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PubMed1987

Modulation of the masking phenomenon by the crossed part of the medial olivocochlear bundle.

The masking phenomenon is used in clinical investigation of the inner ear to determine the frequency selectivity properties of the auditory system. Sectioning of the crossed part of the medial efferent bundle in a guinea pig model decreases the simultaneous masking phenomenon. The efferent effect seen in the forward masking varied with the masking paradigm used. When the masker onset precedes the maskee onset by more than 40 ms, masking diminishes after sectioning the crossed part of the medial efferent bundle. When this duration was shortest (30 ms), no effect was observed. This phenomenon could be explained by the time necessary to stimulate the medial efferent loop. Our results could explain some differences observed in the compound action potential masking curves with different masking paradigms.

Acoustic Stimulation↗
PubMed1998

[First experience with the laryngeal intubation mask].

OBJECTIVE: We report our initial experience with an improved model of the laryngeal mask airway, the intubating laryngeal mask airway (ILMA, commercial name Fastrach), which was designed by A.I.J. Brain to improve blind endotracheal intubation through a laryngeal mask. METHODS: In the ILMA, a number of construction details were newly designed compared to a standard laryngeal mask airway (SLM): 1) the angle between the shaft and the mask plane is changed, and also the radius of the shaft, 2) the internal diameter of the shaft is increased to allow the passage of an 8.0 mm cuffed endotracheal tube, resulting in an outer diameter of 2.0 cm, 3) a stable rubber lip (epiglottic elevating bar) is incorporated instead of the gills to clear the epiglottis out of the lumen during the passage of the endotracheal tube, 4) the shaft is manufactured out of stainless steel covered by silicone with a handle attached for more precise guidance. We prospectively used the ILMA in 80 patients for blind intubation. 51 had normal anatomy (group I), 19 were difficult to intubate (Cormack grade 3-4; group II), 18 had a reduced mouth opening (< or = 2.5 cm awake; group III), 8 of them also were difficult to intubate. RESULTS: Insertion of the ILMA and ventilation was possible in all patients but one with a mouth opening of 1.3 cm during anaesthesia. 4 patients with difficult intubation and one with normal anatomy could not be ventilated with a face mask but could be ventilated with the ILMA. Blind intubation was successful in 69 out of 80 patients (global success rate 87%; group I: 84%; II: 95%; III: 83%), in 38 during the first attempt (initial success rate 48%; group I: 45%; II: 63%; III: 44%), showing no difference for patients with normal anatomy, difficult intubation or reduced mouth opening. For 82 successful intubations, 157 intubation attempts were performed (success rate per attempt 52%; group I: 48%; II: 67%; III: 54%). Success rate per attempt decreased to 42%, if intubation was not successful during the first attempt (106 intubation attempts resulting in 44 successful intubations, including 31 intubation attempts for 11 failures; group I: 35%; II: 56%; III: 38%). Intubation through the ILMA was not possible in 11 patients (14%). Failures to intubate were caused by a reduced mouth opening in 1 patient, an unsuited endotracheal tube in 1 patient, a wrong size of the ILMA may have been the cause in 6 patients, in the remaining 3, lacking personal skill may have been responsible. Endotracheal tubes suited to be used with the ILMA are straight or preformed Woodbridge tubes, whereas standard plastic tubes are too stiff. Manouvres facilitating blind intubation though the ILMA were careful alignment of the ILMA with the handle, an up and down manouvre, rotation of the tube or head movements. CONCLUSION: The ILMA improved ventilation compared to a face mask and almost doubled the success rate of blind intubation compared to a SLM in our hands in a variety of intubation situations. The ILMA has the potential to be useful in difficult to intubate patients--except those with cervical pathology--or in emergency medicine. Handling can be trained during every day routine. Experienced judgement of definite endotracheal tube placement is mandatory.

Anesthesia, Inhalation↗
PubMed1983

Spatial frequency masking and Weber's Law.

The threshold masking effect of one simultaneously presented grating upon another was studied as a function of mask contrast and frequency. The masking function typically obeys Weber's Law with method-of-adjustment psychophysics, and typically does not with forced-choice. This apparent discrepancy was studied in some detail. We suggest that thresholds can be set with at least two different criteria, depending upon experimental conditions. When the mask is unfamiliar, it functions as noise and detection occurs at a constant signal/noise ratio, which yields Weber's Law. When the mask is highly familiar, its masking effect is less and obeys a power law. This power-law masking appears to represent an inherent non-linearity of the visual system.

