Polyphasic analysis reveals correlation between phenotypic and genotypic analysis in soybean bradyrhizobia (Bradyrhizobium spp.).

Soybean bradyrhizobia (Bradyrhizobium spp.) are bacteria that fix atmospheric nitrogen within the root nodules of soybean, a crop critical for meeting global nutritional protein demand. Members of this group differ in symbiotic effectiveness, and historically both phenotypic and genotypic approaches have been used to assess bradyrhizobial diversity. However, agreement between various approaches of assessment is poorly known. A collection (n=382) of soybean bradyrhizobia (Bradyrhizobium japonicum, B. diazoefficiens, and B. elkanii) were characterized by Internal Transcribed Spacer - Restriction Fragment Length Polymorphism (ITS-RFLP), cellular fatty acid composition (fatty acid methyl esters, FAME), and serological reactions to assess agreement between phenotypic and genotypic methods. Overall, 76% of the accessions demonstrated identical clustering with each of these techniques. FAME was able to identify all 382 accessions, whereas 14% were non-reactive serologically. One ITS-RFLP group, containing 36 Delaware isolates, produced multiple ITS amplicons indicating they possess multiple ribosomal RNA (rrn) operons. Cloning and sequencing revealed that these strains contained as many as three heterogenous rrn operons, a trait previously unknown in bradyrhizobia. A representative subset of 96 isolates was further characterized using 16S rRNA and Internal Transcribed Spacer (ITS) amplicon sequencing. ITS sequences showed better inter- and intra-species discrimination (65-99% identity) than 16S sequences (96-99% identity). This study shows that phenotypic and genotypic approaches are strongly correlated at the species level but should be approached with caution. We also suggest using combined 16S and ITS genotyping data to obtain better inter- and intra-species resolution in bradyrhizobia classification.

Metabolic Profile of Supragingival Plaque Exposed to Arginine and Fluoride.

Caries lesions develop when acid production from bacterial metabolism of dietary carbohydrates outweighs the various mechanisms that promote pH homeostasis, including bacterial alkali production. Therapies that provide arginine as a substrate for alkali production in supragingival oral biofilms have strong anticaries potential. The objective of this study was to investigate the metabolic profile of site-specific supragingival plaque in response to the use of arginine (Arg: 1.5% arginine, fluoride-free) or fluoride (F: 1,100 ppm F/NaF) toothpastes. Eighty-three adults of different caries status were recruited and assigned to treatment with Arg or F for 12 wk. Caries lesions were diagnosed using International Caries Detection and Assessment System II, and plaque samples were collected from caries-free and carious tooth surfaces. Taxonomic profiles were obtained by HOMINGS (Human Oral Microbe Identification using Next Generation Sequencing), and plaque metabolism was assessed by the levels of arginine catabolism via the arginine deiminase pathway (ADS), acidogenicity, and global metabolomics. Principal component analysis (PCA), partial least squares-discriminant analysis, analysis of variance, and random forest tests were used to distinguish metabolic profiles. Of the 509 active lesions diagnosed at baseline, 70 (14%) were inactive after 12 wk. Generalized linear model showed that enamel lesions were significantly more likely to become inactive compared to dentin lesions (P < 0.0001), but no difference was found when treatment with Arg was compared to F (P = 0.46). Arg significantly increased plaque ADS activity (P = 0.031) and plaque pH values after incubation with glucose (P = 0.001). F reduced plaque lactate production from endogenous sources (P = 0.02). PCA revealed differences between the metabolic profiles of plaque treated with Arg or F. Arg significantly affected the concentrations of 16 metabolites, including phenethylamine, agmatine, and glucosamine-6-phosphate (P < 0.05), while F affected the concentrations of 9 metabolites, including phenethylamine, N-methyl-glutamate, and agmatine (P < 0.05). The anticaries mechanisms of action of arginine and fluoride are distinct. Arginine metabolism promotes biofilm pH homeostasis, whereas fluoride is thought to enhance resistance of tooth minerals to low pH and reduce acid production by supragingival oral biofilms.

