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PubMed2001

Effects of nitrous oxide on mask induction of anesthesia with sevoflurane or isoflurane in dogs.

OBJECTIVE: To determine the effects of nitrous oxide (N2O) on the speed and quality of mask induction with sevoflurane or isoflurane in dogs. ANIMALS: 7 healthy Beagles. PROCEDURE: Anesthesia was induced with sevoflurane or isoflurane delivered in 100% oxygen or in a 2:1 mixture of N2O and oxygen via a face mask. Each dog received all treatments with at least 1 week between treatments. Initial vaporizer settings were 0.8% for sevoflurane and 0.5% for isoflurane (0.4 times the minimum alveolar concentration [MAC]). Vaporizer settings were increased by 0.4 MAC at 15-second intervals until settings were 4.8% for sevoflurane and 3.0% for isoflurane (2.4 MAC). Times to onset and cessation of involuntary movements, loss of the palpebral reflex, negative response to tail-clamp stimulation, and endotracheal intubation were recorded, and cardiopulmonary variables were measured. RESULTS: Administration of sevoflurane resulted in a more rapid induction, compared with isoflurane. However, N2O had no effect on induction time for either agent. Heart rate, mean arterial blood pressure, cardiac output, and respiratory rate significantly increased and tidal volume significantly decreased from baseline values immediately after onset of induction in all groups. Again, concomitant administration of N2O had no effect on cardiopulmonary variables. CONCLUSIONS AND CLINICAL RELEVANCE: Administration of N2O did not improve the rate or quality of mask induction with sevoflurane or isoflurane. The benefits provided by N2O attributable to concentrating and second gas effects appear minimal in healthy dogs when low solubility inhalation agents such as isoflurane and sevoflurane are used for mask induction.

Anesthesia, Inhalation↗
PubMed2002

Effects of medetomidine-midazolam, midazolambutorphanol, or acepromazine-butorphanol as premedicants for mask induction of anesthesia with sevoflurane in dogs.

OBJECTIVE: To characterize the effects of medetomidine-midazolam, midazolam-butorphanol, or acepromazine-butorphanol as premedicants for mask induction of anesthesia with sevoflurane in dogs. ANIMALS: 10 healthy Beagles. PROCEDURE: The following premedicants were administered intramuscularly: medetomidine-midazolam (20 microg/kg and 0.3 mg/kg, respectively), midazolam-butorphanol (0.1 and 0.2 mg/kg, respectively), and acepromazine-butorphanol (0.05 and 0.2 mg/kg, respectively). Saline (0.9% NaCI) solution (0.1 ml/kg) was administered intramuscularly as a control. Anesthesia was induced in each dog with sevoflurane in a 100% O2 at a flow rate of 4 L/min developed by a facemask. Vaporizer settings were increased by 0.8% at 15-second intervals until the value corresponding to 4.8% sevoflurane was achieved. Time to onset and cessation of involuntary movements, loss of the palpebral reflex, negative response to tail-clamp stimulation, and endotracheal intubation were recorded, and the cardiopulmonary variables were measured. RESULTS: Mask induction with sevoflurane in dogs that received each premedicant resulted in a shorter induction time and milder changes in heart rate, mean arterial blood pressure, cardiac output, and respiratory rate, compared with mask induction without premedicants. Treatment with medetomidine-midazolam resulted in a shorter and smoother induction, compared with acepromazine-butorphanol or midazolam-butorphanol treatment, whereas the cardiovascular changes were greater. Cardiopulmonary variables of dogs during induction following treatment with acepromazine-butorphanol or midazolam-butorphanol were maintained close to the anesthetic maintenance values for sevoflurane, with the exception of mild hypotension that was observed in dogs following acepromazine-butorphanol treatment. CONCLUSION AND CLINICAL RELEVANCE: In dogs use of premedicants provides a smoother and better quality mask induction with sevoflurane.

Analgesics, Opioid↗
PubMed1977

Visual masking and carbon monoxide toxicity.

Thresholds for letters were measured with and without a masking stimulus (presented either to the same eye as the letters or to the other eye) before and after exposure of smokers and nonsmokers to 500 ppm carbon monoxide (CO) in air for 1 hr. Identification of the unmasked letters was not degraded by CO but a number of thresholds of the masked letters were significantly affected among the smokers. The effects of the CO on binocular and interocular masking were similar. These results suggest that the first effects of CO toxicity are neither on the receptors nor central but on the transmission lines in between and that smokers are more susceptible than nonsmokers to short-term increases in the level of CO. The masking phenomenon, however, does not appear to be an unusually sensitive measure of CO toxicity.

