Age-related differences in processing dynamic information to identify vowel quality.

This study examined age-related differences in the use of dynamic acoustic information (in the form of formant transitions) to identify vowel quality in CVCs. Two versions of 61 naturally produced, commonly occurring, monosyllabic English words were created: a control version (the unmodified whole word) and a silent-center version (in which approximately 62% of the medial vowel was replaced by silence). A group of normal-hearing young adults (19-25 years old) and older adults (61-75 years old) identified these tokens. The older subjects were found to be significantly worse than the younger subjects at identifying the medial vowel and the initial and final consonants in the silent-center condition. These results support the hypothesis of an age-related decrement in the ability to process dynamic perceptual cues in the perception of vowel quality.

Effect of speech distinctions and age differences on auditory event-related potentials.

P3 event-related potentials were recorded from 37 subjects in two age groups (19-25 and 61-75 years) listening to tones, stop + vowel (CV) monosyllables, and isolated vowels. P3 latencies were found to be significantly longer for older subjects in all stimulus conditions. CV latencies were also significantly longer than simple tone latencies with the increase being approximately 1.30 msec/year. Latencies to the vowel stimuli were somewhat more variable but also tended to be greater than the tone latencies. P3 amplitudes were greater for the tone stimuli than for the speech stimuli, but no significant age effects were found.

Determining latency and amplitude for multiple peaked P300 waveforms.

The P300 is characterized by a large positive peak occurring approximately 300 msec post stimulus onset. When a single peak is present, the P300 latency is easily identifiable. However, in cases where the P300 waveform is broadly or multiply peaked, ambiguity exists when attempting to determine the latency of the waveform. Amplitude, while less ambiguous, has also been variously measured as baseline to peak, or peak-to-peak, using either N200 or N300 as the trough. The present study was undertaken to evaluate a method for precisely and consistently determining the latency and amplitude characteristics of the P300 waveform. Three independent raters examined 40 records to determine P300 latency and amplitude according to prescribed criteria. Results indicated that the suggested techniques yield good inter-rater agreement on the latency and amplitude characteristics of the P300 waveform regardless of P300 morphology.

Early recognition in the Rhizobium meliloti-alfalfa symbiosis: root exudate factor stimulates root adsorption of homologous rhizobia.

Adsorption of Rhizobium meliloti to alfalfa roots before their infection and nodule formation shows the specificity of the symbiotic association (G. Caetano-Anollés and G. Favelukes, Appl. Environ. Microbiol. 52:377-382, 1986). The time course of specific adsorption of R. meliloti (10(3) to 10(4) cells per ml) to roots shows an initial lag period of 3 h, suggesting that either or both symbionts must become conditioned for the adsorption process. Preincubation of R. meliloti L5-30 for 3 h with dialyzed alfalfa root exudate (RE) markedly increased early adsorption of rhizobia to alfalfa roots. The activity in RE was linked to a nondialyzable, thermolabile, trypsin-sensitive factor(s), very different from the root-exuded flavonoid compounds also involved in early Rhizobium-legume interactions. The lack of activity in the RE from plants grown in 5 mM NO3- suggested its negative regulation by the nitrogen nutritional status of the plant. Preincubation of R. meliloti with heterologous clover RE did not stimulate adsorption of rhizobial cells to roots. A short pretreatment of RE with homologous (but not heterologous) strains eliminated the stimulatory activity from solution. The stimulation of adsorption of R. meliloti to alfalfa roots was strongly dependent on the growth phase of the rhizobia, being greater at the late exponential stage. Nevertheless, the capacity of R. meliloti L5-30 to eliminate from solution the stimulatory activity in RE appeared to be constitutive in the rhizobia. The low concentration of rhizobial cells used in these experiments was critical to detect the stimulation of adsorption. The early interaction of spontaneously released alfalfa root macromolecular factor(s) and free-living R. meliloti, which shows the specificity and regulatory properties characteristic of infection and nodulation, would be an initial recognition event in the rhizosphere which triggers the process of symbiotic association.