Arginine Metabolism in Supragingival Oral Biofilms as a Potential Predictor of Caries Risk.

INTRODUCTION: Ammonia production via the arginine deiminase system (ADS) of oral bacteria can function to reduce the cariogenicity of oral biofilms by neutralizing glycolytic acids that cause tooth demineralization. OBJECTIVES: This cohort study investigated the relationship between ADS activity and bacterial profile changes of supragingival biofilms with caries experience among children over time. METHODS: A total of 79 children aged 2 to 7 y at baseline were assessed every 6 mo for a period of 18 mo. Children were grouped as caries free (CF), caries active with enamel lesions (CAE), or caries active with dentin lesions (CA). Supragingival plaque samples were collected from caries-free surfaces (PF) and from enamel (PE) and dentin (PD) lesions. Plaque ADS activity was measured by monitoring citrulline production from arginine and compared with ribosomal 16S rRNA-derived taxonomic profiles for the same samples. RESULTS: At baseline, 37% of the children were CF, 34% CAE, and 29% CA. At 18 mo, 26% were CF, 41% CAE, 23% CA, and 10% were caries experienced (new restorations but no caries activity). Throughout the study period, ADS activity was significantly higher in the CF group than the CA group (P < 0.0001), and ADS activity in the PF samples was significantly higher than in the PE and PD samples (P < 0.0001). Distance-based redundancy analysis showed that the bacterial communities could be differentiated when plaque samples are grouped into levels of high and low ADS activity. CONCLUSIONS: There is a positive correlation between caries activity and low arginolytic capacity of the supragingival oral biofilms of children and tooth surfaces over time. Measurements of arginine metabolism via ADS may be useful to differentiate the caries risk of individuals and tooth surfaces. KNOWLEDGE TRANSFER STATEMENT: Findings from this study support the development of new strategies for caries risk assessment and prevention based on modulation of the virulence of the oral microbiome through arginine metabolism in supragingival biofilms.

Factors that influence health risk behaviours among teenagers

Objective: To identify the factors that influence health risk behaviours among teenagers in Trinidad and Tobago. Design and Methodology: A quantitative, descriptive research study was carried out among teenagers attending four public secondary schools in the St. George East Educational District in Trinidad. A self administered questionnaire which was modified by the researchers was distributed to 148 study participants to determine how the factors: Sociodemographic characteristics, peer-pressure, social environment/community, media and gender influenced their health risk behaviour. Results:The study found that 31% of the participants were influenced by peer-pressure to engage in bullying while 20% drank alcohol, 15% engaged in illicit sex while 10% of the participants were influenced to smoke and 24% revealed that peer pressure had no influence on their behaviour. The study also determined that the media influenced 41% of the participants to engage in bullying. Participants also reported that their social environment/community had an influence on their health risk behaviours; the most prevalent behaviour was drug use (36%) while bullying accounted for 34%, followed by alcohol consumption (17%), then smoking (10%), and illicit sex (3%). Conclusion: While this study only investigated the factors that influence health risk behaviour among a small sample of teenagers in one school district and a generalization cannot be made to include all teenagers there is sufficient evidence to recommend further research and for school health programmes that promote health education and awareness of health risk behaviours to reduce the likelihood of teenagers engaging in health risk behaviours.

Biology of Oral Streptococci.