Adaptation, Ocular↗
PubMed1991

Metacontrast with masks varying in spatial frequency and wavelength.

In a metacontrast masking paradigm, adult subjects were required to detect briefly presented target lines followed at various delays by a flanking mask varying in spatial frequency and wavelength. Detection accuracy, the dependent measure, was recorded as a function of the delay of the mask. The results showed that long wavelength masks produced maximum masking at a relatively short delay, while short wavelength masks produced maximum masking at a relatively long delay. Results are discussed within the framework of transient/sustained (magnocellular/parvocellular) theory of visual processing, and suggest that low spatial frequency channels respond with shorter latency and/or faster rise time to short wavelength stimuli, and high spatial frequency channels respond with greater sensitivity to long wavelength stimuli.

Adult↗
PubMed1993

The effects of adaptation and masking on incremental thresholds for contrast.

Using a temporal two-alternative forced-choice procedure, we measured thresholds for detecting increments in contrast of a 2 c/deg vertical grating at a wide range of pedestal contrasts, (1) before and after adapting to a grating of the same orientation and spatial frequency, and (2) in the presence of superimposed masks that varied in either orientation or spatial frequency. The adapting grating and all masks were of fixed 40% contrast. The results show that prior adaptation and concurrent masking have qualitatively similar effects on incremental thresholds; both raise threshold at low pedestal contrasts and leave them unaltered at higher contrasts. But masks have greater effects than adaptors, the effect of an orthogonal mask, or one two octaves higher in spatial frequency, being about the same as a parallel adaptor of the same spatial frequency as the pedestal grating. The results are explained by a model of Ross and Speed [(1991) Proceedings of the Royal Society of London B, 246, 61-69] that assumes that masks and adaptors both reposition the transducer function of contrast sensitive mechanisms and that masks, but not adaptors, also stimulate the detecting mechanism.

Adaptation, Ocular↗
PubMed1995

Zero frequency masking and a model of contrast sensitivity.

Stimulating the visual system tends to desensitize it to certain stimulus properties. Such desensitization is usually called adaptation or masking, but the distinction between the two is unclear. Nonspecific desensitization by light is usually regarded as adaptation, whereas pattern-specific desensitization is typically considered masking. Here we unify the treatment of such desensitizing phenomena by handling both in the spatial frequency domain. The amount of adapting light in a stimulus is represented in the spatial frequency domain by the component at zero frequency. To determine whether such adapting light acts like other components in the spatial frequency domain, we compared the effect of masking by the zero frequency component with the effects of masking by components at other frequencies. We show that the zero frequency component acts like other masking components, decreasing sensitivity to nearby test frequencies and thereby producing the insensitivity to low spatial frequencies that gives the contrast sensitivity curve its band-pass shape at high light levels. Treating light adaptation as masking by the zero frequency component leads to a general model that describes visual sensitivity to test gratings of varying spatial frequency at varying mean luminance, in the presence (or absence) of masking gratings of varying spatial frequency. Individual components of the model provide insight into visual processing at the system level.

Adaptation, Ocular↗
PubMed1994

A two-process analysis of pattern masking.

By comparing the masking effects of cosine gratings and uniform fields on spatially narrow-band test patterns, we obtain evidence that pattern masking is mediated by two stages of visual processing: an early process of point-wise luminance adaptation and a late process of spatial-frequency and orientation-selective filtering. The early (presumably receptoral) component of masking is not affected by the polarity (incremental or decremental) or spatial frequency of test patterns, and may either increase or decrease pattern sensitivity based on local light level. The late masking process is orientation and spatial-frequency dependent, implying a cortical origin. For this cortical process, we find competitive interaction between parallel ON and OFF visual mechanisms: on a bright bar of a mask, threshold shift is greater for decremental than incremental tests, and the opposite is true on a dark bar of the mask. We suggest that ON-OFF interactions in pattern masking serve to normalize the gain of ON and OFF mechanisms simultaneously in order to preserve the relative contrast in the image.

Adaptation, Ocular↗
PubMed1984

Interactions between target and masking stimuli: perceptual and event-related potential effects.