Carbon Monoxide Poisoning↗
PubMed1981

Durations of recognition for single letters, with and without visual masking, by dyslexics and normal readers.

Two sets of letters, i.e., those subject to reversal/rotation such as b, d, p, and q, and others, e, i, u, w, x, and o, were tachistoscopically presented singly to 15 normal readers (aged 6 to 14 yr.) and 29 dyslexic children (aged 7 to 13 yr.). Half the presentations were subject to masking by visual noise. Contrary to a prediction based on temporal-sequencing deficiency, the dyslexics required a significantly longer time than normals for recognition. This occurred under both the masked and non-masked conditions. Reversible/rotational letters gave more difficulty than other letters only when masked, for both normals and dyslexics. This supports the hypothesis that these letters have no special relation to dyslexia. The view that dyslexia is a developmental anomaly was supported by the finding of a significant correlation between recognition-speed and age for the normal readers only.

Adolescent↗
PubMed1982

Auditory masking effects on lingual vibrotactile thresholds as a function of age.

Auditory masking effects on lingual vibrotactile thresholds were examined across three groups of 10 subjects each. The first group included children 13 yr. of age or younger, the second group young college age adults, and the third group elderly individuals 66 yr. of age or older. Lingual vibrotactile thresholds were obtained from each group at a frequency of 250 Hz, under experimental conditions of no-masking, narrow-band masking, and wide-band masking. Results showed statistically significant threshold differences between the elderly group and the other two groups for all three experimental conditions. A difference was also detected between experimental conditions when an over-all statistical analysis was performed, but this difference disappeared under more stringent post hoc examination. Results are discussed with respect to current literature on aging and appropriate simplified procedures are recommended for future lingual vibrotactile threshold testing.

Adolescent↗
PubMed2003

Perceptual learning in visual backward pattern masking.

The present study examined the practice effect of backward pattern masking in the context of recent researches on perceptual learning. On the discrimination task, thresholds decreased substantially with practice. In addition, the practice effect was specific to the mask pattern. In contrast, it cannot reasonably be assumed that the practice effect was specific to the eye and the retinal position. These findings suggest that practice diminished the interruption of processing in capacity-limited processing. In addition, the specificity of the practice effect to the mask pattern suggests that participants learned to filter out the specific type of mask from capacity-limited processing. On the detection task, thresholds decreased with practice. But the improvement was smaller on the detection task than on the discrimination task. It may be assumed that the practice effect on the detection task does not involve a decrease in the effect of the interruption in capacity-limited processing.

Discrimination Learning↗
PubMedDate not supplied

Correlations between cochlear hair cell loss and shifts of masked and absolute behavioral auditory thresholds in the house mouse.

Masked and absolute behavioral auditory thresholds were measured in kanamycin-treated C3H mice with a significant damage or loss of outer hair cells (OHCs) near the cochlear base, and in 17--24-month-old NMRI mice with a significant loss of OHCs and inner hair cells (IHCs) in the basal region and of OHCs near the apex. A loss of IHCs is correlated with an increase in absolute thresholds. A moderate damage or loss of OHCs in a cochlear region had no effect on absolute thresholds but correlated with a decrease and other changes of characteristics of masked thresholds in the respective frequency regions. OH C-damage makes masking by continuous white broad-band noise less efficient. It is concluded that masking and critical bands depend on the function of OHCs, which must be assumed to influence intracochlearly the activity of IHCs in processing the information to the brain.

Animals↗
PubMed1995

Short latency vestibular evoked responses to linear acceleration stimuli in small mammals: masking effects and experimental applications.

Different potential were recorded from a site close to the 8th nerve in chinchillas in response to linear acceleration pulses. Acoustic masking allowed us to distinguish between an early response (within 1 ms after initiation of the acceleration) of probable vestibular origin and later responses of probable cochlear origin. The latter were abolished by intense acoustic masking and by surgical ablation of the cochlea. The early potential was slightly reduced by simultaneous acoustic masking with white noise above 65 dB SPL and was most sensitive to 1 kHz narrow band masking. Vestibular neurons seem to be stimulated by high frequency movements of their hair cell cilia, and vestibular compound action potentials can be recorded as soon as a sufficient number of neurons are brought to a synchronized response. These vestibular evoked potentials may provide a tool for experimental studies on vestibular function in laboratory animals.

Acceleration↗
PubMedDate not supplied

Role of the efferent medial olivocochlear system in contralateral masking and binaural interactions: an electrophysiological study in guinea pigs.

