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Biomedical subjects

A J Moffat

Publications and source records attributed to A J Moffat.

10 recordsLinked to original sources
PubMed2006

Micromorphological investigations into root penetration in a landfill mineral cap, Hertfordshire, UK.

A micromorphological study of the soil fabric of a landfill mineral cap in Hertfordshire, UK is described. The study was undertaken to examine the nature of the cap and roots within it, and whether micromorphology could help in the explanation for root penetration. The results from thin section description supported by macro and micro image analysis clearly demonstrated that rooting was associated with zones of weakness in the cap, due to the heterogeneous particle/pore size distribution, low bulk density and presence of organic matter. The study of the material fabric also suggested evidence of pedological activity within the cap, further indicating that the mineral cap had not been engineered sufficiently to prevent the risk of root penetration. A greater uniformity of particle size within the cap material with a reduced clay and stone fraction were identified as potential solutions to avoid the extent of root penetration. The study demonstrated the value of observing and quantifying the undisturbed fabric of the micro structure in the examination of mineral cap fabric, and diagenetic and pedological processes acting on it.

Acer↗
PubMed2005

Sampling the soil in long-term forest plots: the implications of spatial variation.

Long-term monitoring of forest soils as part of a pan-European network to detect environmental change depends on an accurate determination of the mean of the soil properties at each monitoring event. Forest soil is known to be very variable spatially, however. A study was undertaken to explore and quantify this variability at three forest monitoring plots in Britain. Detailed soil sampling was carried out, and the data from the chemical analyses were analysed by classical statistics and geostatistics. An analysis of variance showed that there were no consistent effects from the sample sites in relation to the position of the trees. The variogram analysis showed that there was spatial dependence at each site for several variables and some varied in an apparently periodic way. An optimal sampling analysis based on the multivariate variogram for each site suggested that a bulked sample from 36 cores would reduce error to an acceptable level. Future sampling should be designed so that it neither targets nor avoids trees and disturbed ground. This can be achieved best by using a stratified random sampling design.

Carbon↗
PubMed2001

Effects of rooting and tree growth of selected woodland species on cap integrity in a mineral capped landfill site.

The above and below ground growth of three tree species (Alnus glutinosa, Pinus nigra var. maritima and Acer pseudoplatanus) was studied on a containment landfill site at Waterford, Hertfordshire, UK. Tree root architecture was studied using soil inspection pits excavated next to 12 trees of each species and mapped in detail. Tree height was related to soil thickness over the compacted mineral cap. No roots entered the cap where soil thickness was 1.3 m, but a few roots, especially of alder, were observed within it when the soil cover was 1.0 m or less. Micromorphological analysis of undisturbed samples of the mineral cap suggested that roots exploited weaknesses in the cap rather than actively causing penetration into it. Alder roots were more tolerant of anaerobic conditions within the cap than the other species examined. The results confirm that mineral caps should be covered by 1.5 m of soil or soil-forming material if tree establishment is intended over a restored landfill site, unless protected by other parts of a composite capping system.

Environmental Pollution↗
PubMed1992

Sonar gain control and echo detection thresholds in the echolocating bat, Eptesicus fuscus.

The echolocating bat, Eptesicus fuscus, detects sonar echoes with a sensitivity that changes according to the time elapsed between broadcasting of each sonar signal and reception of echoes. When tested in an electronic target simulator on a two-choice echo-detection task, the bat's threshold improved by 11.5 dB as echo delay changed from 2.3 to 4.6 ms (target ranges of 40 and 80 cm). Earlier experiments measured the change in detection threshold for delays from 1 to 6.4 ms (target ranges from about 17 to 110 cm) and obtained about 11 dB of improvement per doubling of delay. The new experiments used electronic delay lines to simulate echo delay, thus avoiding movement of loudspeakers to different distances and the possible creation of delay-dependent backward masking between stimulus echoes and cluttering echoes from the loudspeaker surfaces. The slope of the threshold shift defines an echo gain control that keeps echoes from point targets at a fixed sensation level--reducing sensitivity by 11 to 12 dB as echo amplitude increases by 12 dB per halving of range during the bat's approach to the target. A recent experiment using loudness discrimination of echoes at 70 to 80 dB SPL (roughly 50 dB above threshold) found a slope of about 6 dB per halving of range, so the gain-control effect may be level dependent. The observed effect is operationally equivalent to forward masking of echoes by the transmission, but any events correlated with vocalization which impair hearing sensitivity for a short interval following transmissions could cause a decline in sensitivity to echoes. Contractions of the bat's middle-ear muscles synchronized to transmissions may account for the observed threshold shift, at least for a span of echo delays associated with the most critical portion of the approach stage of pursuit. Forward masking by the sonar transmissions may contribute to the threshold shift, too, but middle-ear muscle contractions do occur and must be a significant part of the cause.