Bacteria belonging to the genus Streptococcus are the first inhabitants of the oral cavity, which can be acquired right after birth and thus play an important role in the assembly of the oral microbiota. In this article, we discuss the different oral environments inhabited by streptococci and the species that occupy each niche. Special attention is given to the taxonomy of Streptococcus, because this genus is now divided into eight distinct groups, and oral species are found in six of them. Oral streptococci produce an arsenal of adhesive molecules that allow them to efficiently colonize different tissues in the mouth. Also, they have a remarkable ability to metabolize carbohydrates via fermentation, thereby generating acids as byproducts. Excessive acidification of the oral environment by aciduric species such as Streptococcus mutans is directly associated with the development of dental caries. However, less acid-tolerant species such as Streptococcus salivarius and Streptococcus gordonii produce large amounts of alkali, displaying an important role in the acid-base physiology of the oral cavity. Another important characteristic of certain oral streptococci is their ability to generate hydrogen peroxide that can inhibit the growth of S. mutans. Thus, oral streptococci can also be beneficial to the host by producing molecules that are inhibitory to pathogenic species. Lastly, commensal and pathogenic streptococci residing in the oral cavity can eventually gain access to the bloodstream and cause systemic infections such as infective endocarditis.

Morphometric convergence and molecular divergence: the taxonomic status and evolutionary history of Gymnura crebripunctata and Gymnura marmorata in the eastern Pacific Ocean.

To clarify the taxonomic status of Gymnura crebripunctata and Gymnura marmorata, the extent of morphological and nucleotide variation between these nominal species was examined using multivariate morphological and mitochondrial DNA comparisons of the same characters with congeneric species. Discriminant analysis of 21 morphometric variables from four species (G. crebripunctata, G. marmorata, Gymnura micrura and Gymnura poecilura) successfully distinguished species groupings. Classification success of eastern Pacific species improved further when specimens were grouped by species and sex. Discriminant analysis of size-corrected data generated species assignments that were consistently accurate in separating the two species (100% jackknifed assignment success). Nasal curtain length was identified as the character which contributed the most to discrimination of the two species. Sexual dimorphism was evident in several characters that have previously been relied upon to distinguish G. crebripunctata from G. marmorata. A previously unreported feature, the absence of a tail spine in G. crebripunctata, provides an improved method of field identification between these species. Phylogenetic and genetic distance analyses based on 698 base pairs of the mitochondrial cytochrome b gene indicate that G. crebripunctata and G. marmorata form highly divergent lineages, supporting their validity as distinct species. The closely related batoid Aetoplatea zonura clustered within the Gymnura clade, indicating that it may not represent a valid genus. Strong population structuring (overall Phi(ST) = 0.81, P < 0.01) was evident between G. marmorata from the Pacific coast of the Baja California peninsula and the Gulf of California, supporting the designation of distinct management units in these regions.

Interpersonal psychotherapy for late-life depression: past, present, and future.

Interpersonal psychotherapy (IPT) has demonstrated efficacy in protecting against a recurrence of major depression in elderly subjects when used alone on a monthly basis and when combined with antidepressant medication. The authors summarize their experience using IPT over the past 10 years and discuss a variety of treatment correlates. In addition, preliminary results using IPT combined with paroxetine in depressed elders reveals no difference in remission rates between cognitively intact and cognitively impaired depressed elders.

Informational masking in profile analysis: comparing ideal and human observers.

Predictions from an ideal observer model are compared with human thresholds for two profile analysis tasks. Past work has shown that ideal observer models reasonably account for human thresholds when the profile components are fixed in frequency and amplitude. Randomly varying the frequencies of the tones making up the profile leads to higher thresholds. Owing in part to large interobserver variation, the ideal observer model is not successful in accounting for the pattern of psychophysical thresholds associated with increases in frequency uncertainty. The ideal observer also fails to account for the results of a recent profile analysis experiment in which amplitude randomization was studied [Lentz JJ, Richards VM: J. Acoust. Soc. Am. 102: 535-541, 1998]. Overall, the ideal observer predicts smaller effects of uncertainty on thresholds than are observed in psychophysical experiments.

Use of psychotropics in Sydney nursing homes: associations with depression, psychosis, and behavioral disturbances.

