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A model of visual backward masking.

When two successive stimuli are presented within 0-200 ms intervals, the recognition of the first stimulus (the target) can be impaired by the second (the mask). This backward masking phenomenon has a form called metacontrast masking where the target and the mask are in close spatial proximity but not overlapping. In that case, the masking effect is strongest for interval of 60-100 ms. To understand this behaviour, activity propagation in a feedforward network of leaky integrate and fire neurons is investigated. It is found that, if neurons have a selectivity similar to that of V1 simple cells, activity decays layer after layer and ceases to propagate. To combat this, a local amplification mechanism is included in the model, using excitatory lateral connections, which turn out to support prolonged self-sustained activity. Masking is assumed to arise from local competition between representations recruited by the target and the mask. This tends to interrupt sustained firing, while prolonged retinal input tends to re-initiate it. Thus, masking causes a maximal reduction of the duration of the cortical response to the target towards the end of the retinal response. This duration exhibits the typical U-shape of the masking curve. In this model, masking does not alter the propagation of the onset of the response to the target, thus preserving response reaction times and enabling unconscious priming phenomena.

Models, Neurological↗
PubMed1992

Video surveillance of oxygen administration by mask in postoperative patients.

Patients may not receive prescribed oxygen because the oxygen face mask becomes displaced. Using video, we have observed the position of the face mask in 20 postoperative patients and recorded the timing and the events associated with mask displacement. Correct placement of the mask was confirmed at the start of the 8-h study period from 22:00 on the first night after operation. The mask remained on continuously and positioned correctly in only one patient. In the other 19 patients, it was removed 64 times (range 1-10 times per patient). The mask was removed 45 times for nursing tasks such as mouth care and temperature measurement and these represented 70% of the total number of times that the mask was not in position. Other reasons for removal were vomiting, retching and removal by the patient. The mask remained off a median time of 6 min 55 s per episode (range 46 s to 7 h 46 min 57 s) and per patient a median of 1 h 6 min 48 s (range 1 min to 7 h 46 min 57 s). Mask removal resulted in an average decrease in oxygen saturation of 4%. Oxygen by mask at 4 litre min-1 maintained an average saturation > or = 95% in most, but not all, of the patients.

Adult↗
PubMed2004

Detection of masked hypertension by home blood pressure measurement: is the number of measurements an important issue?

BACKGROUND: Office blood pressure (OBP) and home blood pressure (HBP) enable the identification of patients with masked hypertension. Masked hypertension is defined by normal OBP and high HBP and is known as a pejorative cardiovascular risk factor. OBJECTIVE: The objective was to evaluate in the SHEAF study the influence of the number of office or home blood pressure measurements on the classification of patients as masked hypertensives. METHODS: Patients with OBP <140/90 mmHg (mean of six values: three measurements at two separate visits, V1 and V2) and HBP >135/85 mmHg (mean of all valid measurements performed over a 4-day period) were the masked hypertensive reference group. The consistency of the classification was evaluated by using five definitions of HBP values (mean of the 3, 6, 9, 12 and 15 first measurements) and two definitions of OBP values (mean of three measurements at V1 and mean of three measurements at V2). RESULTS: Among the 4939 treated hypertensives included in the SHEAF study, 463 (9.4%) were classified as masked hypertensives (reference group). By decreasing the number of office or home measurements, the prevalence of masked hypertension ranged from 8.9-12.1%. The sensitivity of the classification ranged from 94-69% therefore 6-31% of the masked hypertensives were not detected. The specificity ranged from 98-94% therefore 1-6% of patients were wrongly classified as masked hypertensives. CONCLUSION: A limited number of home and office BP measurements allowed the detection of masked hypertension with a high specificity and a low sensitivity. A sufficient number of measurements (three measurements at two visits for OBP and three measurements in the morning and in the evening over 2 days for HBP) are required to diagnose masked hypertension.

Aged↗
PubMed1991

Contrast adaptation and contrast masking in human vision.

