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PubMed1996

Decay in the effect of vibrotactile masking.

Vibrotactile forward masking experiments were conducted on the thenar eminence of the hand to determine the time course of masking in the Pacinian (P) or a non-Pacinian (NP I) channel. Brief masking and test stimuli that contained energy centered either below 27 or at 500 Hz were used to preferentially activate the NP I or P channels, respectively. Test thresholds were measured at different fixed delays ( Delta t) between the masking and test stimuli that ranged from 5 to 995 microseconds. Masking stimulus level was also varied from 5 to 25 dB above threshold. The masking effect followed an exponential decay with different time constants for each of the channels, but a similar asymptote (residual masking) for both channels. At the higher masking stimulus levels, the time constant for the P channel was about 40 microseconds, while the time constant for the NP I channel was about 100 microseconds. Residual masking in both channels increased about 1 dB for every 5-dB increase of masking stimulus level.

Adult↗
PubMed1996

Loudness enhancement and intensity discrimination under forward and backward masking.

There is a large deterioration in intensity discrimination performance at medium levels for a 30-ms sinusoidal pedestal presented 100 ms before or 100 ms after an intense masker [Plack and Viemeister, J. Acoust. Soc. Am. 92, 3097-3101 (1992)]. It has also been demonstrated that the loudness of a 30-ms sinusoidal tone burst, presented 100 ms after a masking tone burst, is enhanced at mid-levels [Zeng, J. Acoust. Soc. Am. 96, 2127-2131 (1994)]. The present experiment measured intensity discrimination and loudness enhancement in both forward and backward masking. A double-staircase adaptive procedure was used to match the loudness of a 30-ms, 1-kHz standard sinusoid presented in quiet to the loudness of a 30-ms, 1-kHz sinusoid presented 100 ms after (forward masking) or 100 ms before (backward masking) a 110-ms, 90-dB, 1-kHz masking sinusoid. The mean of the thresholds from the two staircases was used to determine the amount of enhancement, and the difference between the thresholds from the two staircases was used to determine the intensity just noticeable difference (jnd). Four listeners were tested at a range of standard levels between 30 and 90 dB. For all listeners, in both forward and backward masking, the jnd and loudness were greatest at mid-levels (40-70 dB). For a given listener, there was no substantial difference between the form of the results under forward and compared to backward masking, although there was considerable variability in the size of the effects between the individual listeners. Combining all the data, for both forward and backward masking there was a positive correlation between the size of the jnd and the magnitude of the loudness enhancement, although the correlation was only significant in backward masking (p < 0.005). Taken with the results of Zeng, these data suggest a link between loudness enhancement and the jnd increase, and a link between the mechanisms underlying the effects of forward and backward masking on intensity discrimination. It is suggested that all these effects may be caused by long-term loudness integration in the auditory system.

Humans↗
PubMed1997

Signal detection and pitch ranking in conditions of masking release.

Masked threshold levels for signal detection or pitch ranking (low, middle, or high) were determined in conditions where one of three possible signal frequencies could be presented on a given trial of a three-interval forced-choice task. Thresholds were determined under conditions investigating binaural masking release (the masking-level difference, or MLD) and monaural masking release in modulated noise. It was assumed that part of the masking release in modulated noise was due to a within-channel analysis of information in the masker dips, and that part of the masking release was due to across-frequency analysis of temporal envelope information (comodulation masking release, or CMR). In the MLD experiment, the masker was a low-pass noise with a frequency cutoff of 1000 Hz. In the modulated-noise experiment, a broadband noise was square-wave amplitude modulated at a rate of 25 Hz and then digitally low-passed filtered at 1000 Hz. The results indicated that when the frequency separation between frequency components was relatively wide, masking release for pitch ranking was similar to that for signal detection. However, as the frequency separation between components narrowed, masking release for pitch ranking decreased. The results are consistent with an interpretation that information regarding signal frequency is relatively coarse under the conditions of masking release examined here.

Adult↗
PubMed1998

Suppression and the upward spread of masking.