The purpose of this study was to determine the prevalence of psychotropic use in nursing home residents, the extent to which psychotropic dosage is consistent with published guidelines, and the relationships between psychotropic class and psychiatric and behavioral disturbances. Six hundred forty-seven subjects, mean age 82.3 years, residing in 11 nursing homes in the eastern suburbs of Sydney, Australia, were assessed using the Behavioral Pathology in Alzheimer's Disease Rating Scale (BEHAVE-AD), the Abbreviated Mental Test Scale, and the Even Briefer Assessment Scale for Depression (EBAS-DEP). Details of psychotropic prescription and diagnoses of depression, dementia, and psychosis were obtained from nursing home charts. Psychotropics were prescribed for 333 (51.5%) residents, 381 (58.9%) if "as required" (PRN) use is included. Prescription of multiple psychotropics was present in 148 (22.7%) residents. Antidepressants were prescribed for 19.8% of residents, with subtherapeutic doses less likely in residents on selective serotonin reuptake inhibitors. On logistic regression, the use of antidepressants was predicted by the affective disturbances subscale on the BEHAVE-AD. Only 30.4% of residents with significant depressive symptoms on the EBAS-DEP were prescribed antidepressants. Antipsychotics were prescribed for 21.3% residents at a mean dosage of 73 mg chlorpromazine equivalence. Residents on antipsychotics had significantly higher scores on the delusions, hallucinations, activity disturbance, and aggressiveness subscales of the BEHAVE-AD. On logistic regression, only the activity disturbance subscale and chart diagnoses of dementia and psychosis were significant predictors. Psychosis (58.8%) and behavioral disturbances (91.9%) were more prevalent in residents prescribed antipsychotics than in residents not prescribed antipsychotics (42.5% and 76.6%, respectively). High rates of behavioral and psychological symptoms of dementia remained in residents prescribed antipsychotics and high rates of depressive symptoms in residents prescribed antidepressants, suggesting a role for nonpharmacological strategies.

Psychosis, depression and behavioural disturbances in Sydney nursing home residents: prevalence and predictors.

BACKGROUND: There is wide variation in the rates of behavioural and psychological symptoms of dementia (BPSD) reported in nursing homes. AIMS: This study aimed to investigate: (1) the prevalence of BPSD in nursing home residents using the BEHAVE-AD; (2) the relationships of BPSD with (a) demographic, (b) dementia, (c) diurnal and (d) nursing home variables; and (3) the inter-relationships between different types of BPSD, as measured by subscales of the BEHAVE-AD. RESULTS: Over 90% of residents exhibited at least one behavioural disturbance. Specifically, there was evidence of psychosis in 60%, depressed mood in 42% and activity disturbances or aggression in 82% of residents. Younger, more functionally impaired residents with a chart diagnosis of psychosis had higher BPSD rates, as did those residing in larger nursing homes. Individual BPSD were significantly intercorrelated. CONCLUSIONS: BPSD are ubiquitous in nursing home residents. Behavioural disturbances are frequently associated with psychosis and/or depression. The findings suggest the need for psychogeriatric services to nursing homes and smaller facilities.

Different auditory filter bandwidth estimates based on profile analysis, notched noise, and hybrid tasks.

Auditory filter bandwidths were estimated in three experiments. The first experiment was a profile-analysis experiment. The stimuli were composed of sinusoidal components ranging in frequency from 200 to 5000 Hz. The standard stimulus was the sum of equal-amplitude tones, and the signal stimulus had a power spectrum that varied up-down ... up-down. The number of components ranged from four to 60. Interval-by-interval level randomization prevented the change in level of a single component from reliably indicating the change from standard to signal. The second experiment was a notched-noise experiment in which the 1000-Hz tone to be detected was added to a noise with a notch arithmetically centered at 1000 Hz. Detection thresholds were estimated both in the presence of and in the absence of level randomization. In the third, hybrid, experiment a 1000-Hz tone was to be detected, and the masker was composed of equal-amplitude sinusoidal components ranging in frequency from 200 to 5000 Hz. For this experiment, thresholds were estimated both in the presence and absence of level variation. For both the notched-noise and hybrid experiments, only modest effects of level randomization were obtained. A variant of Durlach et al.'s channel model ["Towards a model for discrimination of broadband signals," J. Acoust. Soc. Am. 80, 63-72 (1986)] was used to estimate auditory filter bandwidths for all three experiments. When a two-parameter roex(p,r) filter weighting function was used to fit the data, bandwidth estimates were approximately two to three times as large for the two detection tasks than for the profile-analysis task.

