Biomedical subjects

S K Griffiths

Publications and source records attributed to S K Griffiths.

15 recordsLinked to original sources

PubMed2001

Low-dispersion turns and junctions for microchannel systems.

Numerical methods are employed to optimize the geometry of two-dimensional microchannel turns such that the turn-induced spreading of a solute band is minimized. An inverted numerical method is first developed to compute the electric potential and local species motion in turns of arbitrary geometry. The turn geometry is then optimized by means of a nonlinear least-squares minimization algorithm using the spatial variance of the species distribution leaving the turn as the object function. This approach yields the turn geometry producing the minimum possible dispersion, subject only to prescribed constraints. The resulting low-dispersion turns provide an induced variance 2-3 orders of magnitude below that of a comparable conventional turns. Sample results are presented for 180 and 90 degrees turns, and the use of these turns to form wyes and tees is discussed. A sample 45 degrees wye is presented. The use of low-dispersion turns in folding separation columns is also discussed, and sample calculations are presented for folding a column 100 microm in width and up to 900 mm in length onto a region of only 10 by 10 mm. These low-dispersion geometries are applicable to electroosmosis, electrophoresis, and some pressure-driven flows.

Journal Article↗

PubMed2000

Intra-abdominal sound pressure levels during impulse noise exposure in sheep.

Ambient sound pressure levels (SPLs) created with intense blasts were compared with SPLs recorded in the abdomen of euthanized sheep. Hydrophones were placed in the abdominal cavity at locations referred to as proximal, medial, and distal with respect to a shock tube that created 169-dB peak SPL (pSPL). No differences in pSPL, duration, or rise time were found between recordings in air and at the intra-abdominal proximal position. Significant differences were noted in these variables when recordings in air were compared with recordings made at the medial and distal locations. Intra-abdominal pSPL varied by 20 dB depending on recording location.

Abdomen↗

PubMed1998

Psychometric equivalence of recorded spondaic words as test items.

In the determination of the speech-reception threshold (SRT), spondaic words are assumed to be homogeneous with respect to intelligibility; and the assumption of equal intelligibility requires that the words be comparable for all signal levels. Previous attempts to assess the equal intelligibility assumption using word thresholds as the sole criterion are not an adequate basis for specifying the equality of intelligibility. In the present study, the recorded spondaic words (Tillman recording) were analyzed in an attempt to create a more homogeneous set of spondaic words for future laboratory work. To achieve this goal, the data reported by Young, Dudley, and Gunter (1982) and data collected in our laboratory were fitted to a logistic function (psychometric function) from which a 50% point (threshold) and slope were obtained. To specify their acoustical parameters, the recorded spondaic words were digitized and the RMS level and duration of each syllable and word were calculated. None of the RMS or duration measures were correlated with word thresholds, so no attempt was made to equate level or duration. On the other hand, when the threshold of each word was adjusted to equal the mean threshold of the set (n = 36), the dispersion among word thresholds and slopes was greatly reduced. Further, we recommend that small sets of "equally intelligible" spondaic words not be used for clinical testing because set size is a strong factor in determining threshold for spondees (Meyer & Bilger, 1997; Punch & Howard, 1985).

Humans↗

PubMed1998

Reliability issues with the Synthetic Sentence Identification test.

This investigation examined the reliability of the Synthetic Sentence Identification (SSI) test. An adaptive procedure was used to assess speech recognition thresholds (SRTs) for the SSI lists over two separate test sessions. The commercially available Davy Crockett passage and a multitalker babble were used as competition. Subjects consisted of 20 adults with normal hearing sensitivity. Results indicated (1) significantly higher SRTs with the multitalker babble, (2) significant differences in SRTs among SSI lists when using the Davy Crockett competition, (3) no significant learning/practice or fatigue effects across trials in either noise condition, and (4) high test-retest reliability across test sessions. Clinical implications of these data are considered.

Adolescent↗

PubMed1996

Auditory brainstem response in sheep. Part II: Postnatal development.

The auditory brainstem response (ABR) was recorded from 15 chronically instrumented lambs between birth and 7 weeks of age as well as from four adult ewes. Latency changes observed within two days of birth may be related to the dissipation of fluid from the lamb external auditory canal and middle ear cavity. Developmental changes observed in the lambs included an early period of wave latency decrements from 1 to 4 weeks followed by a subsequent increase in wave latencies from 4 to 7 weeks. These changes were significant for the I-IV interwave interval, possibly reflecting the combined effects of increased myelinization and increased neural track length. Responses from 7 week-old lambs were not significantly different from those obtained from the ewes.

