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PubMed1997

CT simulation in stereotactic brain radiotherapy--analysis of isocenter reproducibility with mask fixation.

BACKGROUND AND PURPOSE: CT verification and measurement of isocenter deviation using repeated mask fixation in linac-based stereotactic high dose radiotherapy of brain metastases were performed in this study. MATERIALS AND METHODS: For stereotactic radiotherapy of brain metastases a commercial head mask fixation device based on thermoplastic materials (BrainLAB) was used. A two-step planning-treatment procedure was performed. Immediately before treatment the patient was relocated in the mask and a verification CT scan of the radiopaque marked isocenter was performed and if necessary its position was corrected. The verification procedure is described in detail. Twenty-two CT verifications in 16 patients were analyzed. Deviations were measured separately for each direction. A 3D-deviation vector was calculated. Additionally the average amount of deviation in each of the three dimensions was calculated. RESULTS: The mean deviation and standard deviation (SD) of the isocenter was 0.4 mm (SD 1.5 mm) in the longitudinal direction, -0.1 mm (SD 1.8 mm) in the lateral direction and 0.1 mm (SD 1.2 mm) in the anterior-posterior direction. The mean three-dimensional distance (3D-vector) between the verified and the corrected isocenter was 2.4 mm (SD 1.3 mm). The average deviation (without consideration of direction) was 1.1 mm (SD 1.1 mm), 1.3 mm (SD 1.3 mm) and 0.8 mm (SD 0.9 mm) in the longitudinal, lateral and sagittal directions, respectively. No correlation was found between 3D-deviation and the distance of the isocenter from the reference plane nor between deviation and the position of metastases in the brain (central versus peripheral or between different lobes), or the date of treatment. CONCLUSION: Reproducibility of the isocenter using the presented mask fixation is in the range of positioning reproducibility reported for other non-invasive fixation devices for stereotactic brain treatment. Our results underline the importance of CT verification as a quality assurance method in stereotactic radiotherapy. Under the condition of a preceding CT verification the mask can be used for single dose stereotactic radiotherapy. For fractionated stereotactic irradiation of small target volumes we recommend repeated CT verifications to assure reproducibility.

Brain Neoplasms↗
PubMed2001

Intubating laryngeal mask airway, laryngeal tube, 1100 ml self-inflating bag-alternatives for basic life support?

Insufficient oxygenation, ventilation and gastric inflation with subsequent regurgitation of stomach contents is a major hazard of bag-valve-face mask ventilation during the basic life support phase of cardiopulmonary resuscitation (CPR). The European Resuscitation Council has recommended smaller tidal volumes of approximately 500 ml as an effort to reduce gastric inflation; furthermore, the intubating laryngeal mask airway and the laryngeal tube have been recently developed in order to provide rapid ventilation and to secure the airway. The purpose of our study was to examine whether usage of a newly developed medium-size self-inflating bag (maximum volume, 1100 ml) in association with the intubating laryngeal mask airway, and laryngeal tube may provide adequate lung ventilation, while reducing the risk of gastric inflation in a bench model simulating the initial phase of CPR. Twenty house officers volunteered for our study. When using the laryngeal tube, and the intubating laryngeal mask airway, respectively, the medium-size (maximum volume, 1100 ml) versus adult (maximum volume, 1500 ml) self-inflating bag resulted in significantly (P<0.05) lower mean+/-S.E.M. lung tidal volumes (605+/-22 vs. 832+/-4 ml, and 666+/-27 vs. 887+/-37 ml, respectively), but comparable peak airway pressures. No gastric inflation occurred when using both devices with either ventilation bag. In conclusion, both the intubating laryngeal mask airway and laryngeal tube in combination with both an 1100 and 1500 ml maximum volume self inflating bag proved to be valid alternatives for emergency airway management in a bench model of a simulated unintubated cardiac arrest victim.

Adult↗
PubMed2003

The masking level difference in chinchilla auditory cortex. Effects of inner hair cell loss.