Contralateral broadband noise (BBN) elevates ipsilateral auditory thresholds (central masking) and reduces the amplitude of ipsilateral brainstem auditory evoked potentials (BAEPs). Binaural interactions are complex psychophysical phenomena, but binaural interaction components are easily extracted from BAEPs to monaural versus binaural click stimulation. However, contralateral, or binaural, acoustical stimulation is known to activate simultaneously the crossed and uncrossed medial olivocochlear (MOC) efferent systems and decrease activity in both cochleas. Particularly, contralateral BBN stimulation suppresses in part ipsilateral peripheral activity. What is the role of such contralaterally induced peripheral suppression in the overall changes in central BAEPs observed during contralateral masking or binaural stimulation? Compound action potentials (CAPs) of the auditory nerve and BAEPs were recorded simultaneously in awake guinea pigs from electrodes chronically implanted on the round window of the cochlea and the surface of the brain. Peripheral and central measures of contralateral masking and binaural interactions were obtained from responses to monaural or binaural clicks, with or without contralateral BBN, recorded before, during, and after the reversible blockade of the MOC function following a single intramuscular injection of gentamicin. Contralateral BBN effectively reduced the amplitudes of CAP and of all BAEP peaks. CAP to ipsilateral click did not, however, change significantly from monaural to binaural click stimulation; still, normal binaural interaction components developed in the BAEPs. When the medial efferent function was blocked by gentamicin, the normal contralateral BBN suppression of CAP and of the earliest BAEP peak was lost; however, the later BAEP peaks were suppressed by contralateral BBN as before gentamicin, and the central binaural interaction components were unchanged. In these experimental conditions, the MOC efferent system seems to play little role in centrally recorded contralateral masking and binaural interactions.

Animals↗
PubMedDate not supplied

Remote masking generated by high-frequency two-tone complexes.

Auditory masking generated by two-tone complexes centered around 7 000 Hz was measured in 10 young adults with normal hearing sensitivity as a function of the frequency separation (deltaf) and SPL of the masker's components. Remote masking (1) was evident for test signals in the frequency region corresponding to the masker's deltaf; (2) increased with masker SPL, but at a rate less than that usually observed when lower frequency bands of noise are used as maskers, and (3) was relatively constant in magnitude for a given SPL as a function of the masker's deltaf. The masking produced in low-frequency regions by high-frequency two-tone complexes adds support to the hypothesis that remote masking is primarily a result of aural distortion.

Acoustic Stimulation↗
PubMedDate not supplied

Backward and forward masking in listeners with severe sensorineural hearing loss.

Using synthetic stimuli, backward and forward masking by a first-formant stimulus were measured for two tasks: (1) discrimination of a second-formant transition and (2) pure-tone detection. 16 listeners with moderate to severe sensorineural hearing impairment were studied. Many of the listeners showed backward and forward discrimination masking, sometimes extending as far as 200 ms before or after the first-formant interval. There were large individual differences in degree and temporal extent of masking. The masking contribution of sensitivity shifts, as indicated by pure-tone detection levels, appeared to be limited to a range of only 50 ms before or after the first-formant masker.

Acoustic Stimulation↗
PubMed1985

Influence of interaural level differences on the speech recognition masking level difference.

This study was undertaken to determine the effect of interaural level differences on the magnitude of the speech recognition masking level differences obtained from younger and older subjects with normal hearing and from subjects with asymmetrical sensorineural hearing losses. A ten-word subgroup of CID W-1 spondaic words having the largest masking-level differences was used. Masking level differences decreased systematically with increasing interaural level differences for all three groups. At interaural level differences of 36 and 42 dB, the masking level difference persisted for the younger and older normal subjects (1.6 and 1.9 dB, respectively); similar results were obtained from the subjects with hearing losses at a 24-dB interaural level difference. For all subject groups, the S0N0 thresholds were unaffected by changes in the interaural level difference, whereas the S pi N0 thresholds were inversely related to the interaural level difference.

Adult↗
PubMed1989

Frequency-specific auditory brainstem responses. Effective masking levels and relationship to behavioural thresholds in normal hearing adults.

Ipsilateral masking levels and normal thresholds for tone pip auditory brainstem responses (ABRs) were investigated in normal subjects for the purpose of establishing recording parameters and norms for frequency-specific tone pip ABR testing. White noise was found to effectively mask ABRs to tone pips at mean signal-to-noise ratios of between -1 and -5.5 dB [dB peak-equivalent (pe) SPL/dB SPL] depending on the tone pip frequency. ABR thresholds were established for tone pips in the presence of ipsilateral masking with high-pass filtered noise for 50-Hz tone pips and notched noise for tone pips from 1,000 to 4,000 Hz, at a nominal signal-to-noise ratio of -5 dB (i.e. with the noise SPL measured prior to filtering). Thresholds occurred between 28.6 and 36.6 dB pe SPL, equivalent to 4.4-8.8 dB nHL. ABR thresholds for masked and unmasked tone pips did not differ significantly.