After a preliminary study of visual evoked potentials (VEPS) to a test grating seen in the presence of masks at different orientations, psychophysical data are presented showing the effects of adaptation and of masking on thresholds for detecting the same test grating. The test is a vertical grating of spatial frequency 2 cycles per degree; adapting and masking gratings differ from the test either in orientation or in spatial frequency. The effects of adaptation and masking are explained by a single mechanism model that assumes: (i) adaptation and masking both alter the contrast response (or transducer) function of the mechanism that detects the test; (ii) masks, but not adaptors, stimulate the mechanism that detects the test; and (iii) a test is detectable when it raises response level by a constant amount. The model incorporates two distinct tuning functions, a broad adaptive contrast function and a narrow effective contrast function. It accounts adequately for all the data, including the location and size of the facilitative dip found in some masking functions, the constant slopes of the threshold elevation segments of adaptation functions and the varying slopes of masking functions. It also predicts the sometimes surprising joint effects of adaptation followed by masking and of two masks operating simultaneously.

Adaptation, Ocular↗
PubMed1994

Backward masking in schizophrenia and mania. II. Specifying the visual channels.

BACKGROUND: The backward masking procedures that have been used in psychopathology research have confounded two types of masking mechanisms (integration and interruption) and two types of visual channels (transient and sustained). In an earlier study, we attempted to limit the masking mechanism to interruption. The current study limited the role of sustained (parvocellular) visual channels to masking performance. METHODS: Masking procedures were altered in the following two ways to reduce reliance on sustained visual channels: (1) the spatial frequency was lowered by blurring the target and (2) a location task was used instead of an identification task. Manic patients were included to examine the specificity of deficits on these tasks to schizophrenia and to test the hypothesis that mania is associated with abnormalities on visuospatial tasks. RESULTS: Schizophrenic patients differed significantly from normal controls on both masking conditions. Manic patients also showed deficits relative to normal controls. Manic patients showed a significantly different masking function from that of schizophrenic patients on the location condition. CONCLUSIONS: Schizophrenic deficits within masking paradigms may involve abnormalities in transient, as opposed to sustained, visual channels. Masking performance deficits were also found in manic patients, but the underlying processes are probably different. A reformulation is offered concerning the nature of early visual processing deficits in schizophrenia.

Adult↗
PubMed1995

Complications associated with removal of the laryngeal mask airway: a comparison of removal in deeply anaesthetised versus awake patients.

The purpose of the study was to compare the incidence of complications (coughing, biting, retching, vomiting, excessive salivation and airway obstruction) associated with removal of the laryngeal mask airway. The laryngeal mask airway was used in 100 adults undergoing urological procedures. The patients were randomly assigned to two groups. In 50 patients the laryngeal mask was removed by a nurse when the patient responded to commands in the recovery area. In the other 50 patients it was removed by the anaesthetist with the patient deeply anaesthetized in theatre. The majority of patients were elderly men who had relatively short procedures. The incidence of gastric regurgitation was assessed by measurement of pH of secretions at the tip of the laryngeal mask airway. Complications occurred more frequently in the awake patients (P < 0.01). Most were minor and occurred before removal of the laryngeal mask airway during emergence in the recovery room. Airway obstruction occurred in three patients in whom the laryngeal mask was removed in the recovery room. In two of these patients the oxygen saturation decreased below 80% and the other to 90%. No decrease in arterial oxygenation occurred in the anaesthetised patients in whom the laryngeal mask was removed by the anaesthetist. In 14 patients in the awake group the pH of secretions at the tip of the laryngeal mask was < or = 3 compared with only four patients in the anaesthetised group (P < 0.05). It is concluded that it may be safer to remove the laryngeal mask airway whilst the patients are deeply anaesthetised in the operating room than when they are awake in the recovery room.

Adult↗
PubMed2006

Experiments on the Fehrer-Raab effect and the 'Weather Station Model' of visual backward masking.

The Fehrer-Raab effect (simple reaction time is unaffected by metacontrast masking of the test stimulus) seems to imply that a stimulus can trigger a voluntary reaction without reaching a conscious representation. However, it is also possible that the mask triggers the reaction, and that the masked test stimulus causes a focussing of attention from which processing of the mask profits, thus reaching conscious representation earlier. This is predicted by the Weather Station Model of visual masking. Three experiments tested this explanation. Experiment 1 showed that the masked test stimulus caused a temporal shift of the mask. Experiment 2 showed that the reaction in the Fehrer-Raab effect was not exclusively triggered by a conscious representation of the test stimulus: the mask was involved in evoking the reaction. Experiment 3 again revealed a temporal shift of the mask. However, the shift was only about half as large as the Fehrer-Raab effect. The psychometric functions suggested that the observers used two different cues for their temporal order judgments. The results cast doubts on whether judged temporal order yields a direct estimate of the time of conscious perception. Some methodological alternatives are discussed.