TWo separate experiments were conducted to determinge the nature of the visual event related potential (ERP) at two different cortical areas under conditions of backward visual masking and to see the effects of varied inter-stimulus-intervals (ISIs) upon both masking and the ERP. The results of the first experiment clearly demonstrated visual ERP attenuation at a target-mask ISI of 40 ms, an ISI which consistently produced masking. Both experiments showed that this effect was specific for the occipital recording site since amplitude reductions were not observed at the vertex under this condition. The mask alone condition produced the largest amplitude ERPs in both experiments. Again, this was specific for Oz. In the second experiment target-mask ISIs which did not produce backward masking (i.e., 10 ms and 100 ms) were not accompanied by ERP amplitude reductions. The results may be explained by a cortical excitatory-inhibitory model. That is, when successive stimuli are presented to the visual system, the visual cortical excitation produced by the initial stimuli is inhibited by the presentation of subsequent stimuli when temporal and spatial intervals are appropriate.

Evoked Potentials, Visual↗
PubMed1985

Binaural interaction in auditory brain-stem responses: effects of masking.

Binaural interaction (BI) in auditory brain-stem responses (ABRs) can be shown by comparing a binaurally elicited ABR to the algebraic sum of monaurally elicited ABRs. Subtracting the summed monaural ABR from binaural ABR yields a wave form assumed to contain response components attributable to BI. It has been suggested that acoustic crosstalk accounts for some of the 'BI' seen with this technique and that contralateral masking should be used during monaural stimulation to eliminate crosstalk. However, this practice might in itself confound the results, even if the masking noise were not intense enough to affect the opposite ear, by 'central masking' of brain-stem neural activity. We studied the effects of contralateral wide-band masking on BI/ABR in 10 normal adult subjects. Clicks were presented at levels from 55 to 115 dB peSPL, at 10 dB intervals. Masking was presented at 73 dB SPL (47 dB effective masking level); based on pilot studies of interaural attenuation, this was a level expected to be at, or just below, the threshold of audibility in the contralateral ear. BI/ABR wave forms were not noticeably affected by the addition of contralateral noise in the monaural trials. In addition, BI/ABR was seen (as previously reported) at levels well below any possibility of crossover artifact. Thus, BI/ABR is not simply attributable to crossover. Contralateral masking is not necessary in recording BI/ABR, within the limits of the stimuli and transducers used in this study.

Brain Stem↗
PubMed1993

Effect of surgical mask position on bacterial contamination of the operative field.

The influence of surgical mask usage on bacterial contamination of the operative field was studied during 30 cardiac catheterization procedures. Mask position was varied during each procedure according to a predesigned random table. The number of bacterial colonies recoverable when no mask was worn was significantly higher than that detected when a full mask was worn (P < 0.002). Shedding of Staphylococcus epidermidis was greater when no mask was worn (mean 5.2 colonies 10 min-1) than shedding with full mask (mean 2.7 colonies 10 min-1; P < 0.004). Although mask placement below the nose was associated with higher mean colony counts than that above the nose, these differences were not statistically significant.

Bacteria↗
PubMed1984

Correlates of tone-on-tone masked thresholds in the chinchilla auditory nerve.

In an attempt to determine neural correlates of tone-on-tone masking, discharge patterns of chinchilla auditory-nerve fibers were obtained in response to a set of two-tone stimuli for which behavioral masking had been previously measured (Long, G.L. and Miller, J.D. (1981): Hearing Res. 4, 279-285). The lowest masked thresholds in a sample of fibers were quantitatively similar to the chinchilla's behavioral masked thresholds. In addition, the neural data were in qualitative agreement with other previously-described characteristics of tone-on-tone masking, such as the contribution of cochlear distortion products and the upward spread of masking. It thus appears that the limitations imposed by peripheral frequency analysis determine the tone-on-tone masking pattern.

Animals↗
PubMedDate not supplied

Prevention of oral bacterial flora transmission by using mouth-to-mask ventilation during CPR.

