Immersed false vertical room. A new motion sickness model.

We evaluated a new model of motion sickness--an enclosure decorated with visual cues to upright which was immersed either inverted or "front"-wall down, in Johnson Space Center's Weightless Environment Training Facility (WETF) pool. This "WETF False Vertical Room" (WFVR) was tested with 19 male and 3 female SCUBA diver subjects, aged 23 to 57, who alternately set clocks mounted near the room's 8 corners and made exaggerated pitch head movements. We found that (1) the WFVR test runs produced motion sickness symptoms in 56% and 36% of subjects in the room-inverted and room-front-down positions, respectively. (2) Pitch head movements were the most provocative acts, followed closely by setting the clocks--particularly when a clock face filled the visual field. (3) When measured with a self-ranking questionnaire, terrestrial motion sickness susceptibility correlated strongly (P < 0.005) with WFVR sickness susceptibility. (4) Standing instability, measured with a modified Fregly-Graybiel floor battery, also correlated strongly (P < 0.005) with WFVR sickness susceptibility. This result may reflect a relationship between visual dominance and WFVR sickness. (5) A control study demonstrated that the inverted and front-down positions produced WFVR sickness, but the upright position did not, and that adaptation may have occurred in some subjects with repeated exposure. The WFVR could become a useful terrestrial model of space motion sickness (SMS) because it duplicates the nature of the gravity-dependent sensory conflicts created by microgravity (visual and otolith inputs conflict while somatosensory gravity cues are minimized), and it also duplicates the nature of the provocative stimulus (sensory environment "rule change" versus application of motion to passive subject) more closely than any other proposed terrestrial SMS model. Also, unlike any other proposed terrestrial SMS model, the WFVR incorporates whole-body movement in all three spatial dimensions. However, the WFVR's sensory environment differs from that created by spaceflight in several respects, including the presence of frictional drag on limb movement, magnification at the face-mask-water interface, greater otolith conflict, exhaled bubbles, and the presence of some gravity-dependent somatosensory inputs.

Inverted immersion as a novel gravitoinertial environment.

BACKGROUND: Inverted immersion (II) offers a unique opportunity to swing the orientation of the gravity vector 180 degrees from its usual configuration with vestibular end organs. During II, extrathoracic fluid dynamics are identical to those of upright immersion (UI). II exposes individuals to a novel gravitoinertial environment and, therefore, should produce motion sickness (MS). HYPOTHESIS: II is more provocative of signs and symptoms of MS than UI. METHODS: Nine subjects were exposed once to II and UI. Conventional SCUBA gear was worn. In II, the subject wore a wetsuit which provided 5-7 kg force of positive buoyancy and, with no weight belt, caused him to float while inverted against the underside of a platform. An experiment with UI was identical except that a weight belt was worn which provided 5-7 kg force of negative buoyancy, and the subject stood upright against the bottom of the pool. The experiment was terminated after 3 hours or "upon the onset of the first, clear, persistent feeling of nausea", whichever came first. Throughout the experiment the subject rotated through a series of tasks: assembly of a pipe puzzle, performance of a series of head movements, and ambulation. Immediately post-dive, postural stability was assessed with tandem standing with and without eyes closed and with and without the neck extended 45 degrees. A questionnaire regarding susceptibility to motion sickness was completed pre-dive. RESULTS: No subject terminated the test because of MS during UI; seven subjects terminated the test during II (p < 0.025, McNemar's test). Posture was less stable after II than after UI (p < 0.05, sign test). MS questionnaire results did not predict susceptibility to II. CONCLUSION: II is provocative of MS and postural instability.

Computer-quantified positional nystagmus in normals.

