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Vesta in Latin slang of North-Western Europe means upper cloth, vestibulum – the entrance room of the house, where upper clothes being put off. Corti, who has first described its morphology, had no idea about its function and supposed it to be the entrance part of hearing organ. It was Ernest Mach, who disclosed its function in 1875.

Vestibular system

The vestibular system is most ancient complex of peripheral sensory analyzers and their connections in central nervous system (CNS, which provides posture, spatial orientation, locomotion and spatial interaction (for example, catching the ball). In its peripheral part, labyrinth of the inner ear there are six sensors: movement, gravity, vibration, saccular hearing, magnetic impulse and metabolism[1]. On the contrary to the other analyzers it has also integrative function: auditory system, visual, somatosensory (proprioception), olfactory systems are sending information to vestibular system, forming, thus, sensory sextad, six organs, providing orientation in environment: vision, hearing, proprioception (own body feeling), vestibular sense (position against Earth gravitation field and movement sensation), olfaction (recognition of own house against strange one) and magnetic compass (not in the vestibular periphery, but in the bones of ethmoid sinuses, innervated by trigeminal nerve)[2]. This complex appears first at the level of rhomboid fosse, then spreads at medial longitudinal fasciculus, Deiters and Cajal nuclei, cerebellum, lamina quadrigemina and caudal part of nucleus caudatus. It provides orientation in space, its dysfunction or lesion results in space orientation disorders. Peripheral vestibular organ forms many connections in CNS, which form four projections: vestibulo-cortical, vestibulo-motor, vestibulo-vegetative and vestibulo-limbic. Vestibulo-cortical projection is unique in sending vestibular information into four cortical areas: vestibular representation itself and also visual, hearing, and somatosensory cortex. Therefore, some symptoms we consider to be visual (photopsia, black-outs, teichopsia, flickering, micropsia), somatosensory (numbness) or hearing (tinnitus) by origin, have major input from vestibular analyzer[1]. Vestibulo-motor projection consist of vestibulo-spinal and vestibulo-ocular components, providing posture and locomotion. The lowest level of vestibulo-vegetative projection formation is the connection of lateral, medial and descending vestibular nuclei of rhomboid fosse[3],[4]. At this place the coordinating center is localized, tightly connected with sympathetic and parasympathetic brainstem centers. Its role is constant redistribution of blood volumes, if living being is moving head up, standing up[5]. It plays especially important role in bipedal beings, because, for example in humans its smaller mistakes cause crucial catastrophes in blood pressure. Normal function of this projection improves the activity of all the inner organs: heart, stomach, intestine, even vesica urinaria. Vestibulo-limbic projection is least studied, but in healthy beings it forms high life quality level. In pathology it causes many emotional and behavioral disturbances: bulimia, anorexia, libido decrease, depression and anxiety[6].

Sensitivity

Sensitivity in our case means responsiveness to external conditions or stimulation. In the classic sensology there are classic methods, which are used for the investigation of visual, hearing etc. functions.

1. Threshold method is used to study the absolute sensitivity. The stimulus intensity is regulated by the investigator. First he presents the suprathreshold stimulus, and then he decreases it to the level, when the patient reports the absence of the stimulus. The procedure is proceeded in the inverse scheme: the subthreshold stimulus is increased until the reaction appears.

2. The estimation method is performed in the same manner, but the patient himself is regulating the stimulus intensity. After several adjustment performances the threshold is considered to be the intermediate level between least sensed and insensed stimuli.

3. Adaptive method. The stimulus intensity level is changed smoothly; the direction of the intensity level is regulated by the patient himself with the help of the button. The stimulus intensity is presented as a function of time. The result looks like the ramp shape line, undulating between least sensed and incensed stimuli. The threshold is considered to be the strait line, which is in the area of middle between the extreme points.

4. Method of the standard stimuli means that the series of the standard stimuli of different intensity, following in random order, is presented to the patient. The intensity of the standard stimuli is overlapping all the expected range of the possible threshold stimulus intensities. Each stimulus intensity level is repeated several times during the procedure. As the result the function of reaction frequency on the stimulus intensity is prepared. The curve has typical sigmoid shape and reflects the probability of the reaction occurrence in response to the stimulus in each point of this intensity range.

Other methods are the modifications or combinations of the abovementioned.

General concept

Now we are coming to the idea that vestibular sensitivity might depend on the stimulus parameters. In this situation two opposite tendencies might be identified:

1. Threshold recording in response to intensive, but very short stimulus.

2. Recording of the latent periods in response to slowly increasing continuous stimulus.

Among the authors, who have used the first method, as we have already mentioned there is no one idea about the acceleration threshold sensitivity – the data differs for more, then four orders[7],[8]. As to the second approach all the authors agree that the latent period of movement sensation is decreasing with the increase of acting acceleration. Melcher and Hehn have studied this in details. If the latency is 11 sec for the acceleration of 0,1o/s2, for the accelerations of 1,0o/s2 it becomes 6,9 sec, for 5o/s2 – 2,0 sec, and for 10o/s2 it is already 1,0 sec and less[9].