Aging↗

PubMed1996

Screening for hearing loss in juvenile detention centers.

Results of hearing screenings for 226 incarcerated adolescents are presented. Screenings consisted of visual otoscopy check, pure-tone screening, and tympanometry. Failures were defined as excessive cerumen impeding a clear view of the tympanum, failure to respond at 25 dB HL for 1000 through 6000 Hz, or a Jerger Type B or C tympanogram. An overall failure rate of 35.5 percent was found, with 9.8 percent failing otoscopy, 7.5 percent failing tympanometry, and 25.3 percent failing pure-tone screening. Implications for medical and audiologic evaluation in the population are discussed.

Adolescent↗

PubMed1995

Auditory brainstem response in sheep. Part I: Fetal development.

The Auditory Brainstem Response (ABR) was recorded from fetal sheep in utero between gestational ages of 111 days and 136 days (Normal gestation is 145 days). The ABR was detected at 111 days gestation (dGA) and morphology of the waveform improved rapidly from 116 to 123 dGA. At 123 dGA, the four vertex-positive peaks were similar to those observed in adult ewes. Thresholds of the ABR to clicks and tone bursts improved rapidly between 111 and 123 dGA. From 123 to 136 dGA, ABR thresholds continued to improve, but at a much slower rate. Latencies for the peaks also decreased during development from 111 to 136 days, with latencies for Waves III and IV showing a greater decrease than latencies for Waves I and II.

Animals↗

PubMed1994

Noise induced hearing loss in fetal sheep.

The auditory brainstem response (ABR) was recorded in utero from chronically instrumented fetal sheep prior to and following exposure of pregnant ewes to intense broadband noise (120 dB SPL for 16 h). ABRs were elicited by clicks and tone bursts (0.5, 1, 2, and 4 kHz) delivered through a bone oscillator secured to the fetal skull. Latency-intensity functions for most of the four vertex-positive waves (labelled I-IV) were prolonged and ABR thresholds were temporarily elevated by an average of 8 dB following the noise exposure. Results show that exogenous sounds can penetrate the uterus and result in alterations of the fetal ABR.

Acoustic Stimulation↗

PubMed1994

The perception of speech sounds recorded within the uterus of a pregnant sheep.

The intelligibility of speech stimuli recorded within the uterus of a pregnant sheep was determined perceptually using a group of untrained judges. The intrauterine sound environment of the ewe was intended to simulate that of a pregnant woman. Two separate lists, one of meaningful and one of nonmeaningful speech stimuli, were delivered through a loudspeaker to the side of the ewe and were simultaneously recorded with an air microphone located 15 cm from the flank and with a hydrophone previously sutured to the neck of the fetus. Perceptual test tapes generated from these recordings were played to 102 judges. The intelligibility of the phonemes recorded in the air was significantly greater than the intelligibility of phonemes recorded from the uterus. A male talker's voice was more intelligible than a female talker's voice when recorded from within the uterus, but not so when recorded in the air. An analysis of the feature information transmission from recordings inside and outside the uterus revealed that voicing information is better transmitted in utero than place or manner information.

Animals↗

PubMed1994

Effect of impulse noise on the auditory brainstem response of the fetal sheep and the adult ewe: case study.

Auditory brainstem responses (ABR) were obtained from a sheep fetus in utero and from a nonpregnant ewe before and after a noise exposure generated by 10 105-mm howitzer impulses (160-166 dB peak pressure). Fetal ABR thresholds shifted by less than 15 dB, whereas the adult thresholds shifted more than 50 dB. Comparison of exposed and nonexposed fetal cytocochleograms revealed minimal sensory cell loss in two age-matched animals. Although fetal ABR threshold shifts were noted in this study, the magnitude of shift was modest when compared to the significant elevation of the adult ABR thresholds.

Acoustic Stimulation↗

PubMed1991

Effects of age on the adult auditory middle latency response.

The middle latency components of the auditory evoked response were obtained from a group of normal-hearing, healthy female subjects from 22 to 68 years of age. Recordings were made at several intensity levels to assess the level-dependence of any age-related effects. Cross-sectional analyses revealed that the amplitude of component Pa grows linearly with age, becoming significantly larger in older (50-68 years of age) compared to younger (22-37 years) subjects. The amplitude-intensity function is steeper in the older subjects by a factor of two. Correlational analyses suggested that at higher intensity levels age accounts for about 20% of the variance in the amplitude of Pa. A positive shift in response baseline was observed in the older subjects, and could contribute to the age-related increase in the absolute amplitude of Pa. However, a similar increase in the peak-to-peak and area measures of Pa suggests that some of the increase in the magnitude of Pa is independent of baseline shift. A confounding of age and hearing sensitivity in this study makes it difficult to interpret the age-related effects as strictly central in nature.