INTRODUCTION: Most current authorities agree that, in the absence of visual fixation, low intensity nystagmus can be recorded by electronystagmography (ENG) in clinically normal people. However, there are some notable dissenters, and the reported incidence, intensity, and direction of "normal" spontaneous/positional (S/P) nystagmus vary widely across laboratories. The highly subjective element introduced by manual interpretation of position test records compounds the problem of characterizing "normal" S/P nystagmus. MATERIALS AND METHODS: A computerized ENG analysis system which removes this element of subjectivity is described, and results of a study validating its results against those of experienced ENG interpreters are reported. RESULTS: The computer found an S/P nystagmus behind closed eyelids in at least one of five test positions in 42 of 51 clinically normal subjects (82.3%). This "normal" S/P nystagmus had a statistically significant overall directional bias toward left beating, but was affected by position--tending toward right beating in the left lateral position. The upper 95.4% confidence limits for computer-measured slow phase velocities of "normal" S/P nystagmus were -5.5 degrees/sec (left beating) and +4.4 degrees/sec (right beating). CONCLUSION: Possible explanations for "normal" S/P nystagmus include clinically undetected pathology, normal asymmetries in the vestibulo-ocular reflex, and an imprinted oculomotor pattern such as might be created by reading.

Acoustic distortion products in humans: systematic changes in amplitudes as a function of f2/f1 ratio.

The effects of primary-tone separation on the amplitude of distortion-product emissions (DPEs) at the 2f1-f2 frequency were systematically examined in ten ears of five subjects. All individuals had normal hearing and middle-ear function based upon standard clinical measures. Acoustic-distortion products were elicited at 1, 2.5, and 4 kHz by equilevel primaries at 65, 75, and 85 dB SPL, while f2/f1 ratios were varied in 0.02 increments from 1.01-1.41 (4 kHz), 1.01-1.59 (2.5 kHz), or 1.01-1.79 (1 kHz). A principal outcome reflected in the detailed structure of both average and individual ratio functions was a nonmonotonic change in DPE amplitude as the ratio of f2/f1 increased. Despite the presence of amplitude nonmonotonicities, there was clearly a region of f1 and f2 separation that generated a maximum DPE. The effects of primary-tone separation on DPE amplitudes were systematically related to DPE frequency and primary-tone level. For all three levels of stimulation, the f2/f1 ratio was inversely related to DPE frequency. Thus larger ratios reflecting a greater separation of f1 and f2 were more effective in generating DPEs at 1 kHz rather than at 4 kHz. The optimal ratio for 2.5 kHz fell at an intermediate value. Conversely, acoustic distortion-product amplitude as a function of primary-tone level was directly related to the frequency separation of the primary tones. Regardless of the frequency region of the primary tones, smaller f2/f1 ratios were superior in generating DPEs in response to 65-dB stimuli, whereas larger ratios elicited bigger DPEs with primaries at 75 and 85 dB SPL. Within any specific stimulus-parameter combination, individual variability in DPE amplitude was noted. When all stimulus conditions describing the variations in frequency and level were considered, an f2/f1 ratio of 1.22 was most effective in maximizing DPE amplitude.

Spontaneous otoacoustic emissions in a nonhuman primate. I. Basic features and relations to other emissions.

Otoacoustic emissions in both ears of a rhesus monkey exhibiting stable spontaneous emissions (SOEs) were monitored over a 1-year period. The amplitudes and frequencies of both SOEs and stimulus-frequency emissions (SFEs) were routinely recorded, while transiently evoked (EOE) and distortion-product emissions (DPEs), at the frequency 2f1-f2, were occasionally examined. Between evaluation sessions, both the frequencies and amplitudes of SFEs remained relatively stable in both ears, while the frequencies and amplitudes of SOEs were less constant. Isosuppression contours for SOEs, plotted as a function of frequency and level of tonal maskers, revealed sharp tuning consistent with normal frequency selectivity. Detailed analyses of long-term measurements showed that SOEs occurred most frequently at the peaks of the SFE response. A regular frequency spacing between neighboring amplitude maxima and minima of the SFEs was consistent with the notion that this particular emitted response may result from a periodic disruption of the orderly pattern of sensory cells along the organ of Corti. Intramuscular administration of aspirin abolished SOE and SFE responses, while DPEs remained relatively unchanged suggesting the involvement of separate mechanisms in the generation of different emissions.

Spontaneous otoacoustic emissions in a nonhuman primate. II. Cochlear anatomy.