The purpose of this study is to clarify the role of suppression in the growth of masking when a signal is well above the masker in frequency (upward spread of masking). Classical psychophysical models assume that masking is primarily due to the spread of masker excitation, and that the nonlinear upward spread of masking reflects a differential growth in excitation between the masker and the signal at the signal frequency. In contrast, recent physiological studies have indicated that upward spread of masking in the auditory nerve is due to the increasing effect of suppression with increasing masker level. This study compares thresholds for signals between 2.4 and 5.6 kHz in simultaneous and nonsimultaneous masking for conditions in which the masker is either at or well below the signal frequency. Maximum differences between simultaneous and nonsimultaneous masking were small (< 6 dB) for the on-frequency conditions but larger for the off-frequency conditions (15-32 dB). The results suggest that suppression plays a major role in determining thresholds at high masker levels, when the masker is well below the signal in frequency. This is consistent with the conclusions of physiological studies. However, for signal levels higher than about 40 dB SPL, the growth of masking for signals above the masker frequency is nonlinear even in the nonsimultaneous-masking conditions, where suppression is not expected. This is consistent with an explanation based on the compressive response of the basilar membrane, and confirms that suppression is not necessary for nonlinear upward spread of masking.

Adult↗
PubMed2000

Contributions of suppression and excitation to simultaneous masking: effects of signal frequency and masker-signal frequency relation.

This study investigated the contributions of suppression and excitation to simultaneous masking for a range of masker frequencies both below and above three different signal frequencies (750, 2000, and 4850 Hz). A two-stage experiment was employed. In stage I, the level of each off-frequency simultaneous masker necessary to mask a signal at 10 or 30 dB sensation level was determined. In stage II, three different forward-masking conditions were tested: (1) an on-frequency condition, in which the signals in stage I were used to mask probes of the same frequency; (2) an off-frequency condition, in which the off-frequency maskers (at the levels determined in stage I) were used to mask the probes; and (3) a combined condition, in which the on- and off-frequency maskers were combined to mask the probes. If the off-frequency maskers simultaneously masked the signal via spread of excitation in stage I, then the off-frequency and combined maskers should produce considerable forward masking in stage II. If, on the other hand, they masked via suppression, they should produce little or no forward masking. The contribution of suppression was found to increase with increasing signal frequency; it was absent at 750 Hz, but dominant at 4850 Hz. These results have implications for excitation pattern analyses and are consistent with stronger nonlinear processing at high rather than at low frequencies.

Acoustic Stimulation↗
PubMed1979

Forward masking of auditory nerve fiber responses.

1. Responses of single fibers were obtained from the auditory nerve of chinchillas. Tone-burst stimuli consisted of a masking stimulus followed by a probe stimulus. Forward masking of a fiber's response is defined as a reduction in the magnitude of the probe-evoked response caused by the addition of the masking stimulus. 2. The recovery of probe response magnitude as a function of the time interval between masker offset and probe onset (delta T) follows an exponential time course. A relationship between the time course or magnitude of poststimulus recovery and the characteristic frequency (CF) of a fiber was not detected. 3. The iso-forward masking contour near the threshold of the masking effect across masker frequencies approximates a fiber's frequency threshold curve (FTC). In other words, forward masking tuning curves are essentially the same as frequency threshold curves. 4. The frequency dependence of forward masking is compared to that of two-tone suppression. Tonal stimuli outside the boundaries of a fiber's FTC that produce two-tone suppression are ineffective forward maskers. Certain frequency/intensity combinations within the FTC may produce both suppression and forward masking and tones within the remaining area of the FTC produce no suppression but are effective forward maskers. 5. Both the time course and the magnitude of the forward masking effect are dependent on the discharge rate evoked by the masker regardless of the masker's absolute level or spectral content. An increase in masker-evoked excitation causes an increase in time constant and a greater reduction in probe response magnitude, rd. The function relating rd to masker level parallels the firing rate/masker level function up to 40 dB above response threshold. 6. A decrease in masker duration from 100 ms leads to a decrease in both rd and the time constant of recovery. There is no significant difference between the 100 and 200 ms duration conditions. 7. Forward masking in single fibers is related to the period of poststimulus recovery of spontaneous activity, a component of a fiber's response pattern to the masker, and this component is tentatively identified as a period of recovery from short-term adaptation.

Adaptation, Physiological↗
PubMed2006

Common-onset visual masking in infancy: behavioral and electrophysiological evidence.

