Frameless stereotaxy with scalp-applied fiducial markers for brain biopsy procedures: experience in 218 cases.

OBJECT: The goal of this study was to develop and assess the use and limitations of performing brain biopsy procedures by using image-guided surgical navigation systems (SNSs; that is, frameless stereotactic systems) with scalp-applied fiducial markers. METHODS: Two hundred eighteen percutaneous brain biopsies were performed in 213 patients by using a frameless stereotactic SNS that operated with either sonic or optical digitizer technology and scalp-applied fiducial markers for the purpose of registering image space with operating room space. Common neurosurgical and stereotactic instrumentation was adapted for use with a localizing wand, and recently developed target and trajectory guidance software was used. Eight (3.7%) of the 218 biopsy specimens were nondiagnostic; five of these (2.4%) were obtained during procedures in 208 supratentorial lesions and three were obtained during procedures in 10 infratentorial lesions (30%; p < 0.001). Complications related to the biopsy procedure occurred in eight patients (seven of whom had supratentorial lesions and one of whom had an infratentorial lesion, p > 0.25). Five complications were intracerebral hemorrhages (two of which required craniotomy), two were infections, and one was wound breakdown after instillation of intratumoral carmustine following biopsy. There were only three cases of sustained morbidity, and there were two deaths and one delayed deterioration due to disease progression. Two surgeons performed the majority of the procedures (193 cases). The three surgeons who performed more than 10 biopsies had complication rates lower than 5%, whereas two of the remaining four surgeons had complication rates greater than 10% (p = 0.15). Twenty-three additional procedures were performed in conjunction with the biopsies: nine brachytherapies; five computer-assisted endoscopies; four cyst aspirations; two instillations of carmustine; two placements of Ommaya reservoirs; and one craniotomy. CONCLUSIONS: Brain biopsy procedures in which guidance is provided by a frameless stereotactic SNS with scalp-applied fiducial markers represents a safe and effective alternative to frame-based stereotactic procedures for supratentorial lesions. There were comparable low rates of morbidity and a high degree of diagnostic success. Strategies for performing posterior fossa biopsies are suggested.

Predictors of axillary lymph node metastasis in T1 breast carcinoma.

This study was designed to determine the predictors of axillary lymph node metastasis in T1a (< or =0.5 cm), T1b (>0.5 cm and < or =1.0 cm), and T1c (>1.0 cm and < or =2.0 cm) breast cancers. The charts of 204 patients who underwent axillary lymph node dissections for T1 breast carcinomas at our institution were reviewed. Of these, 23 (11%) patients had T1a cancers, 55 (27%) patients had T1b cancers, and 126 (62%) patients were diagnosed with T1c lesions. Fifty patients (24.5%) had axillary node metastases. Of those with T1a lesions, one (4.3%) patient had axillary node involvement, compared with 9 (16.4%) patients with T1b and 40 (31.7%) patients with T1c lesions. Nodal involvement was significantly increased in T1c cancer compared with either T1a (odds ratio = 8.24; P < 0.05) or T1b (odds ratio = 2.73; P < 0.05). Similar results were found in tumors with grade 3 nuclear pleomorphism (odds ratio = 10.45 versus grade 1 and 3.46 versus grade 2; P < 0.05). The presence of lymphovascular invasion was also associated with an increased likelihood of nodal involvement (odds ratio = 3.15; P < 0.05). Predictors of axillary lymph node metastasis in T1 breast carcinomas include increasing tumor size, grade 3 nuclear pleomorphism, and the presence of lymphovascular invasion. These predictors may have a role in stratifying patients with T1 breast carcinomas into subgroups that may benefit from less invasive methods of evaluating axillary lymph node status.

Comparison of the ABC/2 estimation technique to computer-assisted volumetric analysis of intraparenchymal and subdural hematomas complicating the GUSTO-1 trial.