Sensitivity to changes in level and envelope patterns across frequency.

In the first experiment, two measurements were compared--sensitivity to across-frequency changes in level and sensitivity to across-frequency changes in the modulation phase of SAM tones. For the level task, multi-tone stimuli composed of 2-80 tones ranging in frequency from 200 to 5000 Hz were used. For the phase task, the same frequency range was used, and 2-80 SAM tones were tested. For the level task, observers discriminated between a multi-tone, equal-amplitude standard and one of two signals--a one-step or an up-down signal. The one-step signal had higher levels at low frequencies and lower levels at high frequencies. The up-down signal had components with levels that varied high-low-high-low. For the phase task, the standard was the sum of SAM tones with identical modulator phases across frequency. The one-step signal had a common modulator phase at low frequencies and a different common modulator phase at high frequencies. The up-down signal had modulator phases that varied lag-lead-lag-lead. The results suggest that sensitivity to across-frequency changes in level and modulation phase reflect similar initial processing stages. In a second experiment, SAM tones were used, and psychometric functions were measured for the level task, the phase task, and a condition in which changes in level and modulator phase were both present. The standard was "flat," and an up-down signal was to be detected. For one observer, the data suggest that level and phase information are independently represented. For the other two observers, interactions between the two features of the stimuli are apparent. A multiple-looks model was moderately successful in accounting for the data.

The role of ultrasonography-guided fine-needle aspiration biopsy in the management of nonpalpable and palpable thyroid nodules.

The introduction of highly sensitive imaging techniques has made it possible to detect many nonpalpable thyroid nodules (non-PTN). We investigated the value of ultrasound-guided fine-needle aspiration biopsy (US-guided FNAB) as a diagnostic tool in the management of non-PTN as well as palpable thyroid nodules (PTN) that were considered difficult to aspirate without guidance. US-guided FNAB was performed on a total of 119 nodules (71 palpable and 48 nonpalpable) from 119 patients between 1992 and 1996. All available clinical and follow-up data were reviewed. Surgical follow-up was available in 24 cases. The patients included 100 females and 19 males ranging in age from 9 to 81 years (average, 51 years). FNA diagnoses (PTN versus non-PTN) included papillary carcinoma (12.7% [9/71] versus 4.2% [2/48], follicular neoplasm (16.9% [12/71] versus 0%), medullary carcinoma (1.4% [1/71] versus 0%), atypical cytology (5.6% [4/71] versus 2.1% [1/48], non-neoplastic thyroid (63.4% [45/71] versus 85.4% [41/48]) and unsatisfactory (0% versus 8.3% [4/48]). In 2 cases of occult papillary carcinoma, risk factors included radiation exposure (1 case) and a newly developed nodule during follow-up for hypothyroidism (1 case). Subsequent surgical follow-up (24 cases) confirmed the FNA findings, except for a case of Hürthle cell adenoma and 1 of Hashimoto's thyroiditis diagnosed as papillary carcinoma and follicular neoplasm, respectively. US-guided FNAB in most non-PTN are diagnosed as benign. For most patients with non-PTN and without any high-risk factors, a conservative approach such as clinical follow-up may be a more cost effective and logical approach. In contrast, US-guided FNAB is more useful in diagnosing biologically significant lesions in PTN that may be difficult to aspirate without guidance.