Cognition↗
PubMed1980

Masking of short probe sounds by tone bursts with a sweeping frequency.

The masked threshold of short (less than or equal to 40 ms) probe tone or a short one-third octave probe noise appears to increase if the frequency of a tonal masker is swept. Frequency sweeps were exponential (octave/s) and unidirectional. Probe sounds were presented in the time center of the masker at the center frequency of the masker. The sweep speed, S, appeared to be an important parameter; masker duration was much less important. For 10-ms tonal probes, in-phase with the masker in their common time center, 100-ms maskers, and upward sweeps, increase of the masked threshold appeared to be maximal at a sweep speed of 30 oct/s and the masked threshold was 21 dB higher than the masked threshold found for the stationary masker (S = 0 oct/s). Above 30 oct/s the masked threshold decreased. For downward sweeps masking was maximal at S = 20 oct/s and the threshold was 15 dB higher. Sweeping upward the increase in masking was 12 dB for both noise probes and tonal probes with phase differing by 90 degrees from the masker in their common time center. The results are inconsistent with current models of masking: sweeping the frequency the masked threshold increases whereas the energy within the critical band at the probe frequency decreases.

Humans↗
PubMed1990

Evoked response narrow-band noise masking patterns in the chinchilla.

Narrow-band noise masking patterns were measured at 0.5, 1, 2 and 4 kHz in the chinchilla using the auditory evoked response from the inferior colliculus. At low masker levels, the masking profiles were symmetrical and centered on the masker. However, as masker level increased, the masking profiles spread predominantly toward the high frequencies. The masking profiles obtained at 0.5 and 1 kHz, exhibited a broad plateau extending 1-2 octaves above the masker at the highest masker level (70 dB SPL) whereas those obtained at 2 and 4 kHz showed a peak. In contrast to tone-on-tone masking profiles, none of the narrow-band noise masking profiles contained a low-threshold notch on the high frequency side of the masker. The evoked response masking profiles obtained in the chinchilla were slightly wider, but qualitatively similar to those measured psychophysically in humans. Thus, the evoked response narrow-band noise masking profiles may provide a convenient way of evaluating the spread of masking in difficult-to-test subjects.

Acoustic Stimulation↗
PubMed2005

Visual backward masking: deficits in locating targets are specific to schizophrenia and not related to intellectual decline.

Visual backward masking deficits have been postulated as potential vulnerability markers for schizophrenia. This study investigated the diagnostic specificity of a location and an identification variant of the backward masking task for schizophrenia and analyzed masking performance during the course of the tasks. The influence of schizophrenia patients' intellectual decline on masking performance was also examined. Twenty-eight schizophrenia patients were compared to 28 patients with unipolar depression and 28 healthy controls on a letter location task and a letter identification task applying a low spatial frequency mask. Schizophrenia patients made significantly more detection errors on the location task than depressives at an interstimulus interval (ISI) of 50 ms and healthy controls at ISIs of 16.7, 33.3, 50, and 66.7 ms. Thus, the location masking dysfunction of schizophrenia patients was distinctive at a rather long interstimulus interval (50 ms). On the identification task the performance of schizophrenia patients did not differ from that of the two control groups. Identification but not location masking performance improved during the course of the task for all groups. Intellectual deterioration of schizophrenia patients was not correlated with location or identification masking performance. Schizophrenia patients are characterized by specific impairments in spatial visual processing which appear to be independent of intellectual decline. Potential explanations of the location masking deficit found in schizophrenia are discussed.

Adult↗
PubMed2004

Backward masking and the central performance drop.