BACKGROUND AND PURPOSE: The volume of an intracerebral hemorrhage has been shown to be an important independent predictor of mortality in several reports. A technique for estimating hematoma volume, known as the ABC/2 method, has been proven a reliable, simple bedside technique for the volume measurement of intraparenchymal intracerebral hemorrhage. Subdural hematomas also carry a significant mortality risk but are more amenable to surgical evacuation. A reliable, simple bedside measurement of subdural hematoma volume may prove a valuable tool in prognostication and management of patients with this entity. METHODS: Computed tomographic (CT) brain scans of 244 patients suffering from intracranial hemorrhage in the GUSTO-1 trial were systematically reviewed. The volumes of 298 intraparenchymal hematomas were measured by the ABC/2 technique, and the volumes of 44 subdural hematomas were measured by an adaptation of this technique and compared to computer-assisted volumetric analysis. RESULTS: Excellent correlation between the techniques were achieved for both subdural (r=0.842; slope, 0.982) and intraparenchymal hematoma volume measurements (r=0.929; slope, 1.11). CONCLUSIONS: The ABC/2 method is a simple and accurate technique for the measurement of intraparenchymal hematoma volume, and a simple adaptation allows for a similarly accurate measurement of subdural hematoma volume as well.

Mechanisms of neutropenia involving myeloid maturation arrest in burn sepsis.

OBJECTIVE: To determine the mechanisms that lead to the decrease in bone marrow production of neutrophils during burn sepsis. SUMMARY BACKGROUND DATA: Impaired bone marrow granulopoiesis during burn sepsis often results in neutropenia despite elevated circulating levels of granulocyte colony-stimulating factor (G-CSF). To date, neither the specific stages of neutrophil maturation involved in the bone marrow suppression nor the mechanisms for the impairment have been determined. METHODS: Peripheral blood absolute neutrophil count and G-CSF levels were determined in mice 3 days after randomization to control, burn alone, or burn plus a topical inoculation of Pseudomonas aeruginosa (1000 colony-forming units). Bone marrow aspirates were analyzed for their neutrophil differentiation patterns by Gr-1 antigen expression and their G-CSF receptor status. Histologic analysis of liver, lung, spleen, and wound site was performed. RESULTS: In burn sepsis, absolute neutrophil count was reduced whereas plasma G-CSF levels were elevated, and myeloid differentiation was significantly shifted toward the immature mitotic myeloid cells. Bone marrow G-CSF receptor mRNA levels and G-CSF-stimulated proliferation were substantially decreased in burn sepsis. Histologic analysis revealed no significant neutrophil infiltration into the tissues. CONCLUSIONS: In thermal injury with superimposed sepsis, neutropenia and myeloid maturation arrest, despite the elevated levels of G-CSF, correlate with the reduction in bone marrow G-CSF receptor expression. These observations may provide a potential mechanism for neutropenia in sepsis.

Thrombolysis-related intracranial hemorrhage: a radiographic analysis of 244 cases from the GUSTO-1 trial with clinical correlation. Global Utilization of Streptokinase and Tissue Plasminogen Activator for Occluded Coronary Arteries.