The effects of different envelope patterns and uncertainty for the detection of a tone added to SAM complex tonal maskers.

Thresholds for the detection of a tone added in-phase to the carrier of a fully modulated SAM tone were measured. In some conditions the signal was added to a single SAM tone, and in other conditions the signal was added to the sum of three or more SAM tones. Level equalization ensured that the addition of the tonal signal did not lead to increases in energy. When multiple SAM tone maskers were used, a small number of reproducible maskers were tested, each masker being composed of SAM tones with a variety of relative modulator phases. The maskers were either fixed across intervals and trials, roved across trials but fixed across intervals, or randomly chosen across both intervals and trials. The frequency separation between the signal-centered and off-frequency SAM tones was also varied. For small signal-centered/off-frequency SAM tone frequency separations, a separation ratio of 1.3, thresholds in the fixed condition depend on the relative modulator phases, and a simple mixture model reasonably predicted thresholds in the roving condition based on thresholds in the fixed condition for two of the three observers. For signal-centered/off-frequency SAM tone frequency separation factor of 1.68, effects of relative modulator phases were not obtained. Thresholds in the target-alone condition were generally superior to thresholds measured with the comodulated masker. Comodulated thresholds were better than target-alone thresholds only when level equalization was not used, and so the addition of the signal led to increases in level.

The effects of amplitude perturbation and increasing numbers of components in profile analysis.

In a profile-analysis task, the effect of randomly perturbing the amplitudes of the components of multi-tone stimuli was studied in two experiments. In the first experiment, thresholds for a signal added in-phase to the central component of a standard were measured for different numbers of components in two conditions. In one condition thresholds were measured in blocks for six different "jagged" standards, and in another, thresholds were measured when one of the six standards was chosen randomly on a presentation-by-presentation basis. Regardless of condition, thresholds did not depend on the numbers of components and increased magnitude of perturbation increased thresholds. Moreover, the slope relating thresholds to number of components did not increase with increasing magnitude of perturbation. In the second experiment, the signal consisted of an increase in amplitude of the central components and a decrease in amplitude of the outer components of the standard (a stimulus type which has been shown to maximize the change in threshold with increasing number of components). The amplitudes of component tones were selected randomly on a presentation-by-presentation basis. Thresholds fell with increases in the number of components, but the slope relating thresholds to numbers of components did not change as the magnitude of perturbation increased. The latter result contrasts with that reported by Kidd et al. [J. Acoust. Soc. Am. 90, 1340-1354 (1991)].

The representation of concurrent vowels in the cat anesthetized ventral cochlear nucleus: evidence for a periodicity-tagged spectral representation.

Chopper units of the ventral cochlear nucleus (VCN) provide a rare representation of stimulus spectrum and a temporal representation of fundamental frequency (F0). This dual representation may be useful in segregating competing speech sounds, where differences in F0 are a cue. Responses to the vowel portion of concurrently presented pairs of syllables /bV integral/ with different F0's (88, 98, and 112 Hz) were studied in the VCN of anesthetized cats; 11 English vowels were used for V. Vowels were chosen so that one had a formant frequency just above the unit's best frequency (BF) and the other had a formant just below BF. By changing the stimulus sampling rate, formant peaks were shifted relative to the unit's BF, producing a range of stimuli, varying in the relative power of the two vowels within the unit's tuning curve. Results show that units' discharge rates reflect the energy within their tuning curves and the relative synchronization of units' responses to the two F0's favors the dominant vowel. A method of segregating two vowels is provided in which relative synchronization to the F0's is used to apportion discharge rate between the vowels. Best results were obtained in chopper units, although primarylike units showed similar behavior.

Effects of modulator phase for comodulation masking release and modulation detection interference.