Preliminary studies on the use of an ABR amplitude projection procedure for hearing aid selection.

Hearing aid selection in young nonverbal children is difficult and objective selection procedures are needed. Kiessling (Scand Audiol 1982;11:269-275; Arch Otorhinolaryngol 1983;238:233-240) has proposed an objective hearing aid selection method based on an unaided ABR amplitude projection procedure. However, Kiessling's ABR projection method is based on the assumption that ABR amplitude is directly related to the loudness of a signal--an assumption which has not been tested. This assumption was investigated in a group of ten normally hearing and three hearing-impaired listeners. The results indicated that ABR amplitude measures obtained in a single trial do not always correlate well with perceived loudness, but ABR amplitudes averaged over several trials do correlate well with estimates of perceived loudness. The hearing-impaired listeners then participated in a second phase of the investigation in which hearing aids chosen by the ABR projection procedure were compared with hearing aids chosen by more conventional methods. The results indicated that the projection procedure prescribed appropriate gain and compression characteristics for two of the three hearing-impaired listeners.

Presentation level determination for CV tokens using a computer-controlled adaptive procedure.

The feasibility of using a computer-controlled adaptive procedure to find maximum monotic performance for consonant-vowel (CV) stimuli was evaluated and compared to the procedure typically used to determine a performance-intensity function in two experiments. In experiment I, the computer-controlled adaptive procedure and the typical manual procedure were evaluated on 26 normally hearing listeners. Results indicated that the shape of the performance-intensity functions was similar for both procedures with the computer-controlled adaptive procedure selecting a higher presentation level. The test-retest reliability for the computer-controlled adaptive procedure was good with a mean difference on retest of 1.2 dB. In experiment II, the same procedures were evaluated using 9 sensorineural hearing impaired subjects. Again similar configuration between functions was observed. The primary advantage of the computer-controlled adaptive procedure for both subject groups is efficiency in terms of time of administration and number of CV stimuli used.

Role of Motility and Chemotaxis in Efficiency of Nodulation by Rhizobium meliloti.

Spontaneous mutants of Rhizobium meliloti L5-30 defective in motility or chemotaxis were isolated and compared against the parent with respect to symbiotic competence. Each of the mutants was able to generate normal nodules on the host plant alfalfa (Medicago sativa), but had slightly delayed nodule formation, diminished nodulation in the initially susceptible region of the host root, and relatively low representation in nodules following co-inoculation with equal numbers of the parent. When inoculated in growth pouches with increasing dosages of the parental strain, the number of nodules formed in the initially susceptible region of the root increased sigmoidally, with an optimum concentration of about 10(5) to 10(6) bacteria/plant. The dose-response behavior of the nonmotile and nonchemotactic mutants was similar, but they required 10- to 30-fold higher concentrations of bacteria to generate the same number of nodules. The distribution frequencies of nodules at different positions along the primary root were very similar for the mutants and parent, indicating that reduced nodulation by the mutants in dose-response experiments probably reflects reduced efficiency of nodule initiation rather than developmentally delayed nodule initiation. The number of bacteria that firmly adsorbed to the host root surface during several hours of incubation was 5- to 20-fold greater for the parent than the mutants. The mutants were also somewhat less effective than their parent as competitors in root adsorption assays. It appears that motility and chemotaxis are quantitatively important traits that facilitate the initial contact and adsorption of symbiotic rhizobia to the host root surface, increase the efficiency of nodule initiation, and increase the rate of infection development.

Reliability and performance of the acoustic reflectometer.

Test-retest reliability and performance of an acoustic reflectometer were evaluated for 78 patients in two clinical settings. For a majority of the cases, the repeat measurements did not vary by more than one or two units from the first measurement. The accuracy of the instrument, when compared with standard clinical instruments, indicated that the acoustic reflectometer performed adequately with cases of middle ear effusion and could be used most effectively as a supplement to pneumatic otoscopy.