BACKGROUND AND PURPOSE: Intracranial hemorrhage (ICH) is a serious complication of thrombolytic therapy. We systematically reviewed the radiographic features of 244 cases of symptomatic ICH complicating thrombolysis for acute myocardial infarction in the Global Utilization of Streptokinase and Tissue Plasminogen Activator for Occluded Coronary Arteries (GUSTO-1) trial, correlated these observations with clinical data, and speculated on hemorrhage pathogenesis. METHODS: CT scans from 244 patients suffering symptomatic ICH were systematically reviewed for selected radiographic features, including ICH type, location, hematoma characteristics, mass effect features, hydrocephalus, and preexisting lesions. Hematoma volume was estimated by computer-assisted volumetric analysis. Data from this analysis were correlated with clinical data including hypertension, anticoagulation, age, thrombolytic regimen, and ICH timing. RESULTS: Most hemorrhages were large (median [25th, 75th percentile] volume, 72 mL [39, 118]), solitary (66%), lobar (77%), confluent (80%), and intraparenchymal (82%) with a blood/fluid level (82%) and little edema (median [25th, 75th percentile] volume, 9 mL [5, 16]). Hydrocephalus (P<.001), any one mass effect feature (P<.001), intraventricular hemorrhage (P=.022), mottled hematoma appearance (P=.050), and hematoma blood/fluid level (P<.001) were associated with higher hemorrhage volume in the radiographic analysis, as were older age (P=.005), treatment with combined streptokinase and tissue plasminogen activator (P=.034), and hemorrhage onset 8 to 13 hours after treatment (P=.008) in the clinical analysis. Subdural hemorrhage was a high-volume subgroup whose risk increased with antecedent trauma (P=.026) or syncope (P=.006). Deep intraparenchymal hemorrhage was associated with hypertension (P=.016), and multifocal ICH occurred significantly earlier after treatment (P=.002). CONCLUSIONS: Although the majority of postthrombolytic ICH are large, solitary, and supratentorial, the spectrum is diverse. Features of mass effect reflected the large volumes, and hematoma characteristics of mottling and blood/fluid levels were frequent. Thrombolysis-related coagulopathy and age appear to be the most important identifiable factors in the genesis of postthrombolytic ICH, but the hemorrhage subtype seen may reflect an interaction with other factors such as hypertension, ICH timing, antecedent head trauma, and syncope.

Target and trajectory guidance for interactive surgical navigation systems.

The proliferation of interactive surgical navigation systems (ISN) is evidence of their ability to provide useful information about current location and orientation at surgery. The utility of these systems would potentially expand with intuitive methods to guide the user towards a predefined target(s) along a predefined trajectory. A system of target and trajectory guidance is described comprised of three components: (1) auto-push that displays the plane perpendicular to the present trajectory that contains the preselected target and the distance from probe tip to this plane; (2) concentric circles and collinear spheres where spheres representing the target, entry point and wand tip are projected onto the 'autopush' plane as circles, thereby visually defining the rotations and translations required to reorient the wand to the predefined target and trajectory, and (3) projected trajectories where the ideal wand trajectory is projected onto two orthogonal planes, each of which contain the present wand trajectory. Together, these enhancements enable users of ISN to easily access small, deep intracranial structures along predefined surgical trajectories.

Application of frameless stereotaxy to pedicle screw fixation of the spine.

Interactive frameless stereotaxy has been successfully applied to intracranial surgery. It has contributed to the improved localization of deep-seated brain lesions and has demonstrated a potential for reducing both operative time and morbidity. However, it has not been as effectively applied to spinal surgery. The authors describe the application of frameless stereotactic techniques to spinal surgery, specifically pedicle screw fixation of the lumbosacral spine. Preoperative axial computerized tomography (CT) images of the appropriate spinal segments are obtained and loaded onto a high-speed graphics supercomputer workstation. Intraoperatively, these images can be linked to the appropriate spinal anatomy by a sonic localization digitizer device that is interfaced with the computer workstation. This permits the surgeon to place a pointing device (sonic wand) on any exposed spinal bone landmark in the operative field and obtain multiplanar reconstructed CT images projected in near-real time on the workstation screen. The images can be manipulated to assist the surgeon in determining the proper entry point for a pedicle screw as well as defining the appropriate trajectory in the axial and sagittal planes. It can also define the correct screw length and diameter for each pedicle to be instrumented. The authors applied this device to the insertion of 150 screws into the lumbosacral spines of 30 patients. One hundred forty-nine screws were assessed to be satisfactorily placed by postoperative CT and plain film radiography. In this report the authors discuss their use of this device in the clinical setting and review their preliminary results of frameless stereotaxy applied to spinal surgery. On the basis of their findings, the authors conclude that frameless stereotactic technology can be successfully applied to spinal surgery.