The purpose of the present study was to investigate responses from the unanesthetized chinchilla auditory cortex (AC) to conditions producing a masking level difference (MLD) in perceptual studies, both before and after inner hair cell (IHC) loss caused by carboplatin. Tungsten electrodes were chronically implanted in the right AC (active) and anterior cranium (common) in six adult chinchillas. Following a recovery period, AC responses were obtained from the unanesthetized animal placed in a passive restraint. Toneburst input/output functions were obtained. Tonebursts (500 Hz) ranged from 0 to 80 dB pSPL, in 10 dB steps, and were presented to the left ear, the right ear, binaural (in-phase) and binaural (out-of-phase). For the MLD series, responses to 70 dB pSPL, 500 Hz tonebursts and a continuous broadband noise (40-90 dB SPL, in 5 dB steps) were studied. Three MLD versus non-MLD conditions were obtained: SpiN0/S0N0, SLN0/SLNL, and SRN0/SRNR. Following baseline data collection, each animal was given 75 mg/kg carboplatin intraperitoneally. Four to five weeks later, the electrophysiology protocol was followed again. At 5 weeks post-carboplatin, the animals were sacrificed, the cochleas were harvested, and cochleograms (hair cell loss across cochlear place) were constructed. For all conditions, response latencies increased and amplitudes decreased with decreasing toneburst level and increasing level of masking noise. Masked AC response thresholds were higher (better) for the MLD conditions than their respective non-MLD conditions. AC response latencies across masking noise level did not appear to vary systematically across conditions. Under some conditions, the MLD conditions showed larger amplitudes than their respective non-MLD conditions for equivalent masker levels. Post-carboplatin, cochleograms showed moderate IHC loss (on average, approximately 40% loss in apex) with minimal outer hair cell loss. The differences in MLD versus non-MLD conditions in terms of masked threshold and response amplitude were often reduced post-carboplatin.

Acoustic Stimulation↗
PubMed2003

Spatial spread of neural excitation in cochlear implant recipients: comparison of improved ECAP method and psychophysical forward masking.

This study introduces and evaluates a method for measurement of the longitudinal spread of electrically evoked neural excitation in the cochlea, using the Neural Response Telemetry system (NRT) available with the Nucleus((R)) 24 cochlear implant system. The recently released version of the NRT software (version 3.0) enables presentation of the 'masker' and 'probe' on different electrodes. In the present method the probe position was fixed, while the masker position was varied across the electrode array. The amplitude of the response to the partially masked probe provides a measure of the amount of masking, which is dependent on the extent of overlap of the excitation regions of the masker and probe. These measurements were performed in seven subjects implanted with the Nucleus 24 cochlear implant system (four with straight and three with Contour electrode arrays), for basal, middle and apical probe electrodes. Similar excitation profiles were obtained using either the standard NRT subtraction paradigm or an alternative 'Miller' method. The excitation profiles were compared with those obtained from psychophysical forward masking and good agreement was found. The widths of electrically evoked compound action potential (ECAP) and forward masking profiles did not differ significantly. Whereas the width of the ECAP measure was significantly correlated with both the maximum comfortable level and the distance of the electrode band from the modiolus, the width of the forward masking profile was not.

Adult↗
PubMed1990

[The laryngeal mask].

A new type of airway has been widely used for two years, throughout hospitals in the United Kingdom. Designed and created since 1983 by Dr AIJ Brain, the Laryngeal Mask Airway (LMA) is a compromise between the endotracheal tube and the face-mask. Blindly inserted in an anaesthetized patient, without either a laryngoscope or neuromuscular blockade, it provides a good airway in almost all cases. It is often able to offer an effective alternative to difficult intubation. The LMA can be used with either spontaneous or positive pressure ventilation. Because it doesn't provide a reliable protection of the airway from aspiration, it should never be used in the patient with a full stomach. The spontaneously breathing patient, undergoing elective surgery for 15 to 60 minutes, in supine position, who would ordinarily be managed with a face-mask is the more likely candidate for the LMA. But, longer procedures, in lateral or prone position, with controlled ventilation can usually be carried out using the Brain's device. More effective and less demanding than the facial-mask, much less hurtful than the endotracheal tube, the Laryngeal Mask is potentially an important and valuable addition to anaesthetic care.

Anesthesia, Inhalation↗
PubMed1994

[Diprivan and ventilation: laryngeal mask].

The laryngeal mask ensures a better control of the airway than the face mask, without the disadvantages of an endotracheal tube. Moreover, it provides an effective and simple solution to many problems of difficult intubation and therefore it has a place amongst the equipment required for difficult intubations. The laryngeal mask will most probably become more widely used, provided that its contra-indications are taken into account. These are dominated by its inability to protect the airway from inhalation of gastric contents in patients with a full stomach. Many studies have shown that propofol is suitable for use with the laryngeal mask. However, it should be noted, that there is, as yet, no conclusive evidence of the superiority of propofol over other induction agents. In anaesthesia for day-case surgery, the combination of the laryngeal mask and propofol could prove beneficial.