Kehrer [Spatial Vision 2 (1987) 247] found that texture discrimination performance sometimes peaks in the parafovea rather than at the fovea, and he referred to this phenomenon as the central performance drop (CPD). Kehrer used a backward mask to limit performance and Morikawa [Vision Res. 40 (2000) 3517] argued that in some cases the temporal aspects of the backward mask may be critical to the emergence of the CPD. In one experiment Morikawa showed that the CPD does not emerge when a simultaneous noise-mask (different from the mask used by Kehrer) is used to limit performance. In another experiment Morikawa showed that unmasked texture displays comprising short lines do not elicit the CPD. In both cases, changes in the temporal aspects of the texture displays were accompanied by changes in the spatial structure of the mask or stimulus. For the spatio-temporal theory of the CPD to be sustained one would have to show that noise masks elicit a CPD when used as backward masks and that the short-line textures elicit a CPD when followed by backward masks. Our evidence provides little if any support for either of these predictions. Furthermore, an analysis of a simple filter-rectify-filter model of texture segmentation shows that a greatly attenuated CPD is to be expected when a noise mask is used as a source of spatial noise.

Adult↗
PubMed2005

Configural masking of faces: evidence for high-level interactions in face perception.

The perception of a stimulus can be impaired when presented in the context of a masking pattern. To determine the timing and the nature of face processing, the effect of various masks on the discriminability of faces was investigated. Results reveal a strong configural effect: the magnitude of masking depends on the similarity between mask and target. Masking is absent for non-face masks (noise, houses), modest for scrambled and inverted faces and strongest for upright faces, even when they differ in size, gender or viewpoint from the targets. This suggests an extra-striate location for the masking (possibly FFA). Reduced but significant masking for isolated face parts (internal features or head shape) is consistent with holistic computations in face perception. The duration over which a face mask can impair face discrimination (130 ms) is markedly longer than previously assumed and is sufficient for iterative and feedback computations to be part of face processing.

Contrast Sensitivity↗
PubMed2005

The structure of stereoscopic masking: position, disparity, and size tuning.

The masking effect of a Gaussian blob on detection of a Gaussian target was measured as a function of the position, disparity, width and polarity of the mask. The data reveal a large degree of disparity-specific masking that cannot be explained by the masking of its monocular constituents. At 5 degrees eccentricity, the masking range extends about +/-1 degrees around the lines of sight of the two eyes and 1-3 degrees in disparity, depending on the size of the test stimuli. The masking effects can be modeled as having three additive components, one that has a fixed disparity range and is polarity independent, one with a center/surround form keyed to both the disparity and the polarity of the mask, and one that derives from the monocular masking in each eye. Thus, the profound disparity interaction behavior is not limited to the simple monocular masking properties of the stimuli but reveals extensive connectivity across the disparity domain. Future models of disparity encoding will need to take these properties into account.

Adult↗
PubMed1992

Aerosol penetration through surgical masks.

BACKGROUND: Surgical masks are used in hospitals to reduce postoperative infection in patients. The presence of aerosols containing pathogens makes it desirable to protect the medical staff as well. METHODS: The collection efficiencies of surgical masks measured with two aerosol-size spectrometers. The flow rates through the masks were varied from 5 to 100 L/min to study the flow dependency. For comparison, several industrial-type respirators were also tested. RESULTS: A surgical mask consisting of filter material performed better than did a surgical mask consisting only of a shell with a coarse pore structure. The latter passed 80% of submicrometer-sized aerosols with little flow dependency, whereas the penetration of submicrometer-sized aerosols through the mask made of filter material ranged from 25% at a flow rate of 5 L/min to 70% at 100 L/min. CONCLUSIONS: The mask that has the highest collection efficiency is not necessarily the best mask from the perspective of the filter-quality factor, which considers not only the capture efficiency but also the air resistance. Although surgical mask media may be adequate to remove bacteria exhaled or expelled by health care workers, they may not be sufficient to remove the submicrometer-size aerosols containing pathogens to which these health care workers are potentially exposed.

Aerosols↗
PubMed2004

Comparison of cuff-pressure changes in LMA-Classic and the new Soft Seal laryngeal masks during nitrous oxide anaesthesia in spontaneous breathing patients.