Out of these data the next hypothesis should be assumed:

Measure of sensitivity is threshold

Threshold is understood to be the smallest stimulus causing the reaction studied. Speaking about vestibulo-cortical projection it is possible to study two types of reactions: subjective sensations or vestibular evoked potentials (VestEP). Subjective sensations have been studied at 36 healthy volunteers from 14 to 38 years old. In response to the threshold level accelerations all the persons studied have reported three types of the sensations. At the very low acceleration level the volunteers report the detection of the undiscriminated movement sensation. The increase of the acceleration causes the report of the movement direction, opposite to really acting direction of movement. This phenomenon is actual for both angular and linear movements. At last, starting from some acceleration level, the real direction of acceleration is reported. According to the sensations being described, three types of subjective vestibular thresholds have been identified[1],[10]:

1) Undiscriminated threshold (T1) – 8,3±3,2 cm/s2;

2) Inverted threshold (T2) – 12,2±2,2 cm/s2;

3) Discriminated threshold (T3) – 17,7±2,5 cm/s2

The same results have obtained the team of Prof. Benson from United Kingdom[7].

Coefficients of variability provide much information about the properties of the parameter[11]. They are calculated as:

Cv = s/M

Where Cv – is coefficient of variability, s – sigma, standard deviation, M – average. Both individual (intrapersonal, intraindividual) and interindividual (interpersonal) variability coefficients, either in the same day or within few day intervals have been studies. The biggest variability is demonstrated at the level of T1 accelerations. The interindividual variability coefficients recorded both in the same day and in the different days appeared to be practically identical (24,1 and 25,0% correspondingly). Individual variability coefficients are also close in both cases (16,1 and 19,1% correspondingly). From the other side, individual and interindividual coefficients are different. This indicates that undiscriminated threshold level has high level of individualization.

To study the dependence of sensitivity threshold from individual psychological peculiarities, they use the methods of binary relativity (probabilistic) education. The volunteer is proposed 5 series of 40 signals each, following in occasional, unpredictable mode. Binary – means two types of signal: positive, when the light is switched on during 0,5 s, and negative – pause. The person is proposed to predict the appearance of positive or negative signal. In true relativistic sessions they use different levels of positive signal appearance probability – usually 33, 50 and 67%, the volunteer is informed about the figure in advance. Data proceeding provides the possibility to separate the groups with overestimation and underestimation of the event role. The comparison of the results of the binary relativity study and T1 vestibular sensitivity threshold have shown, that for the group overestimating the event (28,6% of all persons studied) it has been 6,2±1,8 cm/s2, while in the group underestimating the event (14,3% of the persons) – 11,5±1,7 cm/s2. The intermediate group also exists, most numerous (57,1%). These data is important for understanding the sensitivity. First, there are individual psychological peculiarities in the threshold level signal detection, they are determined and stable. They might influence person’s everyday life and professional activities. For example, a hunter waiting for wild animal might start shooting, because he belongs to the overestimator group. Other version, the underestimator hunter doubting if there is or no animal, allows the predator to go and make harm. Second, most important, the proportion of overestimators to underestimators in the population is 2:1. The biological sense of this is that it is better to overestimate the event, then to underestimate it. In pathology this results in anxiety disorders. Their percentage in modern society is increasing because progress is accompanied with plenty of loadings: noise and vibration, information, speeds, "electromagnetic smog", house and industrial chemicals, changes in food preparation[1].

The most stable for all the volunteers appeared to be the level of the inverted sensations threshold T2. This means no statistically significant difference between the individual and interindividual variability coefficients recorded in the same day (10,3 and 12,8% correspondingly) and in different days (11,6 and 14,0% correspondingly). Surprisingly high appeared to be the variability of the discriminated sensations threshold level (T3). In this case the greatest preponderance of the interindividual over individual variability coefficients have been found (in the same day 16,5 and 8,7% and in different days – 33,6 and 22,4% correspondingly). The probabilistic behavior for T1 and T3 has appeared to be the same in the same persons, i.e. the overestimator repeat to be the overestimator in both cases, the same with underestimators. In the T2 case there is no differentiation into groups; this indicates the basic mechanism of formation of this sensation with minor individualization[10].

These sensations have special significance in aviation navigation, forming in the limited vision conditions the illusion of the non-correct direction of movement[12], see Sensory illusions in aviation. Anyways, characteristics of the patients’ complaints and their dynamics need specific attention and detailed investigation.


Conventional and relativistic threshold

There are two strategies for estimation of least stimulus intensity causing the appearance of the response expected: they are called (1) conventional and (2) relativistic or probabilistic. The first one is based on the agreement, convention, about the properties of the response. The responses might be categorized as either 'subjective' or 'objective'. Subjective responses deal with sensations, for example movement sensations, being just described in the case of the vestibular system. Objective responses, on the other hand, are recorded with the help of the special instrumentation. They are minor dependent upon the bias of the tester or the cooperation with the patient. Objective responses might be either direct or indirect. The direct response reflects the function of the system under study while the indirect response reflects the interaction of the system with other systems. Nystagmus, for example, in the case of vestibular analyzer, is an indirect response, because it reflects the interaction of the vestibular and oculo-motor systems. Response might be described in both qualitative and quantitative terms. Speaking about undiscriminated and discriminated subjective sensations at the vestibular sensation threshold level they deal with quality. Using figures to identify the level at which inverted sensations threshold appears, we use quantitative measures. From this point of view many studies might be regarded as multiple approaches.