Both cochleas of a rhesus monkey exhibiting stable spontaneous and stimulus-frequency emissions were evaluated histologically using surface-preparation methods to determine if certain features of these emissions could be related to structural properties of the organ of Corti (OC). The comprehensive assessment included preparation of routine cytocochleograms and a detailed study of the arrangement of cochlear sensory cells, best revealed by the precise positional relationships between stereocilia bundles, in selected areas representing low-, medium-, and high-frequencies. Several additional measurements were made in an area extending from about 25-60% distance from the apex, which was estimated to encompass the cochlear region where emissions were generated. These quantifications included measures, in both micrometers and Hertz, of the distances between irregularities in the lateral border of the OC due to a sporadically occurring fourth row of outer hair cells (OHCs). Measures, in micrometers, of the changes in the radial extent of the corresponding OC in the presence or absence of this extra fourth row of OHCs were also made. A final measure within low-, medium-, and high-frequency OC regions consisted of describing the angles that the tips of the stereocilia bundles were displaced from an axis parallel to the tunnel of Corti. For comparative purposes, similar plots were made in comparable regions of the OC in the normal and experimental cochleas of three additional rhesus monkeys in which one ear had been systematically exposed to noise. In the emitting-monkey cochlea, there was a mild loss of sensory cells scattered throughout the OC which was generally greater for the OHCs. No evidence of small circumscribed lesions, defined as a loss of more than four adjacent hair cells, was found. The most striking observation which varied in degree across the three other monkeys was a generalized irregularity in the cellular organization of the OHC region which was most pronounced in the low- and midfrequency regions of the OC. The notable cellular disorganization specific to the apical half of the cochlea was reflected by an increased variance in the distribution of deviation angles measured for corresponding stereocilia bundles. Outer hair cells in the remaining basal region of the OC were arranged in three regular rows with the usual stereocilia orientation.(ABSTRACT TRUNCATED AT 400 WORDS)

Otoacoustic emissions in ears with hearing loss.

Fifty ears of 37 patients demonstrating several common types of hearing impairment were examined for the presence of spontaneous and evoked otoacoustic emissions to investigate the relationship of acoustic emissions to hearing pathology. Of the 50 ears, 44 exhibited various degrees of sensorineural hearing loss. Evoked otoacoustic emissions to clicks were detected in 34 of 35 sensorineural hearing loss ears with a subjective click threshold less than 55 dB SPL (25 dB nHL). None of nine ears with sensorineural hearing impairment and a subjective click threshold greater than 55 dB SPL demonstrated click-evoked emissions. Spectral analyses revealed that the constituent frequency components of evoked emissions were always within the frequency range where audiometric thresholds were less than 35 dB HL, and in the majority (94%) of cases, thresholds were less than 25 dB HL. In ears with relatively well-preserved hearing within the frequency range of click or 1.5-kHz toneburst stimuli, the basic features of evoked emissions were similar to those described for normal ears. Similarly, for ears demonstrating spontaneous otoacoustic emissions, estimated audiometric thresholds at the emitted frequencies were always less than 20 dB HL. The influence of the type of otologic pathology on acoustic emissions was studied in a subset of ears exhibiting typical high-frequency hearing losses. Ears with a noise-induced impairment showed a significant reduction in the incidence of both spontaneous emissions and spectral peaks in evoked emissions that was not evident in ears with similar patterns of hearing loss caused by other factors.

Acoustic distortion products in rabbit ear canal. I. Basic features and physiological vulnerability.