The transmission of phoneme-level information by multichannel tactile speech perception aids.

Current research on the effectiveness of tactile aids for speech perception by hearing-impaired persons suggests that substantial training, lasting over months or years, is necessary for users to achieve maximal benefits from a tactile device. A number of studies have demonstrated the usefulness of training programs that include an analytic component, such as phoneme training, together with more synthetic tasks such as sentence identification and speech tracking. However, particularly in programs for children, it is desirable to structure training experiences so that easy distinctions are trained first, and more difficult distinctions are approached only later in training. In the present study, a systematic evaluation of phoneme-level information provided by the Tactaid VII, a multichannel tactile aid, was performed. Adult subjects were tested in minimal pairs and closed set phoneme discrimination and identification tasks under tactile aid alone, speechreading alone, and speechreading plus tactile aid conditions, to provide an inventory of stimulus identifiability and permit ranking of discriminations as easy or more difficult. Because these rankings might differ as a function of coarticulation effects, three different vowel contexts were tested for consonant stimuli. Results indicated that there were indeed considerable differences across vowel contexts, and that the /ae/ vowel context yielded the most identifiable stimuli. These data could be used by teachers and therapists to construct viable stimulus sets for training programs for tactile aid users.

Intracranial meningioma resection using frameless stereotaxy.

Thirty-four consecutive patients with intracranial meningiomas underwent 35 resections aided by an interactive surgical navigation system (ISN; "frameless stereotaxy"). System capabilities include real-time display of wand location, orientation, and relationship to nearby structures using multiplanar and three-dimensional presentation of magnetic resonance imaging (MRI) and/or computed tomography (CT) data obtained perioperatively. There were 16 patients with convexity tumors, five patients with sphenoid wing tumors, five patients with falx or parasagittal tumors, and eight patients with skull base tumors (two each: petroclival, cavernous sinus, olfactory groove, and planum sphenoidale). the ISN system was used to locate a minimal craniotomy (i.e., trephine) in 11 (32%) patients, to optimize bone flap design in 13 (38%) patients, to identify the location of parasagittal draining veins in five (15%) patients, and to locate the carotid or basilar arteries in 11 (32%) patients. The techniques provided limited benefit in cranial nerve preservation. No patient had permanent central neurologic morbidity. Where intended preoperatively, tumor resection was complete (i.e., > 98%) in all patients as determined via postoperative MRI. Interactive surgical navigation is a useful adjunct in the operative management of some patients with intracranial meningiomas.

Adaptation of personal projection television to a head-mounted display for intra-operative viewing of neuroimaging.

We describe an adaptation of a personal projection television for intra-operative viewing of conventional neuroimaging and frameless stereotaxy displays. In addition to image display, the device provides surgical magnification and eye protection from body fluids. Image data is first processed by a scan converter to provide a National Standards Television Committee (NTSC) television signal. The adjustable optics of the system display are permanently secured to avoid displacement during surgery. A virtual image of the data is projected to an apparent size of four feet in the inferior visual field of the surgeon's dominant eye. Magnification is provided by the surgical telescopes mounted in the visor. Resolution of the device is consistent with that obtained in a slice of computed tomography or magnetic resonance imaging data. Optimal display of multiplanar data awaits improvement in density of the light emitting diode device used for image generation.

Comparative effectiveness of correction strategies in connected discourse tracking.