Vestibular sensitivity determined by conventional method

For understanding of vestibular threshold the dependence of the parameters of the long latency potentials evoked by the acceleration from stimulus intensity have been studied. Three types of evoked potentials might be recorded: vestibular (VestEP) in the range of 0-20 cm/s2, transitory (TEP), which are mixed vestibular and somatosensory in the range of 20-40 cm/s2, and somatosensory (SEP), at the accelerations higher then 40 cm/s2[10].

In normal control subjects VestEP first appear above the noise level when 16 sweeps are averaged at accelerations of 2-5 cm/s2. The acceleration threshold is defined as the signal-to-noise ratio at which N1-P2 transition exceeds 2:1. With this definition VestEP threshold appears to be 4,9±2,2 cm/s2, with the latencies of the main peaks correspondingly: P1=34,1±8,8 ms, N1=72,6±8,4 ms, and P2=145,9±16,7 ms.

Since the judgment of 2:1 signal-to-noise ratio is the subject of agreement or convention, the attempts to develop more objective approach have been made. For this purpose the least-squares method has been used[13]. Increasing acceleration decreases the latency of peak P1 by 14,3 ms, N1 peak by 17,6 and P2 - by 2,7 ms. The ratio of the differences to their corresponding absolute latencies provides the values of the coefficients (C) of dependence for the peak latencies from stimulus intensity: CP1=42,2%, CN1=24,2%, and CP2=1,8%. So, it appears that peak with shortest latency P1 is the most dependent on the stimulus intensity while peak with longest latency P2 is least dependent on stimulus intensity. Use of the least-squares method provides the equations of linear regression determined as:

y = ax + b

where y – peak latency, x – acceleration, a – slope coefficient and b – is the y-axis intercept. Our data yield the following linear equations:

P1: y = -0,69 x + 33,6,

N1: y = -0,44 x + 64,0,

P2: y = -0,12 x + 145,1.

The y-axis intercept, coefficient b allows us to estimate the peak latencies of the VestEP at the threshold level acceleration; it is possible to name it ideal threshold latency (Fig. 1). Comparison of the data being obtained in such procedure with the experimental threshold recording shows that the data for P1 and P2 coincide nicely (34,1 and 33,6 for P1 and 145,9 and 145,1 for P2). The difference in the data for N1 is due to misestimating of the acceleration threshold, the latter appears to be lower, and then we have identified. This is the shortcoming of the conventional subjectivism.

See also

References

  1. ^ Trinus, K F (2011). "Human sensitivity to electromagnetic fields". Ukr. Med. Alm. №4 (Supplement). 14: 143–145. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  2. ^ Gacek, R R (1982). "Afferent and efferent innervation of the labyrinth". Adv. Oto-Rhino-Laryngol. 28: 1–13.
  3. ^ Gacek, R R (1994). "Anatomy of the central vestibular system". Neurotology. Mosby, St.Luis, Baltimore, Boston: 41–58. {{cite journal}}: |editor1-first= missing |editor1-last= (help)CS1 maint: multiple names: editors list (link)
  4. ^ Bolton, P S (1998). "Influences of neck afferents on sympathetic and respiratory nerve activity". Brain Res. Bull. №413. 47: 19. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  5. ^ Trinus, K F (1996). "Chornobyl vertigo. 10 years of monitoring". Neurootology Newsletter (Suppl. 1): 140.
  6. ^ a b Benson, A J (1986). "Thresholds for the detection of the direction of whole-body linear movement in the horizontal plan". Aviat. Space. Environ. Med. 57: 1088–1096. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  7. ^ Elidan, J (1991). "Short and middle-latency evoked responses to acceleration in man. Electroencephalogr". Clin. Neurophysiol. 80: 140–145. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  8. ^ Melcher, GA (1981). "latency of circular vection during different accelerations of optokinetic stimulus". Perception and Psychophys. 30: 552–556. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  9. ^ a b c Trinus, K F (1986). "Thresholds of long latency evoked potentials and movement sensations perceived during the linear acceleration action on human". Kosm. Biol. Aviakosm. Med (in Russian). 20 (№6). USSR Space Life Sciences Digest: NASA Contractor Report 3922: 62–66. {{cite journal}}: |issue= has extra text (help)
  10. ^ Reed, G F (2002). "Use of coefficient of variation in assessing variability of quantitative assays". Clin. Diagn. Lab. Immunol. 9, 6: 1235–1239. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  11. ^ Malcolm, R (1984). "Pilot disorientation and the use of a peripheral vision display". Aviat. Space Envir. Med. 3: 231–238.
  12. ^ Trinus, K F (1988). Vestibular evoked potentials - a new method for study of the combined effects of environmental factors. Recent advances in researches on the combined effects of environmental factors (Manninen O ed.). Tampere: ISCEF. pp. 143–152.
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