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Biomedical subjects

D Whitaker

Publications and source records attributed to D Whitaker.

At least 19 recordsLinked to original sources
PubMed2006

Construction of resolvable spatial row-column designs.

Resolvable row-column designs are widely used in field trials to control variation and improve the precision of treatment comparisons. Further gains can often be made by using a spatial model or a combination of spatial and incomplete blocking components. Martin, Eccleston, and Gleeson presented some general principles for the construction of robust spatial block designs which were addressed by spatial designs based on the linear variance (LV) model. In this article we define the two-dimensional form of the LV model and investigate extensions of the Martin et al. principles for the construction of resolvable spatial row-column designs. The computer construction of efficient spatial designs is discussed and some comparisons made with designs constructed assuming an autoregressive variance structure.

Analysis of Variance↗
PubMed2004

CrossOver: an algorithm for the construction of efficient cross-over designs.

A cross-over experiment involves the application of sequences of treatments to several subjects over a number of time periods. It is thought that the observation made on each subject at the end of a time period may depend on the direct effect of the treatment applied in the current period, and the carry-over effects of the treatments applied in one or more previous periods. Various models have been proposed to explain the nature of the carry-over effects. An experimental design that is optimal under one model may not be optimal if a different model is the appropriate one. In this paper an algorithm is described to construct efficient cross-over designs for a range of models that involve the direct effects of the treatments and various functions of their carry-over effects. The effectiveness and flexibility of the algorithm are demonstrated by assessing its performance against numerous designs and models given in the literature.

Algorithms↗
PubMed2004

Sensory uncertainty governs the extent of audio-visual interaction.

Auditory signals have been shown to exert a marked influence on visual perception in a wide range of tasks. However, the mechanisms of these interactions are, at present, poorly understood. Here we present a series of experiments where a temporal cue within the auditory domain can significantly affect the localisation of a moving visual target. To investigate the mechanism of this interaction, we first modulated the spatial positional uncertainty of the visual target by varying its size. When visual positional uncertainty was low (small target size), auditory signals had little or no influence on perceived visual location. However, with increasing visual uncertainty (larger target sizes), auditory signals exerted a significantly greater influence on perceived visual location. We then altered the temporal profile of the auditory signal by modulating the spread of its Gaussian temporal envelope. Introducing this temporal uncertainty to the auditory signal greatly reduced its effect on visual localisation judgements. These findings support the view that the relative uncertainty in individual sensory domains governs the perceptual outcome of multisensory integration.

Acoustic Stimulation↗
PubMed2003

Inverse relationship between density of cutaneous hair and pigmented lesion count in patients with malignant melanoma.

The aim of this study was to investigate whether there is an inverse relationship between the density of cutaneous hair and the number of pigmented lesions in patients with malignant melanoma, and to review the clinical and experimental findings with regard to this relationship. Cutaneous hair density and pigmented lesion counts were determined at nine sites by two physicians using specified criteria in 10 patients with a history of malignant melanoma and 22 control subjects. Study participants were Caucasian males of similar age, height and weight, and had similar occupational, ethnic, socioeconomic and sun exposure backgrounds. Statistical analysis was performed using combined data from all the sites and individual site data to determine whether an inverse relationship between cutaneous hair density and pigmented lesion counts exists. The results demonstrated statistically significant inverse relationships for the chest, upper back, upper arm and forearm in the melanoma patients, but not in the controls. For all the sites combined, both groups demonstrated statistically significant inverse relationships. The study findings are consistent with the hypothesis that some pigmented lesions may arise from the melanocytes of the hair follicle. This hypothesis may lead to new approaches to studying and treating melanoma.

Adult↗
PubMed2001

Expression and integrity of DNA topoisomerase II isoforms does not explain generic drug resistance in malignant mesothelioma.