The Emergency Cardiac Care Committee of the American Heart Association has recently recommended utilizing protective barrier precautions during CPR (1,2). We assessed 17 mask and faceshield resuscitation devices for adequacy of barrier protection. Eight of the devices were faceshields (CPR Microshield, Hygenic, MedCare Mask, Resusci, Samaritan, Sealeasy, Portex); 8 were mask devices (Laerdal, Dyna Med, MTM Emergency Lung Ventilator, MTM Emergency Resuscitator, Res-Q-Flo, Rightway Mouth-to-Mask Resuscitation, Trufit), and one of the devices did not meet the criteria for either faceshield or mask (Lifesaver). All masks were disinfected, applied to the investigator's face as directed by the manufacturers' instructions, and then cultured for oral aerobic bacterial flora on the rescuer side. No mask devices cultured positive for oral aerobic bacterial flora, while 6 of 8 faceshield devices cultured positive for oral aerobic bacterial flora (P less than 0.007). The CPR Microshield and the Portex faceshield were the only devices that did not develop a positive culture. We conclude that all ventilation devices with a one-way valve, except the Sealeasy device, provide adequate barrier type protection from oral aerobic bacterial flora when simulating mouth-to-barrier type protection when performing mouth-to-mouth ventilation.

Cross Infection↗
PubMedDate not supplied

Role of the laryngeal mask airway in the immobile cervical spine.

STUDY OBJECTIVE: To determine whether the laryngeal mask airway has a useful role in the airway management of patients whose cervical spines are immobilized in a rigid cervical collar. DESIGN: A randomized study comparing the difficulty, rapidity, and success rate of ventilating patients with immobilized cervical spines using a laryngeal mask airway and an endotracheal tube. SETTING: Medical center surgical unit. PATIENTS: Twenty-eight ASA physical status I and II women scheduled to undergo elective gynecologic surgery requiring general anesthesia. INTERVENTIONS: Tracheas of all anesthetized patients were sequentially intubated with an endotracheal tube and had a laryngeal mask inserted in random order. MEASUREMENTS AND MAIN RESULTS: Mouth opening was measured, and a Mallampati classification was made in each subject both with and without a rigid Philadelphia collar in situ. The view at laryngoscopy was recorded. The time taken to insert both devices to allow for satisfactory ventilation and the degree of difficulty encountered were determined. With a cervical collar in situ, mouth opening was reduced up to 60%. The Mallampati assessment and laryngoscopic view were shifted to one suggestive of a more difficult intubation. The time taken to ventilate these patients and the difficulties encountered were significantly less when using the laryngeal mask (p = 0.0001). A successful outcome was more likely following insertion of the laryngeal mask than when attempting intubation with an endotracheal tube. CONCLUSIONS: The laryngeal mask airway compared favorably with an endotracheal tube in success rate, difficulty of insertion, and time to position correctly in this patient population. Although the laryngeal mask does not reliably protect against aspiration, we believe it may play a useful role if more conventional methods of airway management fail. Further studies in the trauma scenario are indicated.

Adult↗
PubMed2006

Spatial dynamics of masked picture repetition effects.

The aim of this study was to further elucidate the mechanisms of early and automatic object processing using a masked picture priming paradigm with both identity and exemplar repetition in functional MRI (fMRI). Masked repetition priming has been commonly used with words to isolate automatic, rapidly occurring mechanisms involved in visual word recognition; however, studies using the technique of masked priming with rapid presentation of pictures have been limited. This study demonstrates how the masked priming technique can be used to study early, automatic processing of rapidly presented complex objects. Temporal-occipital regions previously found to be sensitive to repetition priming in both masked word and unmasked picture studies were found to show repetition suppression for the identity primes only. Most notably, when divided into anterior and posterior divisions, the fusiform gyrus showed anatomically specific repetition suppression only in the posterior portion. This finding is comparable to that found in a previous study of masked word priming, and the similarity may suggest an overlap in the early identification processes for visual word form and visual object processing in this region. Finally, masked repetition of different exemplar objects did not result in reliable neural effects, suggesting that the underlying mechanisms of the more semantic-based, different exemplar priming may occur later or require the intervention of conscious processes.

Adult↗
PubMed2006

Lateral masking, levels of processing and stimulus category: a comparative study between normal and dyslexic readers.

The lateral masking effect results in lower performance on letter recognition when items are flanked by other stimuli. Using a new paradigm based on discrimination (feature analysis) and categorization (memory access) tasks, we investigated the influence of level of processing (as addressed, respectively, by these two tasks) and stimulus type (Latin letters, Korean letters and geometrical figures) on lateral masking. In addition, performance of dyslexic and non-dyslexic adult readers was compared. The non-dyslexic participants demonstrated a classical lateral masking effect with lower performance for flanked items than isolated ones. In addition, lateral masking was stronger in the categorization than in the discrimination task and was restricted to familiar items, i.e., Latin letters and geometrical figures. Dyslexic participants showed poorer performance than non-dyslexics on processing isolated items, and the pattern of decrease in performance for lateral masking was similar to non-dyslexics. However, they also showed a stronger decrease in performance in categorization and a stronger decrease related to the lateral masking for this categorization task. Our results in normal readers suggest that lateral masking relies on the interference between the target and the flankers during feature integration that may result in marked impairment of memory access (categorization task). Poorer performance in dyslexic readers may reflect impaired parafoveal/peripheral low-level processing during feature integration that may have worsened during the flanked condition due to a target selection/spatial-attentional disorder. Moreover, dyslexic subjects presented an additional categorization deficit that may relate to a specific left-hemispheric disorder.