Common-onset visual masking (COVM) occurs when a mask and a target have common onset but delayed offset, with the mask persisting beyond the duration of the target [Di Lollo, V., Enns, J. T., & Rensink, R. A. Competition for consciousness among visual events: The psychophysics of reentrant visual events. Journal of Experimental Psychology: General, 129, 481-507, 2000]. We report the first behavioral and electrophysiological evidence of COVM in infants. An initial behavioral study included a familiarization phase during which a visual pattern (the target) surrounded by four black dots (the mask) was flashed 15 times to the infant. In the "unmasked" condition, the mask disappeared with the target. In the "masked" condition, the mask remained on the screen after deletion of the target for a further 93 msec. During the test phase, the familiar target pattern was paired with a new pattern. Infants in the unmasked condition showed a significant familiarity preference, suggesting that they had encoded the target during familiarization, whereas those in the masked condition showed no preference, suggesting that they had not encoded the target during familiarization. In the second experiment, high-density event-related potentials were used to investigate the electrophysiological pattern of activity that accompanies COVM. Six-month-old infants viewed both masked and unmasked conditions. Electrophysiological data indicated that over posterior channels the masked condition elicited a larger amplitude positive wave around 300 msec after stimulus onset than trials in the unmasked condition.

Attention↗
PubMed1999

Backward masking in unmedicated schizophrenic patients in psychotic remission: possible reflection of aberrant cortical oscillation.

OBJECTIVE: Patients with schizophrenia consistently show performance deficits on measures of visual backward masking, but the nature of these deficits is not well understood. Performance deficits on backward masking tasks may indicate an underlying predisposition instead of the presence of illness, because deficits are present in unaffected first-degree relatives. Performance deficits in remitted patients would constitute converging support for this hypothesis. METHOD: Eleven patients with recent-onset schizophrenia who were in a period of no medication use during remission of psychosis were compared with a matched normal group on three visual masking conditions. These conditions included target identification tasks with a high-energy mask, a low-energy mask, and a blurred target. RESULTS: Patients in psychotic remission showed significant deficits across all conditions. In addition, trend analyses revealed significant group differences in the shape of the masking functions: the comparison group showed an oscillating performance pattern across all masking conditions, whereas the patients did not exhibit this pattern on any condition. CONCLUSIONS: These data from patients in well-documented psychotic remission add converging support for the hypothesis that deficits on backward masking procedures are indicators of vulnerability to schizophrenia. Because visual masking procedures may reflect underlying neural oscillations of 30 to 70 Hz in the visual cortex, the pattern of results is consistent with the theory that visual masking deficits in schizophrenia stem from an underlying failure to establish cortical oscillations.

Adult↗
PubMed2005

Efficacy of three face masks in preventing inhalation of airborne contaminants in dental practice.

BACKGROUND: Up-to-date studies are needed on the protection provided by face masks used by dentists. We assessed the relative filtering efficacy of two currently used surgical face masks (one a molded mask, the other a tie-on mask) and a certified personal particulate respirator, all made by a single manufacturer. METHODS: The authors sprayed bicarbonate particulate against a porcelain surface (representing the patient's mouth) and collected it via a mannequin head (representing the dentist's head) placed 40 centimeters away and a tube with two airflow rates (0.5 cubic meters per hour and 9 m3/hour). They calculated the dry residue weight. They performed three separate runs for each mask and three runs with no mask at the two airflow rates with and without aerosol. RESULTS: With no mask (control), the authors recorded significant weight gains at both airflow rates with and without vaporization. With vaporization, the three masks were associated with different dry residue weights (P < .03 with the Kruskal-Wallis test at both flow rates), the respirator propviding the lowest amount. The respirator provided an efficiency of 94 to 96 percent, compared with 90 to 92 percent and 85 to 86 percent for the molded and tie-on surgical masks, respectively. CONCLUSIONS: These data provide independent evidencependent evidence that a certified personalthat a certified personal respirator can be more effective than high quality surgical masks in dental settings. CLINICAL IMPLICATIONS: Dentists should be aware that a certified particulate respirator can provide them with superior filtering protection.