The effectiveness of four correction strategies commonly used in connected discourse tracking was investigated in the present study. The strategies were 1) verbatim repetition of a word or phrase; 2) use of antonyms or synonyms as cues; 3) use of phonemic cues, with no whole word repetition; and 4) going back or ahead in the text, with no repetition of the missed segment. Four normal-hearing adults served as listeners. Live-voice presentation of text by two female talkers was employed for all conditions. Listeners were tested in two stimulus presentation modes, speechreading alone and speechreading plus a multichannel tactile aid. Results indicated that strategy 1, repetition of the missed segment, produced the highest tracking rates, significantly higher than any of the other strategies. Strategy 2 produced the lowest tracking rates. Strategies 1 and 3 yielded the lowest percentage of initially missed words, or blockages, and strategy 4 the highest percentage. Significantly higher tracking rates were found under the speechreading plus tactile aid presentation mode, compared with speechreading alone. Further, tracking rates increased significantly from the beginning to the end of training. Data were compared with a more typical CDT task, in which all correction strategies were operative, and results showed little difference in tracking rates between this task and the constrained CDT employing only strategy 1. Overall, results suggest that simple repetition of missed segments is an effective correction strategy for CDT and argue for its inclusion in computer-assisted tracking implementations.

Vibrotactile temporal masking: effects of multiple maskers.

Previous investigations of temporal masking effects in vibrotactile detection tasks have generally produced results suggesting considerable similarities in stimulus processing between the tactile system and the auditory system. Auditory presentation of combinations of maskers (e.g., two forward maskers) has yielded masking effects in excess of the level of masking predicted by a simple intensity sum of each masker's individual effect. This "additional masking" has led to predictions of compressive nonlinearities in auditory stimulus processing. In the present study, two experiments were conducted to investigate further temporal masking phenomena for the tactile system. In the first experiment vibrotactile temporal masking functions for single forward and backward maskers were generated to examine trade-offs among values of masker duration, masking intensity, and interstimulus interval to maintain constant detectability of a target. Results suggested that stimuli in very close temporal proximity to the target might follow somewhat different trading equations from those farther removed. Overall, results were consistent with previous findings of temporal integration in the tactile system, and support the notion of an energy integrating mechanism. In the second experiment, pairs of maskers were presented to determine whether additional masking effects occurred in vibrotactile detection. In contrast to findings for auditory presentation, in which all masker configurations generated additional masking, the tactile results showed substantial amounts of additional masking only for pairs of backward maskers. Several possible explanations for this results are evaluated, and results are discussed in terms of similarities and dissimilarities in auditory and tactile temporal processing.

Astrocytoma resection using an interactive frameless stereotactic wand: an early experience.

18 patients with varying grades of astrocytoma had tumour resection using a new frameless, interactive sterotactic localizing wand. The system enables localizing information to be presented to the operator in multiple two-dimensional or three-dimensional displays in real time. In all cases the wand was used to help outline tumour boundaries in an attempt to resect solid tumour completely. Other uses included placing a minimal craniotomy or modifying an existing craniotomy in 17 (94%) patients, for intraoperative physiologic mapping in 2 (11%) and electrode placement in 1 (6%). All patients had a complete (98-100%) resection by postoperative MRI, and in 8 (44%) the tumour was removed from eloquent areas. Evaluation at approximately 6 weeks after surgery showed that 3 (16%) were improved, 14 (78%) were the same, and 1 (6%) was worse. On the second postoperative day no patient was better than their preoperative status, 10 (56%) were the same, and 8 (44%) patients were clinically worse. At 3 months the figures were 2 (11%), 11 (61%), and 4 (22%) respectively, and 1 patient had expired. The extent of resection has been shown to be an important prognostic factor in both low and high grade astrocytomas. This system allows accurate volumetric near en bloc resection and this was achieved in 17 (94%) cases. In one case of hippocampal tumour the tumour was removed in a piecemeal fashion. This system is an effective low cost alternative to frame volumetric systems and is associated with minimal morbidity at 6 weeks.

Use of a frameless, armless stereotactic wand for brain tumor localization with two-dimensional and three-dimensional neuroimaging.