Adult↗
PubMed2003

Extending the shine-through effect to classical masking paradigms.

A vernier, presented for a short time, shines through a following grating if the grating contains nine and more elements but remains largely invisible for smaller gratings. Therefore, extended grating masks yield, surprisingly, less masking than smaller ones. Here, we show that this mask size effect is not unique to grating masks. Masking diminishes if the size of classical pattern-, noise-, light-, and metacontrast masks increases and if these masks are regular, i.e. highly ordered.

Afterimage↗
PubMed2003

100% inspired oxygen from a Hudson mask-a realistic goal?

BACKGROUND: The administration of oxygen at a high-inspired concentration is often required in medicine, particularly in resuscitation of critically ill patients. However, there is a lack of evidence-based guidance on how to achieve this using currently available apparatus. The aim of this study was to assess how maximum inspired oxygen concentrations can be delivered using existing equipment. METHODS: Ten healthy female volunteers breathed oxygen through two types of Hudson non-rebreathing mask with reservoir bag, one with a safety vent in the mask body and the other with a valve replacing this safety vent (3-valve mask). Oxygen flow was adjusted to either 10 or 15 l min(-1) and the masks were fitted to the face either loosely or tightly. The expired oxygen concentration was measured using an oxygen analyzer. FINDINGS: By using the Hudson non-rebreathing mask with three valves, increasing the oxygen flow to 15 l min(-1), and fitting the mask tightly to the face the average expired oxygen fraction could be raised to 0.85. This equates to an average inspired oxygen fraction of 0.97 in these subjects. INTERPRETATION: The three simple measures mentioned above result in a significant improvement in the performance of the Hudson non-rebreathing mask. Together they allow the delivery of an inspired oxygen concentration close to maximum.

Adult↗
PubMed2002

The relative role of beats and combination tones in determining the shapes of masking patterns: II. Hearing-impaired listeners.

Masking patterns were measured for hearing-impaired subjects with varying degrees of hearing loss. In one set of conditions, three subjects were tested using narrowband noise ('noise') and sinusoidal ('tone') maskers and narrowband noise signals. The maskers had centre frequencies of 0.25, 0.5, 1.0 and 4.0 kHz and levels of 60, 80 and 100 dB SPL. Masking patterns for both the noise and tone maskers showed irregularities ('dips'), especially for signal frequencies up to 500 Hz above the masker frequency. The irregularities occurred for all masker levels and for all subjects for at least one masker frequency and they occurred for a relatively constant range of masker-signal frequency separations, suggesting that they were the result of beat detection. In another set of conditions, masking patterns were measured using two subjects, for a 2.0-kHz tone masker with a level of 100 dB SPL and tone and noise signals. For the tone masker alone (baseline condition), the masking patterns again exhibited prominent dips above, and sometimes below, the masker frequency. The addition of a lowpass noise to the masker, intended to mask combination tones, had little effect for one subject. For the other subject, who had near-normal absolute thresholds at low frequencies, the noise elevated thresholds for masker-signal frequency separations between 500 and 1500 Hz. For this subject, an extra tone with a frequency equal to the masker-signal frequency separation, added in place of the lowpass noise, had a very similar effect to that produced by the lowpass noise, suggesting that he was detecting a simple difference tone in the baseline condition. The addition of a pair of high-frequency tones (MDI tones - intended to reduce the detectability of beats) to the masker elevated thresholds for signal frequencies from 1500 to 2500 Hz for one subject and from 1500 to 3500 Hz for another subject. The addition of lowpass noise and MDI tones to the masker produced masking patterns very similar to those observed when the MDI tones alone were added to the masker. Overall, the results suggest that the irregularities in the masking patterns were caused mainly by the detection of beats and not by the detection of combination tones.

Acoustic Stimulation↗