PURPOSE: Malignant mesothelioma is a tumour that is highly resistant to a number of different chemotherapy agents, yet the mechanisms by which resistance occurs are poorly understood. The pattern of resistance is consistent with disruption of topoisomerase function or expression. Coupled with this, we have previously noted a common serological reaction to the beta isoform of topoisomerase II, suggesting that it may be aberrantly expressed in patients with mesothelioma. METHODS: We assessed the expression of topoisomerase II isoforms in sections of primary tumour. We tested a panel of five mesothelioma cell lines for sensitivity to the known topoisomerase-targeting drugs, doxorubicin and etoposide. We sequenced expressed segments of the topoisomerase genes from these cell lines that have previously been associated with drug resistance. We then investigated other potential resistance mechanisms. RESULTS: We found that the beta isoform of topoisomerase II was more frequently expressed in primary tumours. Only one of the five cell lines was highly resistant to etoposide and this cell line was found to have a point mutation in the gene for topoisomerase IIalpha. Protein levels of topoisomerase IIalpha and beta did not correlate with sensitivity to either doxorubicin nor to etoposide. Semiquantitative analysis suggested that there was marked variation in the levels of mRNA expression of MRP, gamma-GCS and MDR1. None of these findings could be associated with resistance to chemotherapy. CONCLUSION: We conclude that mutations in topoisomerase IIalpha can be associated with extreme resistance of mesothelioma to etoposide. The generic drug resistance of this tumour requires further investigation.

Adult↗
PubMed2001

Optimal reading speed in simulated cataract: development of a potential vision test.

Previous studies suggest that optimal reading speed is unaffected by cataract, yet is significantly reduced in age-related macular disease. This raises the question of whether a reading speed test could be developed to assess potential vision after cataract surgery. In this study, 20 young subjects, with and without a simulation of dense cataract, read aloud Bailey-Lovie word charts. From the results, critical print size, optimal reading speed and word acuity were calculated. The simulated cataract reduced word acuity and increased the critical print size, yet optimal reading speed remained unchanged. Measurements were also taken with the charts in reversed contrast polarity. Reversing the contrast polarity of the charts improved the word acuity and optimal reading speed with the cataract simulation. The results suggest that optimal reading speed could be used as a potential vision test. Further investigations using patients with cataract and ARMD are required.

Adult↗
PubMed2001

Development of a reading speed test for potential-vision measurements.

PURPOSE: Previous studies suggest that optimal reading speed is unaffected by cataract, yet is significantly reduced in age-related macular degeneration (ARMD ). This raises the question of whether a reading speed test could be developed to assess potential vision after cataract surgery. METHODS: Nineteen subjects with cataract, 15 with ARMD, and 13 control subjects with normal, healthy eyes read Bailey-Lovie word charts aloud, and subsequently, critical print size and optimal reading speed were calculated. Measurements were also taken with the charts in reversed-contrast polarity and after pupillary dilation. RESULTS: Although the subjects with cataract had reduced word acuity and increased critical print size, optimal reading speed was similar to that of the control group at a mean of approximately 100 wpm. Optimal reading speed in the subjects with ARMD was substantially worse (mean of 39 wpm). Reversing the contrast polarity of the charts slightly increased the word acuity and optimal reading speed of the subjects with cataract. CONCLUSIONS: The results suggest that optimal reading speed would be useful as a potential-vision test. The proposed test would use text size of at least 1.32 degrees (1.2 log minimum angle of resolution [logMAR]), and pupil dilation would be unnecessary. A reading test with black letters on a white background would be adequate, because charts with reversed-contrast polarity made minimal difference in reading speed.

Aged↗
PubMed2000

Low milk yields.

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Animal Feed↗
PubMed2000

Mesothelial regeneration is not dependent on subserosal cells.

