-digit numbers slarting with 900, but in the revarsed order (900, 899, 898 ...).
The extraction of the vocal 0 from the context of the word "eighr" ("osam" in Serbian), numbers 838 -I number and 638 -11 number, was done by means of a computer programme "Sound forge 4.5c".The speech signals of the vocal 0 Out of the utterance of the given numbers were chosen for the analysis due to the appropriate lime inteval between words.prolonged exp osurc to hypoxia (about 10 minutes) and bec:lUse of the The differences in the intensily :'Ire various: F==27 .811;p<O.OI.There was no greater effect on lhe intensity of formant's vocal 0 on the prolonged effecl of hypoxia at 5 500 m altilude.compared to the intensity of formanlS in the initial stage of hypoxic effect at the above rnelllioned height (Figure 3).The differences of the intensity of the initial formant and the prolonged effeci of hypoxia were important just for the !)ccond [orm::1Ot (F 2 was lessened in the initial phase.where the effecl of hypoxia was 6 700 m in allitude.Hypoxill til 6 700111 altitude In the inilial stage of hypoxia effect al the altitude of 6700 m the formant's intensity was lessened: F=27.127; p<O.OI (Figure 4).However, the longer Ihe exposure 10 hy• poxiu, the greater the formam's imcnshy.For that reason the statistical relevance of the intensity due La the prolonged exposlIre LO the altitude of 6 700 m compared to "the earth" (Figure 5 and 6) was losl.

Hypoxia ar 5 500 111 altitude compared to hypoxia at 6 700 111 altilllde
There was the initial effect of hypoxia at 5 500 m altitude on the formant's structure of the vocal 0 compn.red to the initial effect of hypoxia at 6700 m altitude.The intensily of the vocal 0 formant was significantly greater al hypoxia "' 5 500 m altitude: F= 18.299: p<O.OO I (Figure 7), This was probably a case of gelling out of shock.and passing into the phase of reconvalescence and adapting oneself to the strcs~d situalion thai was provoked by hypoxia at such altitude.Due to the changes in the intensity of formants, during longer exposure to hypoxia at 6700 m n.ltiwde.there was no difference in the formant's intensity when cn.mc to the production of vocal 0 in hypoxia at these two altitudes for the first two formants (Figure 8).The differences of intensity were significant for the third (F=6.519;p<O.OI) and the fourth (F=23.623;p<O.OI) formant.

Discussion
Acute hypoxia at the altilUde of 3 500 m caused the changes of man's psychophysiological performances (13), and with the changes of height (hypoxia) the effects became more evident.The trekkers, who climb too high and too fast, may

Impulses from the chemoreceptors in the aortal and carotid bodies increase as paz falls and change the fnclional state of the respiratory center. These chages cause
the changes in pulmonary ventilation, and phonation.Additionally, reticular activation system stimulates cortex of the brain and center of the speech produclion (Broca) and have consequences to the process of phonation and articulation.
3. The low atmospheric pO, at high altitude change the characteristics of the resonators of the cavities (pharynx, nose, mouth) of the articulator apparalus.
Additionally, vocal effort of overworking causes the modification of the quantity of energy (20.21) at the frequeneies above 2000Hz (II).Hypoxia selectively damages certain brain Slructures such as hippocampus, cerebellum, layers III.IV and VI of neocortex and basal ganglia.The decrease of motor speech control due hypoxia might be caused by degradation of palhways from subcortical ganglia to the prefrontal cortex (22).
The mutual effects of these mechanisms specify the characteristics of the speech signal, e.g.formant structure, in the condition of hypoxia.
For these reasons, it is import01nt to develop a method of distant estimation of psychophysiological and neurophysiological performances by computer speech analysis in real time.

BJsed on the analysis of the results, the following can be concluded:
There are no significant changes in the frequencies of vocal a formants in the hypoxic conditions, compared to normoxia.
The rise of formants intensities when compared tot he normoxia was found at the allitude of 5500 m.
Hypoxia at the altitude of 6700 m caused the fall of intensities in the initial period compared to normoxia.
In the initial period of hypoxia at the alatilude of 5500 m the formants intensities are greater, compared to the formants speech intensity at the altitude of 6 700 m.
The above-mentioned data proved that hypoxia at these heights innuenced the changes of the speech formants structures in a different way when compared to normoxia.Hypoxia at the altitude of 5 500 m had a stimulalive effect on speech production, while hypoxia at the altilude of 6 700 m decreased.

Fig. I -
Fig. I -The averaged values of the vocal 0 formants from I number in hypoxic condilion al 5 500 m alliwde (5 500J) and normoxia

Fig. 2 -
Fig. 2 -The averaged V~IJUCS of the vocal a formants from II number in hypoxic condition at 5 500 m altilUde (5 50011) and normoxia Fig. 6 -The averaged values of the vocal 0 formonls from I and II number in hypoxic condition al 6 700 m (6 7001 : 6700flJ
Fig. 5 -The averaged values of the vocal 0 formants from II number in hypoxic condition at 6700 m (6 7001I) and normoxia suffer from initial acute mauow.insickness (AMS).with highã ltitude cerebral edema (HACE) as an outcome.It includes headache, confusion.focal neurologic:l1 deficits and occasionally progression to coma.In addition.high-altitude encephalopathy is often associmcd with the coexistent high-altitude pulmonary edema (14).The diagnosis of AMS was made in 29.8% of study population at 4234 m in the Everest region (15).Hacken et a!.(16) identified a clinical imaging correlale for high-altitude encephalopathy and changes in the white matter of the splenium of the corpus callosum.which sugests vasogcnic edema as the physiopathologic mechanism.The moulltJineer (otherwise a healthy man), at an Jltitude of 5000 m, who experienced two attacks of a fully reversible right sided hemisymptomatology and had a difficulty in finding word and impairing sJX:ech, was suspect for the migraine with aura (digitolingual syndrome), transient ischemic auack of cerebrovascular stroke (17).The reversal damage of cognitive functioning at these Jilitude conditions comprises the decreased ability to give information verbally and the decrease of cognitive functioning (18), abilities necessary for performing Lotig's tesl of counting in the reversed order (19).The characteristic spectral changes of the speech sig-Ilal in hypoxia are probably the results of the following mechanisms and their synergic action: I.The low atmospheric pOl at high altitude leads to the pOl fall in lhe alveoli and consequently to the decreased oxygen concentration in the blood.Insufficient oxygenation of the brain may cause the dysfunction of lhe interneuronal connections important for generation and transmission of the nerve impulses in the polysinaptic chain which determines the muscles activities engaged in phonation and articulation.Due 10 hypoxiJ, lhe dysfunction of phonation and articulation leads to characteristic changes of speech signal, firstly, 10 the changes of its formant Siructure.