Animals↗
PubMed1988

Clutter interference along the target range axis in the echolocating bat, Eptesicus fuscus.

The sensitivity of the echolocating bat, Eptesicus fuscus, for detection of a sonar target is impaired by the presence of additional targets located at similar distances. At a range of 54 cm, sensitivity to one target declines if the range separation to other targets is smaller than 8-9 cm. This loss of sensitivity is an example of clutter interference along the range axis, created by simultaneous masking of one set of echoes by another. Echoes that fall within an experimentally defined critical range band may sum together to contribute collectively to detection in that band. Echoes falling into separate bands may be detected independently. Acoustic glints within a band appear to be grouped together to be perceived as a single range-extended target of complex structure. Range bands may thus define what a "target" is by specifying within-target and between-target differences in range. The width of critical range bands appears to depend upon target range, with wider bands at greater ranges.

Animals↗
PubMed1985

Sonar tracking of horizontally moving targets by the big brown bat Eptesicus fuscus.

When following a moving target, echolocating bats (Eptesicus fuscus) keep their heads aimed at the target's position. This tracking behavior seems not to involve predicting the target's trajectory, but is achieved by the bat's pointing its head at the target's last known position. The bat obtains frequent position updates by emitting sonar signals at a high rate. After the lag between head and target positions and the nonunity tracking gain were corrected for, bats' tracking accuracy in the horizontal plane was +/- 1.6 degree.

Animals↗
PubMed1984

Noise masking of tone responses and critical ratios in single units of the mouse cochlear nerve and cochlear nucleus.

Responses of single units in the cochlear nerve and cochlear nucleus to tone bursts in a background of continuous white broadband noise were recorded. Tone and noise intensities ranged from threshold to saturation levels. Masking of the tone response by the noise was demonstrated by comparing peristimulus-time histograms and spike rates recorded during the tone and between tone presentations. The response of a unit to masking was found to be predictable based upon the difference in its rate of response to the tone and to the noise when the tone was masked. Several nonlinearities of the masking process are described. The most prominent one is an increase in the difference between tone and noise levels at the threshold of masking with increasing tone levels, i.e. neural critical ratios increase with increasing tone level. On the average, the frequency dependence of single unit effective bandwidths and of critical ratio bandwidths is similar to that of mean behavioral critical ratio bands.

Action Potentials↗
PubMed1983

Two-tone suppression in auditory nerve fibers of the green treefrog (Hyla cinerea).

The phenomenon of two-tone suppression was studied quantitatively in the peripheral auditory system of the green treefrog (Hyla cinerea). Linear relationships were found between best excitatory and best suppressor frequency, between response thresholds at these frequencies, between Q 10dB-values of excitatory and suppressor tuning curves and best excitatory frequency, and between both Q 10dB-values.

Animals↗
PubMed1972

Serum protein levels in primary biliary cirrhosis.

Serum levels of albumin, transferrin, alpha(2)-macroglobulin, beta(1)C/beta(1)A, IgA, IgG, and IgM have been determined in 73 patients with primary biliary cirrhosis and in age- and sex-matched controls. A highly significant fall in albumin was demonstrated, and there were highly significant increases in alpha(2)-macroglobulin and all three immunoglobulin levels. Transferrin and beta(1)/Cbeta(1)A levels were unchanged. No significant correlations were found between the titre of antimitochondrial antibody, the duration of symptoms, and any of the serum proteins estimated. A highly significant positive correlation was present between serum albumin and transferrin levels in both patient and control groups.

Adult↗