Adult↗

PubMed1991

The amplitude modulation-following response as an audiometric tool.

The amplitude modulation-following response (AMFR) is an auditory scalp-recorded potential, evoked using continuous, amplitude-modulated tones. The current study was designed to explore the audiometric utility of the AMFR by demonstrating the frequency specificity of the response and assessing the relation between behaviorally measured pure-tone thresholds and AMFR detection thresholds. Subjects in this study were six normal-hearing and four hearing-impaired adults. High-pass masking results in the normal hearing subjects demonstrated that the AMFR is associated with a narrow range of activation along the cochlea around the carrier frequency. Frequency-specific results from the hearing-impaired subjects confirmed this finding. Thresholds for the AMFR, defined in spectral terms, were consistent with the behavioral estimates in both the normal and the hearing-impaired subjects.

Acoustic Stimulation↗

PubMed1989

Correlations between various measures of head size and auditory brainstem response latencies.

The literature is mixed concerning the degree to which between-subject variance in the latencies of the auditory brainstem response (ABR) relates to differences in the length of the auditory pathway. Most investigations have used one of several measures of head size to indirectly index brain size and neural pathway length. While some studies have found a positive correlation between head size and the latencies of the ABR, others have reported little or no relationship. We hypothesized that the differences between studies result from differences in the head dimensions measured, the precision of measurement technique, and issues of sampling. We therefore decided to use the International 10-20 system of electrode placement to provide reproducible skull benchmarks on which to base head size measures, to obtain measures of head size via two procedures, and to control external variables that might influence the ABR. The results show that head size has a moderate positive influence on the latencies of the ABR given precise head size measures.

Adult↗

PubMed1989

Statistical dependence among the wave latencies of the auditory brain stem response.

If the interpeak intervals in the auditory brain stem response (ABR) are assumed to be statistically independent variables that represent the neural transmission time between loci, then the absolute latency of an ABR wave is the sum of the transmission times between successive loci. Consequently, the correlations between the absolute latencies of the ABR waves would be part-whole correlations whose expected values are appreciably different from r = 0. We derived the expected values of the correlations between the latencies of the ABR waves on the assumption that their latencies are the sum of independent elements (transmission times) and found them to be 0.707, 0.577, and 0.816 for I.III, I.V, and III.V, respectively. In addition, the expected values of the correlations between the latencies of the ABR waves assuming that the absolute wave latencies themselves are independent variables (e.g., rI.III = 0) were derived. Several of the correlations among interpeak and between the interpeak and absolute latencies were demonstrated to be appreciably different from r = 0. It would appear that it is more reasonable to assume that the elements (transmission times) rather than the component latencies are the variables of choice for statistical analysis of ABR data.

Brain Stem↗

PubMed1970

Physiological comparison of L-serine dehydratase and tryptophanase from Bacillus alvei.

Tryptophanase from Bacillus alvei also possesses serine dehydratase activity. A comparison of this enzyme with l-serine dehydratase [l-serine hydro-lyase (deaminating), EC 4.2.1.13] in toluene-treated whole cell preparations of the organism was undertaken. Tryptophanase is a constitutive enzyme in B. alvei. The dehydratase undergoes a repression-derepression-repression sequence as the l-serine level in the growth medium is increased from 0 to 0.1 m. Tryptophanase activity is decreased in organisms grown in medium containing glucose. Both enzymes are repressed in organisms grown in glycerol-containing medium. l-Serine dehydratase has a pH optimum of 7.5 in potassium phosphate buffer; tryptophanase functions optimally in this buffer at pH 8.2. Both enzymes lose activity in the presence of tris(hydroxymethyl)aminomethane buffer. Either K(+) or NH(4) (+) is required for full tryptophanase activity, but Na(+) is markedly inhibitory. These three cations are stimulatory to l-serine dehydratase activity. Both enzymes are subject to apparent substrate inhibition at high concentrations of their respective amino acids, but the inhibition of tryptophanase activity can be completely overcome by the removal of indole as it is formed. The dehydratase does not catalyze cleavage of d-serine, l-threonine, or alpha-substituted serine analogues at the concentrations tested. However, activity of the enzyme in cleaving l-serine is competitively inhibited by d-serine, indicating that the d-isomer can occupy an active site on the enzyme. The enzyme catalyzes cleavage of some beta-substituted serine analogues.

Ammonia↗