In contrast to evoked otoacoustic emissions, acoustic distortion products (DPs) recorded from the ear canal are present at predictable frequencies with respect to their primary tones, f1 and f2. Such specificity may provide detailed frequency-place information concerning the functional state of limited regions of the organ of Corti following experimental intervention. However, to date, it is not clear whether emitted DPs solely reflect activity at the basilar-membrane regions of primary tones or if the remote DP site makes a significant contribution to the emitted signal measured in the ear canal. We have investigated a number of the general features of acoustic-DP generation in the rabbit so that, in later experiments, the contributions of specific basilar-membrane regions involved in generating these DPs can be identified using techniques designed to manipulate their normal properties. The first report describes the outcome of systematic manipulations of a number of stimulus conditions and alterations to the physiological state of the cochlea by exposure to fatiguing sound or anoxia. Experimental findings for the 2f1-f2 DP showed that, in general, the relations of the levels and frequency of the primary tones to DP magnitude were consistent with previously published data from other mammalian species. Additional observations for other odd-order intermodulation DPs at the 3f1-2f2 and 2f2-f1 frequencies suggested that the basic attributes of the acoustic DPs were similarly affected by systematic manipulation of the basic parameters of the primary tones and the general metabolic state of the cochlea. General anesthesia, however, did not affect DP amplitude. A companion paper describes the results of a series of subsequent experiments using response-suppression, interfering-tone, and temporary threshold shift techniques which address more directly the issue of which basilar-membrane sites contribute to the generation of different acoustic DPs.

Acoustic distortion products in rabbit ear canal. II. Sites of origin revealed by suppression contours and pure-tone exposures.

Previous work on acoustic distortion products (DPs) recorded from the ear canal has not established unequivocally whether emitted DPs principally reflect basilar-membrane nonlinearities at the frequency sites of the primary tones, f1 and f2, or if the DP-frequency place itself makes a significant contribution to the emitted response. Results from some studies on acoustic emissions attribute generation of the emitted DP almost exclusively to the regions of maximum primary-tone interaction, while the findings of other investigations implicate reemission of the response from the DP locus as a significant contributor to response magnitude. Using suppression, interfering tones, and temporary threshold shift (TTS) procedures, the work reported here was designed to establish more definitively the precise contributions of the basilar-membrane regions involved in generating acoustic DPs in rabbits. Suppression tuning curves and interfering-tone experiments indicated that for the DP at 2f1-f2, regions near the f1 or f2 frequencies were the major contributors to the emitted response. However, for the higher-frequency DP at 2f2-f1, the basilar-membrane region just basal to the DP site was implicated as the generator. Following brief episodes of TTS at frequencies related to either the DP or the primary tones, the locus of the exposure stimulus that most effectively reduced the magnitude of the 2f1-f2 response also implicated the region of maximal primary-tone interaction in the generation of the acoustic DP. In contrast, for the DP at 2f2-f1, basilar-membrane sites nearer the DP were identified as the primary contributors to the emitted response. Both sets of results imply that different DPs recorded from the ear canal may originate from unique regions of primary-tone interaction along the basilar membrane.

The normal summating potential recorded from external ear canal.

With a "plastic leaf" electrode, cochlear summating potential (SP) and auditory nerve action potential (AP) responses to rectangular-pulse clicks were recorded from the ear canal skin surface of 96 normal-hearing ears of 48 subjects. The main goals of this investigation were to develop a more precise characterization of the relationship between SP and AP amplitudes across normal ears and to determine the confidence limits of this relationship so that a more accurate "normal limit" could be established for clinical testing. The results suggest that the across-subjects SP-AP amplitude relationship is linear. Also, SP scatter increases as AP amplitude increases, but the scatter is equalized by log transforming the data. The distance of the SP from the log-transformed SP-AP estimating line in SE ("AP-normalized SP amplitude") was found to be superior to the SP/AP amplitude ratio as a method of adjusting SP to AP amplitude, because the SP/AP ratio varied significantly with AP amplitude both across subjects and with different ear canal electrode positions.

Spontaneous, click-, and toneburst-evoked otoacoustic emissions from normal ears.