The acoustic reflectometer as a screening device: a comparison.

Sixty children (120 ears) were evaluated using reflectometry, pure tones, immittance, otologic examination, and pure-tone air and bone conduction thresholds. Comparisons were made between the various screeners. Reflectometry was found to be the least sensitive and the least specific of the procedures. Results were then examined within each diagnostic category to determine test agreement with the otologists' judgments. The reflectometer categorization was found to be inconsistent with the otologic findings. Finally, the reflectometer was compared to each of the other screener results and pure-tone thresholds. The test results were significantly correlated but showed high over- and under-referral rates for the reflectometer. Seventy-eight children and adults (156 ears) were screened in a second study. Again, the acoustic reflectometer did not perform adequately as a general screening tool. When only cases of middle ear effusion were screened, the acoustic reflectometer did not perform as well as immitance.

Post-noise-exposure auditory sensitivity and temporal integration recovery functions.

Normal-hearing young adults (N:12) were exposed to a band-limited noise (1.4-2 kc/s) at 100 db SPL for 15 min. Pre- and post-exposure threshold measurements were made at 2 and 4 kc/s at durations of 500 and of 20 msec, at time intervals of TTS2, TTS15 and every 30-min interval thereafter for 4 hrs. Greater TTS occurred at both frequencies with the 500-msec than with the 20-msec duration. Threshold recovery patterns over time were similar for both durations but thresholds for the 20-msec duration recovered sooner. There was a delayed recovery function at 4 kc/s for the relative differences (20 vs 500 msec). Implications of the findings were discussed.

Four spondee threshold procedures: a comparison.

Two ascending and two descending procedures were used to obtain spondee thresholds from normally hearing and sensorineural hearing impaired subjects. Minimal differences were found when comparisons of threshold were made among all four procedures. Results indicated slightly more sensitive thresholds when the descending procedures were used and slightly poorer pure-tone average and spondee threshold agreement. Test-retest reliability was good for all four procedures, but large differences existed among the procedures in the amount of time and in the number of words required to establish threshold. Advantages and clinical implications are discussed.

Test-retest reliability of a forced-choice procedure for determining most comfortable loudness level for speech.

A new forced-choice procedure for measuring comfortable loudness for speech was evaluated on a group of 30 normal-hearing subjects. The test-retest reliability was determined for the forced-choice ascending and descending approach and found to be high (r = 0.83 and 0.84). Clinical reliability indicated that 80 to 83% of the subjects varied no more than 5 dB upon retest. In a second experimental session, comparisons between the forced-choice procedure and the method of limits (ascending and descending approaches) were evaluated on a group of 14 sensorineural subjects. Results indicated the forced-choice procedure eliminated the measurement differences found with the method of limits (ascending and descending approaches) and the forced-choice measurements remained stable across varying degrees and configurations of hearing loss. The procedure is fast, reliable, and has several advantages over the method of limits procedure.

Temporal integration and presbycusis.

On 8 presbycusic Ss with similar sensorineural hearing losses, temporal integration (TI) was estimated by comparing HTLs for signal durations of 500 and 20 msec, at frequencies .5-6 kc/s. Normal-hearing young adult controls were also tested. Degree of group hearing loss for the presbycusic Ss was related to group TI data, but did not account for the amount of decrease in TI with these older Ss. Slightly reduced TI (6 db) was found in Ss' area of normal hearing even though the test frequency and adjacent frequencies were within a normal-hearing range. Reduced TI occurred most consistently in Ss with HTLs greater than or equal to 40 db. Individual variance suggested caution in using TI as a diagnostic clinical procedure.

The value of noise-excluder shells in reducing overreferrals from a school audiometric screening program.

Audiometric screening conducted in noisy environments can result in overreferrals to specialty clinics. A follow-up study, conducted to investigate referral rates as related to the type of headset used in screening programs, showed that noise-excluding shells reduced overreferrals without increasing underreferrals.