Aerosols↗
PubMed1983

Backward contralateral masking by light and by random pattern.

Letter-stimuli as targets were presented to the right or left visual fields and followed either by a flash of light or by a flash of light plus a patterned mask. The patterned mask always appeared in the opposite visual field of the letter targets. Analysis showed that masking occurred for both types of masks but that subjects produced more errors at each of five intervals between onset of the target and onset of the mask for the flash of light plus a patterned mask in the opposite visual field than for the flash of light alone. A pattern mask, when presented to the opposite visual field of a target stimulus, interferes with target processing at short target-mask intervals. These findings suggest that central backward masking may involve target-mask interactions beyond the visual cortex (Area 17).

Adult↗
PubMed1988

Performance of the Hudson Multi-Vent oxygen mask.

The performance of the Hudson Multi-Vent oxygen mask was assessed by attaching it to a model of a head and generating varying inspiratory flows of oxygen through the mouth and nasal orifices. The oxygen concentration that was delivered by the mask varied with the inspiratory flow rate at all oxygen concentrations. This effect was proportionately similar at all levels of set oxygen concentration: about 50% of the increment of inspired oxygen was lost as the inspiratory flow rate increased from 20 L/min to 100 L/min. The performance of the mask was not affected by the route (oral or nasal) of inspiratory flow. Rotation of the mask in the coronal plane produced marked changes in delivered oxygen concentration; with the oxygen concentration set at 50%, and the inspiratory flow rate at 40 L/min, the delivered oxygen concentration was 45% when the mask was parallel to the coronal plane of the model, was 41% when it was angled 10 degrees towards the face, and was 27% when it was angled 10 degrees away from the face. The measurement of oxygen concentrations at different points inside the mask confirmed that the oxygen-enriched gas is directed in a localized stream in the centre of the mask, and that air is entrained through the large ports that are sited at the sides of the mask to satisfy inspiratory flow. The measurement of inspired oxygen concentrations in 12 hospital inpatients confirmed that the model reflected accurately the performance of the mask when used in a routine way. We conclude that, in clinical use, the Hudson Multi-Vent mask is unlikely to deliver the set oxygen concentration, and that the delivered oxygen concentration is not predictable.

Equipment Design↗
PubMedDate not supplied

[Comparative study of two methods for laryngeal mask insertion].

OBJECTIVE: To compare two ways of inserting laryngeal airway masks: uninflated and partially inflated to 75% of the volume, as recommended by manufacturers. PATIENTS AND METHOD: We studied 60 ASA I-II patients scheduled for outpatient surgery under general anesthesia with numbers 3 or 4 laryngeal masks, after having obtained informed consent from the patients (or parents in the case of minors). The patients were randomly assigned to two groups. In group A the masks were inserted inflated to 75% of volume as recommended by manufacturers, whereas in group B deflated masks were inserted as described by Brain. Anesthesia was standardized for all patients. One patient was withdrawn from the study when a technical error was detected. We recorded the presence of criteria predictive of difficult airway management, systolic and diastolic blood pressures, heart rate at four times (baseline, before and after induction and after insertion of the mask), number of insertion tries, final mask volume needed for adequate ventilation, need for an additional dose of the hypnotic drug and complications. RESULTS: No statistically significant differences between the two insertion methods were found. Difficult airway management criteria were unrelated to difficulty of mask insertion. Correct insertion of an uninflated mask proved impossible in one patient, in whom we were then able to insert an inflated mask. CONCLUSION: No differences were found between inflated insertion and Brain's uninflated insertion technique. We believe that inflated mask insertion might be useful when uninflated insertion proves impossible.

Adult↗
PubMed1983

Effect of a contralateral masking stimulus on auditory response performance.