Anesthesia, Intravenous↗
PubMed2001

Negative priming in schizophrenia: effects of masking and prime presentation time.

Beech et al. [Br. J. Clin. Psychol. 28 (1989) 109--116] previously reported attenuated negative priming in schizophrenic patients that was interpreted as a sign of dysfunctional cognitive inhibition. However, subsequent research has provided mixed results. In the present study, it was investigated whether reduced negative priming in schizophrenics may be an experimental artifact. Based on evidence from backward masking studies in schizophrenia, it was hypothesized that brief prime presentation times and pattern masking as used by Beech et al. and others may have impaired the visual perception of the prime display in schizophrenics. 20 schizophrenic patients and 20 matched healthy controls participated in the study. Subjects completed four negative priming experiments varying in prime presentation time (100 or 250 ms) and masking (a mask or a blank screen followed prime presentation). In line with prediction, reduced negative priming in schizophrenics only occurred for trials with 100 ms prime presentation time followed by a mask. Neither psychopathology nor any sociodemographic variable correlated substantially with negative priming. Results strongly suggest that reduced negative priming in schizophrenics may not be due to reduced cognitive inhibition but mirrors perceptual deficits.

Adult↗
PubMed2000

Event-related potential indices of semantic priming using masked and unmasked words: evidence that the N400 does not reflect a post-lexical process.

Several authors have contended that the N400 is a reflection of a post-lexical event such as that proposed by Neely and Keefe [J.H. Neely, D.E. Keefe, Semantic context effects on visual word processing: a hybrid prospective/retrospective processing theory, in: G.H. Bower (Ed.), The Psychology of Learning and Motivation: Advances in Research and Theory, Vol. 23, Academic Press, New York, 1989, pp. 207-248.], whereby the subject compares the word on the current trial to the "context" provided by the word on the preceding trial [M. Besson, M. Kutas, The many facets of repetition: A cued-recall and event-related potential analysis of repeating words in same versus different sentence contexts, Journal of Experimental Psychology: Learning, Memory and Cognition, 19 (5) (1993), 1115-1133; C. Brown, P. Hagoort, The processing nature of the N400: Evidence from masked priming. Journal of Cognitive Neuroscience, 5(1) (1993), 34-44; P.J. Holcomb, Semantic priming and stimulus degradation: Implications for the role of the N400 in language processing, Psychophysiology 30 (1993), 47-61; M.D. Rugg, M.C. Doyle, Event-related potentials and stimulus repetition in indirect and direct tests of memory, in: H. Heinze, T. Munte, G.R. Mangun (Eds), Cognitive Electrophysiology, Birkhauser Boston, Cambridge, MA, 1994]. A study which used masked primes to directly test this possibility has been reported by Brown and Hagoort [C. Brown, P. Hagoort, The processing nature of the N400: evidence from masked priming. Journal of Cognitive Neuroscience, 5(1) (1993), 34-44]. When the primes were masked, no priming effect was observed on the N400. When behavioral data were collected in the same paradigm, from another group of subjects, the usual priming effect on RT was obtained. Considered together, the data from the two groups of subjects indicated that activation of semantic representations had occurred without conscious awareness. As no N400 priming effect was observed, it was suggested that N400 must reflect a post-lexical process. This interpretation, however, is at odds with the findings of other studies which have reported N400 priming effects under conditions where post-lexical processes would not be thought to operate[J. Anderson, P. Holcomb, Auditory and visual semantic priming using different stimulus onset asynchronies: an event-related brain potential study. Psychophysiology 32 (1995), 177-190; J. Boddy, Event-related potentials in chronometric analysis of primed word recognition with different stimulus onset asynchronies, Psychophysiology 23 (1986), 232-245; D. Deacon, T. Uhm, W. Ritter, S. Hewitt, The lifetime of automatic priming effects may exceed two seconds, Cognitive Brain Research 7 (1999), 465-472; P.J. Holcomb, Automatic and attentional process: an event-related brain potential analysis of semantic priming. Brain and Language 35 (1998) 66-85]. The present study replicated Brown and Hagoort using a repeated measures design, a shorter SOA (stimulus onset asynchrony), and a slightly different threshold setting procedure. Significant priming effects were obtained on the mean amplitude of the N400 regardless of whether the words were masked or unmasked. The findings imply that the processing subserving the N400 is not postlexical, since the N400 was manipulated without the subjects being aware of the identity of the words.