Adolescent↗
PubMedDate not supplied

Relative effective frequency response of bone versus air conduction stimulation examined via masking.

An adaptation of the sensorineural acuity level procedure was employed to obtain thresholds under bone- (BC) vs. air-conducted (AC) white noise masking. For the BC masking condition, a Radioear B71 was placed on one mastoid. An Etymotic ER3 with a foam tip placed in the ear on the same side was used to deliver the pure-tone probe stimulus. This transducer was chosen to approximate the Telephonics TDH-39 earphone response characteristic while reducing occlusion effect. For the AC masking condition, the masker and probe were mixed electrically and delivered to the earphone. Masked threshold data, transformed into frequency response curves, demonstrated greater variance of BC vs. AC response across frequency but less high-frequency roll-off than expected from coupler measurements obtained using an artificial mastoid and 6-cm3 cavity, respectively.

Acoustic Stimulation↗
PubMed1979

Further interpretation of the threshold of octave masking (TOM) test.

The threshold of octave masking (TOM) test was administered to normal-hearing and sensorineural-impaired listeners at four test frequencies; 500, 1000, 2000 and 4000 Hz. The TOM value was found to be inversely proportional to the degree of hearing loss at the masker frequency. Results indicate that the TOM test is capable of distinguishing subjects with sensorineural involvement from those with normal hearing and of providing a measure of the degree of sensory dysfunction. Examination of the slope of octave masking dysfunction. Examination of the slope of octave masking revealed that once the influence of hearing loss is overcome at higher intensities the sensorineural ear performs essentially the same as the normal ear in a tone-on-tone masking task.

Audiometry↗
PubMed1982

High-pass masking in the classification of low-frequency hearing loss.

Invalid BC determinations may be responsible for many diagnostic misinterpretations in the classification of low-frequency hearing losses. To reduce the influence from harmonic distortion, BC thresholds are determined during simultaneous, ipsilateral HP masking and composed with the conventionally obtained BC thresholds. Based on normative data a change of greater than or equal to 10 dB HL caused by HP masking is used as the criterion for judging the validity of the conventional BC testing. In 55% (11/20) of ears with low-frequency hearing loss, a decrease in air-bone gap was obtained with HP masking, and it is concluded that HP masking is of clinical significance for the diagnosis in these patients.

Adolescent↗
PubMed1986

Effects of signal duration on the 500-Hz masking-level difference.

Two experiments were conducted to study the masking-level difference (SoNo-S pi No thresholds) for 2-128-ms, 500-Hz tones in 32-dB pressure-spectrum level, continuous broadband noise. In Experiment 1 (N = 8) the Békésy tracking procedure was used, whereas in Experiment 2 (N = 3) an adaptive, two-interval forced-choice (2IFC) psychophysical procedure was used. The data from both experiments indicated that as the signal duration was shortened from greater than or equal to 100 to approximately 10 ms the SoNo and S pi No threshold integration functions diverged, thereby producing an increase in the masking-level difference. For signal durations less than 10 ms, however, the SoNo and S pi No threshold functions converged and the masking-level difference decreased. In Experiment 2, the mean masking-level difference decreased significantly by 1.7 dB as the signal duration was shortened from 10 to 2 ms. This new finding, which was not anticipated, has recently been confirmed in another laboratory (Cokely & Hall, 1985).

Acoustic Stimulation↗
PubMed1989

Masked threshold changes associated with angular separation of noise and signal sources.

Threshold changes associated with separating a signal source and a masking white noise source from 0 degree to 90 degrees were determined for 0.5, 1 and 8 kHz pure tones and click trains. No changes occurred for the 0.5 and 1 kHz pure tones. Masked thresholds of 8 kHz pure tones and click trains decreased linearly by 9 and 13 dB respectively as angular separation was moved from 0 degree to 90 degrees. Changes in click train stimuli masked thresholds did not change significantly when the ear directed toward the masking source was occluded (11 dB drop at 90 degrees). The absence of changes at low frequencies and the similarity in magnitude of the changes in signals containing high frequency components with the responses to the monaural click trains, suggests that the threshold changes can be attributed to a head shadow effect. The casting of a sound shadow effectively lowers the noise level on the shielded side. These findings question the importance of cross-correlation techniques when detecting signals in noise.

Acoustics↗