HTLs at 1 kc/s were obtained from the L ear of 5 normal-hearing adults by Bekesy, Up-Down, and Ascending procedures, as a function of a contralateral narrow-band noise mask centered at 1 kc/s, at 70 db SL. Insert receivers were used in each ear. Bekesy tracing width was also examined, and in the Up-Down and Ascending procedures also response-level performance and the latency of the response decision. Masked and masking stimuli were presented simultaneously, both in either continuous or pulsed form. HTLs differed in quiet as a function of the temporal characteristics of the test tone, and in noise, as a function of the temporal presentation of the masked and masking stimuli. All Ss yielded higher threshold shift with contralateral (i.e., central) masking under the pulsed/pulsed than under the continuous/continuous presentation in the Bekesy procedure. The pulsed masking noise slightly decreased Bekesy tracing widths, while the continuous masking signal slightly increased them. Neither the temporal characteristics of the test tone, nor the presence/absence of the contralateral masking signal led to any systematic change in either the performance-level curves or the latency of the response decision. However, both the performance-level curves and the latency of the response decision were differentially affected by the experimental procedures. The Bekesy may be the procedure of choice for investigating central masking.

Acoustic Stimulation↗
PubMed1978

Backward and forward masking with reproducible noise bursts.

This study investigated the effects of two "frozen" narrow-band waveforms with different waveform envelopes 50 msec in duration and centered at 250 Hz, on temporal masking at short masking intervals and, further, assessed the effects of phase shifts on backward and forward masking. Four normal-hearing experienced Ss detected a monaurally presented 250-Hz tone burst, 12 msec in duration, that either preceded, occurred simultaneously with, or followed a burst of the narrow-band noise. A 2IFC technique was used with a transformed up-down procedure for threshold estimation. The masking intervals from onset of masker to onset of tonal pulse ranged from -30 to +70 msec. The 3 phasic conditions for the signal were in-phase, 90 degrees out-of-phase, and 180 degrees out-of-phase. Results of this experiment substantiated the data of other researchers who have shown greater backward than forward masking at short masking intervals and greater masking was found for all simultaneous conditions than for any of the backward or forward masking conditions. In addition, differences between the two noise waveforms, as well as the phase shifts, had a significant effect on the thresholds obtained for the backward and forward masking conditions, as well as for the simultaneous masking conditions. Criterion change and waveform storage are discussed as possibly having an effect up these variables.

Acoustic Stimulation↗
PubMed1996

Does wearing a face mask reduce bacterial wound infection? A literature review.

Current practices of operating room management and sterile technique are direct descendants of the elaborate principles of antisepsis and asepsis set down by Lister. The surgical face mask has become an integral part of the uniform for theatre personnel since its introduction in an attempt to reduce the rate of clinical wound infections (von Mikulicz 1897). The Medical Research Council's (1968) recommendations on aseptic procedure advise donning a new mask for each patient and changing the mask part-way through long procedures (four hours or more). In most hospitals no one is allowed to enter an operating room without wearing a face mask. Anecdotal evidence in the author's own area of practice suggests that surgeons and other theatre personnel consider wearing face masks a nuisance but feel it is the "thing to do". Increasing costs of medical services is now a real problem. In one major teaching hospital in 1990, 10,000 pounds was spent on masks for theatre use (Leyland and McCloy 1993). Although the available clinical data suggests that the present generation of masks does not protect staff either from airborne bacteria or Hepatitis B virus (Ransjo 1986, Reingold 1988), theatre personnel may adopt self-protection as a reason for wearing a mask. It is not the intention of this literature review to examine self protection as a reason for wearing a face mask in theatre but to investigate whether masks do reduce bacterial infections in the postoperative patient.

Bacterial Infections↗
PubMed1997

[Influence of the dead space induced by the face mask on the measure of heart rate variability].