In an effort to evaluate the importance of across-frequency comparisons of envelope patterns in comodulation masking release (CMR) experiments and to compare joint effects of target-masker frequency separation for both CMR and modulation detection interference (MDI) tasks, thresholds were measured for three tasks. These tasks were: (a) the detection of sinusoidal amplitude modulation (SAM) of a tone, (b) the detection of a reduction in the modulation depth of a fully modulated SAM tone, and (c) the detection of a tone added to a narrow band of noise. Thresholds were obtained for the target alone and for the target presented with two maskers. For the detection of SAM, thresholds did not depend on whether the modulation patterns of the target and masker elements were the same or random. For the latter two tasks, modulator phase effects were apparent for target-masker frequency separations less than 1-2 oct. In contrast, past work has shown that observers can compare modulator envelope phases across frequency separations larger than 1-2 oct [Strickland et al., J. Acoust. Soc. Am. 86, 2160-2166 (1989); Yost and Sheft, J. Acoust. Soc. Am. 85, 848-857 (1989)]. In a second experiment, thresholds for the detection of SAM were obtained after prolonged exposure to a fully modulated SAM tone. For four of the five observers, modulation-rate specific adaptation was obtained for test/adapting carrier-frequency separations approaching 2 oct below and 1 oct above the adaptor.

Sensitivity to changes in overall level and spectral shape: an evaluation of a channel model.

Two experiments involving level and spectral shape discrimination which test an optimal channel model developed by Durlach et al. [J. Acoust. Soc Am. 80, 63-72 (1986)] are described. The model specifies how the auditory system compares and/or combines intensity information in different frequency channels. In the first experiment, psychometric functions were obtained for the discrimination of changes in level and discrimination of changes in spectral shape for an eight-tone complex sound. A variety of different base spectral shapes were tested. In some conditions, level randomization was introduced to reduce the reliability of across-interval changes in level. Increasing the amount of level variation degraded performance for the level discrimination task but had no effect on the shape discrimination task. In all conditions, sensitivity to changes in spectral shape was superior to sensitivity to changes in level. Consequently, two models of central noise are evaluated in an attempt to explain these results; one in which central noise acts prior to the formation of the likelihood ratio and one in which central noise degrades the likelihood ratio. The former model is more successful in accounting for the data. In a second experiment, the detectability of a level increment to one component of a multitone complex was measured. The frequency content of the complex was varied by systematically removing six components from a 23-component complex. Thresholds were measured for increments at three different signal frequencies. A common trend in the data was that when there was a spectral gap directly above the signal frequency, thresholds were lowest. This result differs from the predictions of a simple channel model, and contrasts with results presented by Green and Berg [Q. J. Exp. Psychol. 43A, 449-458 (1991)].

The effects on comodulation masking release of systemic variations in on- and off-frequency masker modulation patterns.

Detection thresholds were obtained for a 500-Hz tone added to a masker comprised of an amplitude-modulated tone centered at the signal frequency (on-frequency masker) and an array of amplitude modulated tones centered at 300, 700, 800, 900, 1000, and 1100 Hz (off-frequency maskers). The shapes of the amplitude modulation patterns of the on- and off-frequency maskers were either matched or mismatched. In the shape-matched conditions the on- and off-frequency masker modulation patterns were the same, either sinusoidally or square-wave amplitude modulated. In the shape-mismatched conditions, the on-frequency masker was sinusoidally amplitude modulated and the off-frequency maskers were square-wave amplitude modulated. The rate of modulation was either 10 or 20 Hz, and the duty cycle of square-wave modulation was systemically varied. The relative phases of the on- and off-frequency modulators were either in-phase, out-of-phase, or random-phase. Comodulation masking release (CMR) was defined as the difference between thresholds in the in-phase and random-phase conditions. CMRs as large as 12 dB were obtained for the shape-matched as well as the shape-mismatched conditions. Thresholds in the out-of-phase condition were on average 2.6 dB higher than those in the random-phase condition. Results are consistent with a cued listening model where off-frequency modulation minima trigger sampling at the output of the auditory filter centered on the signal frequency.