Preliminary experience with a frameless, armless stereotactic localization system in brain tumor surgery is presented. The localizing wand emits ultrasonic pulses that are detected by a table-mounted array of microphones--with triangulation of the emitter positions. The wand tip and trajectory are determined by proprietary computer software. Real-time display of this information is presented in multiple, two-dimensional or three-dimensional displays. Forty-eight patients underwent 52 craniotomies for brain tumors. The wand was used to assist in placing a minimal craniotomy in 48 cases, to determine the tumor/brain interface in 27 cases, to localize subcortical tumors in 14 cases, and to correlate the physiological mapping with the surface anatomy in 5 cases. In 12 instances, the wand was used in conjunction with frame stereotaxy and found to be comparable or superior. Triplanar (coronal, sagittal, transverse) two-dimensional images provided sufficient information for the detection of tumor boundaries but proved difficult to use to access a subcortical lesion; two-dimensional or three-dimensional images along the localization axis were more helpful. Frameless stereotaxy with this sonic wand system proved to be a useful adjunct to open-tumor biopsy or resection.

Intraoperative localization using an armless, frameless stereotactic wand. Technical note.

A technique of "frameless" stereotaxy that allows real-time intraoperative neurosurgical localization is described. The system is composed of four components: a hand-held probe containing two ultrasonic emitters, a microphone array that is rigidly affixed to the operating table in proximity to the surgical field, hardware to control and detect timing of signal production and reception, and a color graphics computer workstation with software to calculate and present the location of the probe tip on reconstructed neuroimaging studies. Unlike previously reported mechanical or sonic navigational devices, this system is adaptable to a wide array of neurosurgical instruments, allows free movement of the operating table and conventional patient draping, and has accuracy in the hostile operating room environment that rivals that of frame stereotaxy. In the operating room environment, using four pulse pairs with the wand positioned optimally, reproducibility of a point in space is +/- 0.6 mm. The wand has a broad range of orientations that maintain error at or below 1.0 mm. The mean error when measuring distances within a 1000-cu cm cube is 1.1 +/- 1.0 mm (1.0% +/- 0.7%). The ability to localize a fourth point (a target) in space is typically within 1.5 mm (using computerized tomography scans with a 1-mm slice thickness) but is dependent on several variables. This technology provides a powerful yet flexible tool in the neurosurgical operating room.

Evaluation of a principal-components tactile aid for the hearing-impaired.

Principal component analysis, a statistical data reduction technique which can be used to eliminate redundant information, has shown promising results as a speech coding strategy in auditory perceptual studies. The present study describes the development, modification, and evaluation of a principal components-based tactile aid for speech perception by the hearing-impaired. In this device, the first two principal components of an input speech signal were displayed on two-dimensional arrays of vibrators contacting either the fingertip or the forearm. Initial testing of the device with closed-set recorded speech tokens showed fair recognition performance, reaching 57% for three consonants and 56% for four vowels. Modifications to the processor algorithm designed to improve vowel recognizability resulted in higher levels of performance (66% for eight vowels). A real-time prototype was constructed implementing the revised algorithm. Live-voice testing was conducted with six normal-hearing subjects, three of whom had previous training with the Queen's University vocoder, a multichannel tactile vocoder that has shown promising results. Performance of these "trained" subjects for both single-item and connected speech tasks was excellent, equalling levels obtained with the Queen's vocoder. These results suggest that a principal components design may be a promising alternative to a vocoder strategy for a tactile aid. Results for the "naive" subjects did not reach the levels attained by the trained subjects, a finding partially attributed to the short training period available to the naive subjects. The higher level of performance for the trained subjects, together with the similarity of performance for the principal components aid and the Queen's vocoder for these subjects, suggests that they were able to transfer previous learning with the Queen's vocoder to the principal components device.

Evaluating tactile aids for speech perception and production by hearing-impaired adults and children.