It has been proposed that after mesothelial injury, resident cells within the subserosal connective tissue proliferate, differentiate, and migrate to the serosal surface. The aim of this study was to examine the temporal and spatial changes of proliferating cells in a murine model of testicular mesothelial healing and assess the potential of submesothelial cells to reconstitute the damaged mesothelium. Histology and autoradiography were employed to determine the number of cells within the submesothelial connective tissue, as well as the proportion of cells undergoing DNA synthesis on and beneath the injured serosa. Mesothelial cells surrounding the wound demonstrated maximal DNA synthesis 48 h after injury (27. 82+/-5.64% SEM, compared with 0.17+/-0.16% (3)H-TdR labelled cells for resting mesothelium), whereas a significant increase in proliferating submesothelial cells was not seen until day 4 post-injury (7.79+/-3.31% compared with 0.85+/-0.64% (3)H-TdR labelled cells at day 2). Furthermore, this small number of dividing submesothelial cells must include cells other than the proposed mesothelial precursors, indicating a very low proportion of precursor cells in the submesothelial cell population. As large numbers of mesothelial cells were seen at the wound centre by 3-4 days after injury, it is unlikely that submesothelial cells contributed significantly to the repopulation of the injured mesothelium. It is hypothesized that regenerating mesothelium is more likely to originate from the surrounding uninjured mesothelial cell population.

Animals↗
PubMed2000

Compound grating discrimination in extrafoveal and amblyopic vision.

Compound grating discrimination was measured in normal foveal and extrafoveal vision as well as in central vision of amblyopic subjects. Two types of discrimination were examined: 0 degrees versus 180 degrees and 90 degrees versus 270 degrees phase shift of the second harmonic relative to the fundamental. In common with several previous studies, we found that both 0/180 and 90/270 discriminations are possible in extrafoveal vision. However, we show that differences in foveal and extrafoveal sensitivity can be eliminated for both types of discrimination by scaling the stimulus size appropriately. The extent of spatial magnification necessary to equate foveal and extrafoveal performance differed markedly, with 90/270 discriminations requiring much more magnification. In the amblyopic subjects, the magnitude of the 90/270 deficit was greater than the 0/180 deficit in all six amblyopes tested. In common with previous investigations, we suggest that the visual system adopts a discrimination strategy, based upon differences in local features, between the patterns to be discriminated. One process registers positional relationships, while a second process registers local contrast differences. In this context, the reduced ability of the normal periphery and amblyopic fovea to perform mirror-symmetric discriminations is explained in terms of losses in positional acuity.

Amblyopia↗
PubMed2000

Contrast matching across spatial frequencies for isoluminant chromatic gratings.

Contrast matching was performed with isoluminant red-green and s-cone gratings at spatial frequencies ranging from 0.5 to 8 c/deg. Contrast threshold curves were low-pass in shape, in agreement with previous findings. Contrast matching functions resembled threshold curves at low contrast levels, but became flat and independent of spatial frequency at high contrasts. Thus, isoluminant chromatic gratings exhibited contrast constancy at suprathreshold contrast levels in a similar manner as has been demonstrated for achromatic gratings.

Color Perception↗
PubMed2000

Continuing education for cytotechnologists: the Australian experience.

The development of a continuing education scheme for cytotechonologists in Australia is described. The process involved the establishment of a working party with Terms of Reference to review current practice in Australia and existing schemes in other parts of the world. The scheme developed takes the form of a continuing education diary that provides guidelines on the various forms of continuing education activity and corresponding credit points. The diary also provides for a record of activity to be kept. The scheme requires bi-annual submission of personal activity which is logged into a national database. A peer profile is provided and successful achievement is marked by the issue of a certificate of participation. The programme has achieved a 57% compliance in its first year of operation.

Australia↗
PubMed1999

Watery milk.

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Animal Feed↗
PubMed1999

Perceptual distortions in the neural representation of visual space.

The visual mechanism by which human observers determine the separation between objects has long been of interest. This study examines the extent to which separation in visual space can be misperceived in foveal and extrafoveal vision. Foveally, vertical separations were consistently overestimated relative to horizontal separations, a result which is consistent with the well-documented horizontal-vertical illusion (HVI). Extrafoveally, much larger misrepresentations of visual space were perceived. In addition, separations tangential to fixation were consistently perceived as being greater than separations in a radial direction. These marked misperceptions of visual space which occur in extrafoveal vision take the form of a radial/tangential anisotropy combined with an overestimation of vertical distance. The results have important implications for meridional anisotropies which have previously been documented in a number of visual performance tasks.

Adult↗