Evoked and spontaneous otoacoustic emissions were recorded bilaterally in a group of normal subjects (n = 14) using clicks and tonebursts at four frequencies (0.5, 1, 1.5, and 3 kHz). All ears (n = 28) demonstrated evoked emissions, but not to every stimulus type. The 0.5-kHz toneburst evoked emissions in only 10 (36%) ears, the 1.5-kHz toneburst in all ears, and the remaining stimuli in at least 80% of ears. Two distinct patterns of evoked emissions were identified. Five (18%) ears showed short, broadband click-evoked emissions lasting less than 20 ms after stimulus onset. In these ears, toneburst-evoked emissions were often more prominent than click-evoked emissions and no spontaneous emissions were detected. Twenty-three (82%) ears showed click-evoked emissions lasting longer than 20 ms poststimulus onset. Spectral analysis of these emissions demonstrated several (2-10) narrow frequency peaks. Highly similar peaks were present in the spectra of toneburst-evoked emissions within the range of toneburst spectra. Spontaneous emissions were recorded in 12 of the 23 ears. In these ears, at the frequencies of spontaneous emissions, prominent peaks in both click- and toneburst-evoked emission spectra were always present. Otoacoustic emission characteristics correlated significantly between the ears of individual subjects inferring that a symmetrical cochlear mechanism generates otoacoustic emissions.

Spontaneous otoacoustic emissions in the nonhuman primate: a survey.

A number of reports have described a relatively high incidence of spontaneous otoacoustic emissions (SOAEs) in recordings made from the sealed human ear canal. Our attempt to detect similar emissions in 122 presumably normal-hearing ears from 61 monkeys revealed SOAEs in 5% of the primates and 2.5% of the ears tested.

Auditory evoked potentials--the cochlear summating potential in detection of endolymphatic hydrops.

With an expanded clinical and normative SP amplitude series, we have re-examined the incidence of SP enlargement in clinically defined Meniere and cochlear ears. The overall incidence of SP enlargement in the expanded series is somewhat less than that found in our previous study (56% versus 68%). This difference may be due to considerable preselection of patients included in the previous smaller clinical population, since many of these patients were recruited for the study because of diagnosed Meniere's disease. In contrast, the present, larger clinical population was selected from our normal service referral base, and thus contains a much higher percentage of diagnostic problems. The comparison between SP enlargement and hearing levels demonstrates that the SP test for Meniere's disease must be interpreted in the light of high-frequency hearing levels. If the patient has normal or near-normal hearing (4- to 8-kHz hearing levels 25 dB or less), SP enlargement will not be a good criterion for detecting the presence of Meniere's disease. Gibson and colleagues and Kumagami and associates have reported similar observations. Also, in the presence of severe high-frequency hearing loss (above about 70 dB), SP enlargement occurs relatively infrequently among ears with clinical diagnoses of Meniere's disease. However, in the mid-range (25 to 70 dB) of high-frequency loss, SP enlargement appears to provide a reasonably accurate test for Meniere's disease (with a "hit rate" among clinically defined Meniere ears of about 71%).(ABSTRACT TRUNCATED AT 250 WORDS)

Compound action potentials and single unit responses to a 1 kHz haversine in the cat.

The single-cycle 1 kHz haversine (one cycle of a 1 kHz sine wave beginning at -90 degrees) is a low-frequency impulsive stimulus which has been little use, but which has significant potential applications both as a clinical and a research tool. The auditory nerve compound action potential (CAP) and single unit discharge patterns evoked by a single-cycle 1 kHz haversine stimulus were studied in anesthetized cats. The haversine CAP waveform consisted of two or three short latency peaks with peak to peak intervals of about 1.0 ms. Latencies of the CAP peaks decreased with increased stimulus intensity and were also strongly dependent on stimulus polarity. Typically, CAP peak latencies changed by about 0.5 ms with stimulus polarity reversal. Single unit responses were classified by the peak latency pattern of their haversine post-stimulus time histograms (PSTHs). Low CF units had low thresholds and PSTHs resembling their click responses. High CF units had high thresholds and PSTHs comprised of one or two short latency peaks whose latencies were polarity-sensitive. Some units in an intermediate CF range (approximately 1.5-3.0 kHz) had PSTHs which were a transitional form between the high and low CF types of response. The unit discharge patterns strongly suggested a low frequency origin for the haversine CAP at all intensities.

Subcortical correlates of the auditory brain stem potentials in man: bipolar EEG and multiple unit activity and electrical stimulation.