The respiratory pattern influences the power spectrum of heart rate variability and therefore control or monitoring of respiration is needed. The effect of the dead space induced by the face mask used in the expiratory gas exchange analysis on the measurement of heart rate variability was evaluated in 22 subjects (15 males and 7 females aged 39-76 years, mean 42 years) using electrocardiography recorded for 5 min during spontaneous respiration under the conditions of supine rest, sitting on the bicycle ergometer with and without a face mask. The power spectrum of heart rate variability was obtained by the coarse-graining spectral analysis (CGSA) method and the total power (TP : 0-0.5 Hz), low frequency component (LF: 0-0.15 Hz), high frequency component (HF: 0.15-0.50 Hz), HF/TP and LF/HF were measured. The volume of the face mask was 320 ml and the tidal volume during sitting with the face mask was 596 +/- 110 ml (444-808 ml). The value of LF/HF increased from supine rest to sitting in accordance to the change of body position (p < 0.05), but the value of LF/HF when sitting with the face mask decreased to the level during supine rest. The value of HF/TP decreased from supine rest to sitting (p < 0.05), but when sitting with the face mask returned to that during supine rest. To evaluate the effect of tidal volume, the subjects were divided into two groups (each of 11 subjects) by the median value of tidal volume (570 ml). The value of LF/HF decreased from supine rest to sitting with the face mask in the smaller tidal volume group (tidal volume < 570 ml) (p < 0.02). Moreover, there was a significant correlation between the change of the value of LF from supine rest to sitting with the face mask and the tidal volume (r = 0.44, p < 0.05). There results suggest that the power spectrum of heart rate variability is strongly influenced by the dead space induced by the face mask used in expiratory gas exchange analysis. In particular, the sympathetic activation from supine rest to sitting in subjects with the smaller tidal volume is unclear. Thus, interpretation of the results of heart rate variability with or without the face mask used in expiratory gas exchange analysis requires care.

Adult↗
PubMed2006

Masking selected sequence variation by incorporating mismatches into melting analysis probes.

Hybridization probe melting analysis can be complicated by the presence of sequence variation (benign polymorphisms or other mutations) near the targeted mutation. We investigated the use of "masking" probes to differentiate alleles with similar probe melting temperatures. Selected sequence variation was masked by incorporating mismatches (deletion, unmatched nucleotide, or universal base) into hybridization probes at the polymorphic location. Such masking probes create a probe/target mismatch with all possible alleles at the selected polymorphic location. Any allele with additional variation at another site is identified by a lower probe melting temperature than alleles that vary only at the masked position. This "masking technique" was applied to RET protooncogene and HPA6 mutation detection using unlabeled hybridization probes, a saturating dsDNA dye, and high-resolution melting analysis. Masking probes clearly distinguished all targeted mutations from polymorphisms when at least 1 base pair (bp) separated the mutation from the masked variation. We were able to mask polymorphisms immediately adjacent to mutations, except in certain cases, such as those involving single-base deletion probes when both adjacent positions had the same polymorphic nucleotides. The masking probes can also localize mutations to specific codons or nucleotide positions. Masking probes can simplify melting analysis of complex regions and eliminate the need for sequencing.

Alleles↗
PubMed1992

The masking effect of sialic acid on Con A, PNA and SBA ectoderm binding sites during neurulation in the bantam chick embryo.

The masking effect of sialic acid on cell surface carbohydrates localized on the ectoderm in stage 6-11 bantam embryos was examined using fluorescein isothiocyanate-labeled Con A, PNA, SBA, LFA, and LPA before and after neuraminidase treatment. The results showed selective lectin binding on both the neuroectoderm and the surface ectoderm. In general, these lectin-binding sites increased or were at least expressed on neuroectoderm during neurulation. On the apical surfaces of the developing neuroectoderm, masked Con A-binding sites were evident from the earliest stage and rapidly increased. These sites coexisted with unmasked binding sites which gradually increased. Masked PNA sites were rarely observed but became abundant in later stages, even though coexistent unmasked sites also rapidly increased. Masked SBA sites were poorly observable in the early stage and gradually increased thereafter, whereas unmasked sites were expressed at later stages. On the basal surfaces masked Con A sites were evident in the early stages but gradually decreased in later stages, whereas unmasked sites were relatively abundant and increased thereafter. Masked PNA sites were evident and increased very rapidly, whereas unmasked sites became observable up to the latest stage. Masked SBA sites were minimal in all three stages, and unmasked sites expressed themselves slightly at later stages. The change in composition of carbohydrates on the developing neuroectoderm was obviously different from that on the developing surface ectoderm. On the contact surface of the neural ridge, the number of masked sites of penultimate sugars was large at Con A sites, slight at PNA and SBA sites, which coexisted with unmasked sugar chain terminals in the areas where Con A sites were moderate and where PNA and SBA sites were poor. Finally, the role of masking on binding sites for Con A, PNA and SBA during neural tube closure is discussed, and the observation that the apparent masking effect on three lectin binding sites did not correspond to the content of sialic acid detected by LFA and LPA is a subject for further study.

Animals↗