Adolescent↗
PubMed1996

The laryngeal mask: an essential part of emergency airway management.

In this article I have endeavoured to introduce the laryngeal mask airway to those who have not come in contact with it in their clinical experience. Those who have and want to know more will, after reading it, form a knowledge base which, I hope, will encourage the acquisition of the skill of insertion. The background and development of the mask are described along with a description of the mask's insertion into the airway and its positioning. I have also included our experience gained whilst introducing the mask into our Hospital Trust, the Royal Berkshire & Battle Hospital NHS Trust (UK), and the training programme which we set up to train an retrain necessary personnel. The aim of the article is not to usurp the endotracheal tube's position in emergency airway management, but to enlighten the reader as to how the mask can dovetail in with its use. The endotracheal tube, of course, remains the gold standard in the isolation of the adult patient's airway in cardiopulmonary arrest.

Adult↗
PubMed2003

Simple method to determine the size of the laryngeal mask airway in children.

BACKGROUND AND OBJECTIVE: The size of the laryngeal mask airway in children is determined by the patient's weight. However, in some instances an alternative method may be wanted. The aim was to search for a new method that would be easy to perform at the bedside. METHODS: The size of the laryngeal mask airway was determined in 183 children by choosing the laryngeal mask that best matched the combined widths of the patient's index, middle and ring fingers. The results were compared with the standard method recommended by the manufacturer's weight-related guidelines. The patients were classified in different groups depending on the laryngeal mask airway sizes determined by both methods. A kappa coefficient evaluated the agreement between both techniques. RESULTS: The kappa coefficient was 0.81, showing an 'excellent agreement' between both methods. The size was the same for both methods in 142 children (78%). The disagreement between both techniques was only of one size in the remaining 41 patients (22%). In such patients, the weight was a borderline value that would indicate a change in the size of the laryngeal mask airway using the classic method. CONCLUSIONS: This new approach is of valid and practical use in children, particularly as an alternative in those situations where the patient's weight is unknown, such as in emergency situations or in those borderline instances where an alternative measurement would be useful.

Adolescent↗
PubMed2004

Three-dimensional cervical spine movement during intubation using the Macintosh and Bullard laryngoscopes, the bonfils fibrescope and the intubating laryngeal mask airway.

BACKGROUND AND OBJECTIVE: Cervical spine movement may be limited for morphological reasons or through injury. The major goal of the present study was to evaluate the three-dimensional cervical spine movement during intubation with a Macintosh or Bullard laryngoscope, a Bonfils fibrescope or an intubating laryngeal mask using an ultrasound-based motion system. METHODS: Forty-eight patients without any history of cervical spine problems who had to undergo elective surgery in general anaesthesia were intubated using a Macintosh or Bullard laryngoscope, a Bonfils fibrescope or an intubating laryngeal mask airway. During intubation, cervical motion as well as overall time to intubation, number of attempts, and postoperative complaints were noted. RESULTS: The range of cervical spine motion during intubation, especially concerning extension, using the Macintosh laryngoscope was much greater (22.5 degrees +/- 9.9 degrees) than using Bullard (3.4 degrees +/- 1.4 degrees), Bonfils (5.5 degrees +/- 5.0 degrees) or intubating laryngeal mask (4.9 degrees +/- 2.1 degrees). Time to intubate the trachea using Bonfils (52.1 +/- 22.0 s) and intubating laryngeal mask (49.8 +/- 18.7 s) were much longer than with Macintosh (18.9 + 7.1s) and Bullard laryngoscope (16.1 + 6.2 s) (significance level: 0.05). CONCLUSIONS: Our findings suggest that the Bullard laryngoscope may be a useful adjunct to intubate patients with cervical spine injuries. In elective situations when time to intubation is not critical Bonfils as well as intubating laryngeal mask airway should also be considered as serious alternatives to direct laryngoscopy.

Anesthesia, General↗
PubMed2005

Insertion of the endotracheal tube, laryngeal mask airway and oesophageal-tracheal Combitube. A 6-month comparative prospective study of acquisition and retention skills by medical students.