The type and degree of benefit provided by tactile aids for the hearing-impaired varies from device to device, as a function of such variables as the number of tactile transducers, type of stimulation, location of stimulation, and the nature of processing of the input acoustic waveform. In this paper the results from evaluations of tactile devices in our laboratory are discussed, to provide some insights into the amount of assistance that might be expected from the use of different tactile aids in different listening tasks. A number of perceptual tasks have been evaluated, ranging from simple detection of a stimulus to the tracking of connected speech. The results of these evaluations suggest that tactile aids, particularly multichannel devices that employ a number of tactile transducers and convey information about the spectral content of the speech signal, can be of significant benefit in speech perception. Further studies with profoundly hearing-impaired children indicate that aspects of speech production can also be improved through the use of a multichannel tactile aid, as evidenced from judgments of videotaped productions shown to teachers of the hearing-impaired. These findings suggest that even a relatively brief period of training with a tactile aid can lead to improvements in speech production by hearing-impaired children.

Relative performance of single-channel and multichannel tactile aids for speech perception.

Although the results from a number of studies of the performance of multichannel tactile aids for speech perception have suggested that such devices might provide more benefit to hearing-impaired persons than single-channel tactile aids (3,4), recent studies involving direct comparisons of multichannel and single-channel vibrotactile aids (5,6) indicated otherwise. In fact, for some types of speech information, such as rhythm and stress perception, single-channel aids were shown to be superior. The present study attempted to address this apparent discrepancy by comparing the performance of two single-channel devices with two multichannel devices in a variety of speech perception tasks including both single-item and connected speech stimuli. Results indicated that the two classes of tactile device performed similarly in rhythm and stress perception, but that the multichannel aids in many cases showed better performance for tasks in which the identification of fine-structure phoneme information was required (both single-item and connected speech). Results are discussed in terms of the possibility that the performance of a specific multichannel tactile aid cannot be considered indicative of all devices of the same class.

Masker fringe and binaural detection.

Yost [J. Acoust. Soc. Am. 78,901-907 (1985)] found that the detectability of a 30-ms dichotic signal (S pi) in a 30-ms diotic noise (No) was not affected by the presence of a 500-ms dichotic forward fringe (N pi). Kollmeier and Gilkey [J. Acoust. Soc. Am. 87, 1709-1719, (1990)] performed a somewhat different experiment and varied the onset time of a 25-ms S pi signal in a 750-ms noise that switched, after 375-ms, from N pi to No. In contrast to Yost, they found that the N pi segment of the noise reduced the detectability of the signal even when the signal was temporally delayed well into the No segment of the noise and suggested that the N pi segment of noise acted as a forward masker. To resolve this apparent conflict, the present study investigated the detectability of a brief S pi signal in the presence of an No masker of the same duration as the signal. The masker was preceded by quiet or an N pi forward fringe and followed by quiet, an No, or N pi backward fringe. The present study differs from most previous studies of the effects of the masker fringe in that the onset time of the signal was systematically varied to examine how masking changes during the time course of the complex fringe-masker-fringe stimulus.(ABSTRACT TRUNCATED AT 250 WORDS)

Evaluation of two multichannel tactile aids for the hearing impaired.

Two multichannel tactile devices for the hearing impaired were compared in speech perception tasks of varying levels of complexity. Both devices implemented the "vocoder" principle in their stimulus processing: One device had a 16-element linear vibratory array worn on the forearm and displayed activity in 16 overlapping frequency channels; the other device delivered tactile stimulation to a linear array of 16 electrodes worn on the abdomen. Subjects were tested in several phoneme discrimination tasks, ranging from discrimination of pairs of words differing in only one phoneme under tactile aid alone conditions to identification of stimuli in a larger set under tactile aid alone, lipreading alone, and lipreading plus tactile aid conditions. Results showed both devices to be better transmitters of manner and voicing features of articulation than of place features, when tested in single-item tasks. No systematic differences in performance with the two devices were observed. However, in a connected discourse tracking task, the vibrotactile vocoder in conjunction with lipreading yielded much greater improvements over lipreading alone than did the electrotactile vocoder. One possible explanation for this difference in performance, the inclusion of a noise suppression circuit in the electrotactile aid, was evaluated, but did not appear to account for the differences observed. Results are discussed in terms of additional differences between the two devices that may influence performance.