Bipolar EEG and multiple unit activity (MUA) responses related to the vertex auditory brain stem potentials (ABSP) were studied in different subcortical loci in parkinsonian and epileptic patients with implanted electrodes used as an electrophysiological procedure for surgical treatment. In addition, the subjective responses of patients to the electrical stimulation of these subcortical loci were studied for their possible auditory sensations. (1) Two types of bipolar EEG responses to monoaural stimulation were recorded from a limited number of subcortical loci: type A responses were formed by two consecutive components of opposite polarity peaking at 5.2 and 8.5 msec post-stimulus in 3 different loci contra- and ipsilateral to the stimulated ear: mesencephalic reticular formation (MRF), medial geniculate (MG) and lateral geniculate (LG) nuclei thalami. The polarity of the first component was everted while the second component was inverted between MRF and MG. Type B responses were formed by one component mainly peaking at 15.5 msec post-stimulus with no polarity reversal in either side within the ventral portion of the hippocampus bilaterally (CnA). (2) MUA responses were recorded only within the basal portion of MG bilaterally. Contralateral MUA responses consisted of 3 consecutive activation bursts peaking at 5.5, 7.2 and 9.0 msec post-stimulus and correlated in latency with waves IV-V, VI and VII of ABSP respectively. Ipsilateral MUA responses consisted of two consecutive activation bursts peaking at 14.0 and 16.0 msec post-stimulus and correlating with SN following ABSP. (3) Threshold electrical stimulation of the MG elicited subjective auditory responses referred to the contralateral ear.

Ménière's disease and the summating potential. III. Effect of glycerol administration.

We investigated the effect of administering hypertonic glycerol on cochlear and auditory nerve responses to rectangular-pulse clicks and on pure-tone behavioral thresholds. Eleven ears with Ménière's disease, and 20 ears without the disease were studied. Among the ears with Ménière's disease summating potential (SP) amplitudes decreased and 250 to 1,000-Hz thresholds improved after glycerol administration. Action potential amplitudes from the ears with Ménière's disease also decreased after glycerol ingestion, but to a lesser degree. In contrast, among the ears without Ménieère's disease, 250- to 1,000-Hz behavioral thresholds did not change significantly after glycerol nor did any of the electrophysiologic responses. Analogous group differences were found when the ears were regrouped according to the presence of absence of pathologic SP enlargement. These results demonstrate a tendency for three variables-(1) clinical diagnosis of Ménière's disease, (2) enlarged SP, and (3) positive glycerol test result-to cluster together in the same population of ears.

Meniere's disease and the summating potential. II. Vestibular test results.

Results of electronystagmographic (ENG) position and bithermal caloric tests from patients with abnormally enlarged summating potentials (SPs) were compared with results from a unilateral weakness (UW)-matched ENG-comparison group and with a group of patients with unilateral cochlear deficits and dizziness but without enlarged SPs. The patients with enlarged SPs had significantly higher incidence of caloric UW on the involved side than did the patients with cochlear deficits but without enlarged SPs. Also, the patients with SP enlargement had a significantly higher incidence of directional preponderance toward the involved side than did either of the comparison groups. Across-group differences in position test results were predictable from the caloric test results, but these differences were not statistically significant.

The summating potential and Meniere's disease. I. Summating potential amplitude in Meniere and non-Meniere ears.

We investigated the relationship between Meniere's disease and the abnormally enlarged summating potential (SP) in a series of patients and normal subjects. Cochlear and auditory nerve responses to rectangular-pulse clicks, delivered at a rate of 8/s and an intensity of 115 dB peak-equivalent sound pressure level, were recorded from ear canal skin. We performed a parabolic regression for SP amplitudes as a function of auditory nerve action potential (AP) amplitudes in normal ears and obtained the 95% confidence interval (+/- 2 SD) for normal scatter around the fitted curve. Sixty-eight percent of SP amplitudes from ears with Meniere's disease exceeded the upper normal confidence limit. In contrast, only 7% of the cochlear ears and none of the retrocochlear ears had SP amplitudes above this normal upper limit. Cochlear microphonic potentials and APs from the ears with Meniere's disease also tended to be larger than normal, but this tendency was not strong enough to be of diagnostic use.