OBJECTIVE: To assess the ability of medical students to learn and retain skills of airway manipulation for insertion of the endotracheal tube, the laryngeal mask airway (Laryngeal Mask Company, Henley-on-Thames, UK) and the oesophageal-tracheal Combitube (Kendall-Sheridan Catheter Corp., Argyle, NY, USA). METHODS: A 6-month prospective study was conducted among fifth-year medical students attending a 3-week clerkship in the Division of Anesthesiology and Critical Care Medicine in the Soroka Medical Center. All the students viewed a demonstration of insertion technique for the endotracheal tube, the laryngeal mask airway and the Combitube, followed by formal teaching in a mannikin. At the end of the program, the insertion skills were demonstrated in the mannikin, the success rate on the first attempt was registered and the students were requested to assess (by questionnaire) their ability to execute airway manipulation (phase 1). Six months later, the students were requested to repeat the insertion technique, and a similar re-evaluation applied (phase 2). RESULTS: The success rate, during the first phase, at first attempts was 100% for the laryngeal mask airway and the Combitube, compared to 57.4% for the endotracheal tube (P < 0.02), and 92.6%, 96.2% and 62.9% (P < 0.02) respectively for the second phase of the study. CONCLUSION: Learning and retention skills of medical students, in a mannikin, are more accentuated with the laryngeal mask airway and the Combitube than seen with an endotracheal tube.

Clinical Clerkship↗
PubMed2006

Feasibility of flexible lightwand-guided tracheal intubation with the intubating laryngeal mask during out-of-hospital cardiopulmonary resuscitation by an emergency physician.

BACKGROUND AND OBJECTIVE: We tested the feasibility of using the intubating laryngeal mask airway Fastrach (ILMA) as a ventilatory device and for flexible lightwand-guided tracheal intubation for out-of-hospital cardiopulmonary resuscitation by an emergency physician. METHODS: After completion of a training programme, a single experienced emergency physician used the technique for all patients requiring out-of-hospital tracheal intubation over a 10-month period. If access to the head and neck was limited, the intubating laryngeal mask airway was inserted from below and to the side, otherwise it was inserted from above the head. Data about the time for the ambulance to reach the patient, whether or not access to the head and neck was limited, whether or not circulation was successfully restored, and the insertion and intubation success rates were noted. RESULTS: The mean (range) time for the ambulance to reach the patient was 12 (10-20) min. Access to the head and neck was limited in 8/37 (22%). Circulation was successfully restored in 10/37 (27%). The intubating laryngeal mask airway was successfully inserted at the first attempt in 35/37 (95%) and at the second attempt in 2/37 (5%). The tracheal tube was successfully inserted in 25/37 (67.5%) at the first attempt, 7/37 (19%) at the second attempt and 5/37 (13.5%) at the third attempt. There were no overall failures for intubating laryngeal mask airway insertion or tracheal intubation. There were no differences in success rate between positions. Oesophageal intubation was detected and corrected in 2/37 (5%). CONCLUSION: The intubating laryngeal mask airway has a high success rate as a ventilatory device and as a flexible lightwand-guided airway intubator during out-of-hospital cardiopulmonary resuscitation by a well-trained emergency physician. This technique may be particularly useful when there is limited access to the head and neck.

Adult↗
PubMed2005

The effects of direct sequential masking on evoked potentials in the auditory area of the cortex in guinea pigs.

Thresholds for the appearance of evoked potentials to clicks in test series were determined in the quiet and in conditions of sequential masking of one stationary and four moving sound images. Differences in the thresholds for the appearance of evoked responses to signals in the quiet and in the presence of the marker were taken as a measure of the extent of masking. The overall responses of the auditory area of the guinea pig cortex showed direct masking as a clear reduction in the magnitude of the response to the first click in the series as compared with the response in the quiet. The extent of masking decreased with increases in the time delay and was independent of the position of the sound image corresponding to this click in different test signals. This observation may provide indirect support for the hypothesis that the initial part of the signal is subject to greater masking than the final part. The magnitude of responses to subsequent clicks in series were determined by a combination of two factors--the action of the sequential masker in the "masker-signal" system and the interaction between the responses to individual clicks in the series making up the test signal.

Acoustic Stimulation↗
PubMed2006

Delayed masking and the auditory attentional blink: A test for retrieval competition and bottleneck models.