BACKGROUND AND OBJECTIVE: There are concerns over the intra-cuff pressure of the laryngeal mask and laryngopharyngeal morbidity. In a randomized study, the authors compared cuff-pressure changes in the LMA-Classic and the new disposable Soft Seal laryngeal mask during nitrous oxide anaesthesia. METHODS: Two-hundred adult patients were randomly assigned to a size 4 laryngeal mask in two equal-sized groups for airway management: (a) the re-usable LMA-Classic, or (b) the new disposable Soft Seal laryngeal mask. Anaesthesia was administered with fentanyl, propofol, nitrous oxide, O2 and sevoflurane. The cuff pressures, adjusted to 45 mmHg at insertion, were monitored continuously until the end of the operation without any further attempt to reduce cuff pressure. On removal of the laryngeal mask, any blood at all was considered positive. Patients were requested to report any sore throat at 2 and 24 h postoperatively. RESULTS: During nitrous oxide anaesthesia, cuff pressures increased in the LMA-Classic group from 45 to 100.3 mmHg and from 45 to 46.8 mmHg in the Soft Seal laryngeal mask group (P < 0.001). The incidence of sore throat was significantly higher at 2 h postoperatively when using the LMA-Classic, although there was no difference at 24 h following the operation. Macroscopic blood was only seen on four occasions in the LMA-Classic group (not significant). CONCLUSIONS: During nitrous oxide anaesthesia, cuff pressure increases in the LMA-Classic mask were significantly higher than those of the Soft Seal laryngeal mask. Trauma to patients, as assessed by the incidence of sore throat in the early postoperative period was significantly higher in the LMA-Classic group. Cuff pressures should be monitored during nitrous oxide anaesthesia when LMA-Classic is used but to do so is of less importance when using the disposable Soft Seal laryngeal mask.

Adult↗
PubMed1995

Perceiving exterior letters of words: differential influences of letter-fragment and non-letter-fragment masks.

Previous research shows letter-fragment masks and non-letter-fragment fields have different effects on performance with briefly presented alphabetic targets. However, popular accounts of these differences ignore mask configuration. Over a series of experiments, configurational effects of letter-fragment (LF) and non-letter-fragment (NLF) masks were compared. When the configuration of LF masks matched word boundaries, performance with exterior letter pairs from words improved, whereas performance with illegal exterior-letter pairs and single letters was unaffected. When the same changes were made to NLF masks, only an overall drop in performance occurred, with no selective effect on target type. Thus, although LF mask configuration selectively affected lexical processing, NLF mask configuration produced substantially different effects, indicating problems with contemporary accounts of masking differences that ignore influences of mask configuration.

Attention↗
PubMed1980

Effect of depth separation on metacontrast masking.

In most investigations of visual masking, spatially adjacent contours are located in the same depth plane, a practice derived from the assumption that depth is not a critical variable in contour formation. In nine experiments, the dependence of masking on the relative depth positions of the target and mask was examined. To permit facile manipulation of depth without introducing confounding changes in proximal stimulation, the stimuli were stereoscopic contours formed from dynamic random-element stereograms generated as anaglyphs on a projection color-television display. The target was a briefly presented Landolt C with gap position randomly varied; the mask was an annulus that surrounded the target. Masking with stereoscopic contours was robust and subject to the same spatial and temporal variables influential in masking with physical contours. Depth separation of the target and mask produced significant reductions in masking and interacted with temporal order and egocentric stimulus position. These results have implications for models of visual masking and indicate that perceived depth is important in contour formation.

Attention↗
PubMed1983

The influence of metacontrast masking on detection and spatial-choice judgments: an apparent distinction between automatic and attentive response mechanisms.

Several studies of metacontrast masking in the 1960s apparently showed that the latency of simple detection responses was uninfluenced by the phenomenal dimming of the target induced by the mask. More recent studies using more suitable methodologies have clearly shown that such is not the case for situations in which the masking is a monotonically decreasing function of stimulus onset asynchrony. Experiment 1 investigated this issue for the situation in which masking is a U-shaped function of stimulus onset asynchrony. Contrary to the results obtained in monotonic masking situations, simple detection responses were not slowed by the masking. Experiment 2 demonstrated that although detection responses are not slowed in the U-shaped masking situation, spatial-choice judgments are. Experiments 3 and 4 indicated that this masking effect on spatial-choice reaction time is lost relatively rapidly with practice. However, changing the stimulus-response assignments reinstates the effect. The experiments suggest that for the situation in which U-shaped masking functions are obtained, responses that require attention (spatial-choice judgments early in practice or after stimulus-response relationships have been switched) are influenced by the metacontrast-induced phenomenal dimming, whereas responses that are automatic (i.e., detection responses; practiced spatial-choice judgments with consistent stimulus-response mappings) are not.

Adolescent↗