The attentional blink (AB) corresponds to a transient deficit in reporting the second (T2) of two targets embedded in a rapid sequence of distractors. The retrieval competition (Shapiro, Raymond & Arnell, 1994) and bottleneck models (Chun & Potter, 1995; Jolicoeur, 1998) predict the attenuation of the deficit with the extension of the delay between T2 and its mask. This prediction was tested using auditory sequences of nonverbal stimuli in which the T2-mask interval was systematically varied. The magnitude of the auditory AB diminished with the lengthening of the interval from 50 to 150 ms while no time-locked deficit was observed with the longest (350 ms) and the shortest (10 ms) intervals. These results suggest that presenting a mask after T2 is not sufficient to produce an auditory AB: The mask must be perceivable as an auditory event distinct from the target and occur before T2 consolidation. The present study also provides evidence that as in vision, AB deficits take place in the auditory domain when T2 is masked by interruption but not by integration. Our findings are best accounted for in terms of bottlenecked processing limitations.

Attention↗
PubMed2000

Combining a beta2-agonist with a face mask to prevent exercise-induced bronchoconstriction.

BACKGROUND: In this study, we aimed to test the effect of combining a P2-agonist with a heat- and moisture-exchanging cellulose face mask in patients with bronchoconstriction induced by exercise in cold air. METHODS: Nine allergic, asthmatic patients with a history of adverse reaction to exercise were tested on an ergometric bicycle at a temperature of approximately -10 degrees C. They were in turn given no treatment, given premedication with a beta2-agonist, allowed to breathe through a heat- and moisture-exchanging cellulose face mask, and given both premedication and the face mask. After each treatment regimen, they were subjected to provocation with exercise and cold air. RESULTS: The decrease in FEV, was greatest with no therapy (mean maximal change: 27%), and this diminished when the face mask was used (mean maximal change: 12%,) or premedication with a 32-agonist was given (mean maximal change: 7%); no decrease in FEV1 occurred with the combination of agonist and face mask. CONCLUSIONS: The results suggest that different mechanisms are involved in the pathophysiology of exercise-induced bronchoconstriction (EIB). This finding may be of importance for asthmatic athletes who train and compete in a cold climate and are affected by small changes in their lung function.

Adrenergic beta-2 Receptor Agonists↗
PubMed2000

Light-guided intubation via the intubating laryngeal mask using a prototype illuminated flexible catheter. Clinical experience in 400 patients.

BACKGROUND: The transillumination of the soft tissues of the neck using lighted stylets has been used as an aid for tracheal intubation. We evaluated the efficacy and safety of a prototype illuminated flexible catheter to facilitate light-guided intubation through the intubating laryngeal mask. METHODS: The illuminated flexible catheter consists of a completely flexible thin plastic catheter with a bulb attached to its distal end. The device was placed into a silicone tracheal tube in such a way that the bulb was adjusted at the distal end of the tracheal tube. The tracheal tube preloaded with the device was inserted through the intubating laryngeal mask and, by observing the glow on the neck, was advanced into the trachea. We report our experience with light-guided intubation through the intubating laryngeal mask in 400 ASA grade 1-3 patients undergoing general anaesthesia. RESULTS: The intubating laryngeal mask was inserted successfully in all patients. The overall intubating success rate was 99.8% (399/400); in 367 (91.8%) cases at the first attempt, in 28 (7%) at the second, in 4 (1%) at the third and in one case (0.2%) at the fifth attempt. There were 27 patients with potentially difficult airways. All these cases were intubated successfully; in 23 of 27 (85.2%) at the first attempt, in 3 of 27 (11.1%) at the second and one of 27 patients (3.7%) at the third attempt. CONCLUSION: We conclude that the use of the illuminated flexible catheter facilitates the intubation through the intubating laryngeal mask. The suggested light-guided intubating method proved to be a simple, safe and effective technique.

Adolescent↗
PubMed1976

Effects of texture on visual noise masking.

Visual noise masks, composed of letter fragments scattered randomly over the visual field, disrupt the recognition of briefly viewed words when presented immediately before and/or after these words. According to the widely held feature-similarity assumption, maximal disruption is predicted when the mask's letter fragments are equal in size and shape to the target letters. This assumption is not supported by the present experiment, which measured the masking effectiveness of visual noise patterns varying in texture (achieved by photographic reduction or enlargement). For target letters with stroke widths subtending 7.4' or less, the optimally disruptive mask did not vary with the stroke width of the target letters, but rather, was invariably composed of letter fragments with stroke widths subtending about 7'. For larger target words, however, this mask was not optimally disruptive. These results suggest that recognition of letters less that about 7' is mediated by a single size-tuned mechanism.

Discrimination, Psychological↗