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Biomedical subjects

R A Baxter

Publications and source records attributed to R A Baxter.

8 recordsLinked to original sources
PubMed1997

Contrast-detail detectability analysis: comparison of a digital spot mammography system and an analog screen-film mammography system.

RATIONALE AND OBJECTIVES: The authors compared the contrast-detail detectability properties of an optically coupled charge-coupled device (CCD) digital mammography system with those of a conventional analog screen-film mammography system. METHODS: A proprietary contrast-detail phantom was imaged with both systems. Twelve observers viewed analog and digital images obtained at seven different mean glandular doses. Contrast-detail-dose curves were generated on the basis of the readers' visual perceptions. Interobserver errors were also calculated. In addition, the authors applied a theoretical model for lesion detectability to calculate and compare the digital and analog mammography systems. RESULTS: The readers' contrast-detail detectability was significantly superior with the digital system in the detail and contrast ranges evaluated (P < .001). CONCLUSION: The performance of the readers paralleled the performance predicted by the theoretical model for the two imaging systems. The following features account for the superior performance of the digital mammography system: (a) higher quantum efficiency of the phosphor, (b) wider dynamic range, (c) better contrast transfer characteristics, and, most important, (d) greater photon flux at the phosphor entrance.

Contrast Sensitivity↗
PubMed1996

Energy efficient neural codes.

In 1969 Barlow introduced the phrase "economy of impulses" to express the tendency for successive neural systems to use lower and lower levels of cell firings to produce equivalent encodings. From this viewpoint, the ultimate economy of impulses is a neural code of minimal redundancy. The hypothesis motivating our research is that energy expenditures, e.g., the metabolic cost of recovering from an action potential relative to the cost of inactivity, should also be factored into the economy of impulses. In fact, coding schemes with the largest representational capacity are not, in general, optimal when energy expenditures are taken into account. We show that for both binary and analog neurons, increased energy expenditure per neuron implies a decrease in average firing rate if energy efficient information transmission is to be maintained.

Action Potentials↗
PubMed1996

Context codes and the effect of noisy learning on a simplified hippocampal CA3 model.

This paper investigates how noise affects a minimal computational model of the hippocampus and, in particular, region CA3. The architecture and physiology employed are consistent with the known anatomy and physiology of this region. Here, we use computer simulations to demonstrate and quantify the ability of this model to create context codes in sequential learning problems. These context codes are mediated by local context neurons which are analogous to hippocampal place-coding cells. These local context neurons endow the network with many of its problem-solving abilities. Our results show that the network encodes context on its own and then uses context to solve sequence prediction under ambiguous conditions. Noise during learning affects performance, and it also affects the development of context codes. The relationship between noise and performance in a sequence prediction is simple and corresponds to a disruption of local context neuron firing. As noise exceeds the signal, sequence completion and local context neuron firing are both lost. For the parameters investigated, extra learning trials and slower learning rates do not overcome either of the effects of noise. The results are consistent with the important role played, in this hippocampal model, by local context neurons in sequence prediction and for disambiguation across time.

Artifacts↗
PubMed1975

The degradation of polychlorinated biphenyls by micro-organisms.

An investigation is described concerning the effects of two micro-organism cultures on single compounds and multi-component mixtures of polychlorinated biphenyls (PCBs). Several low chlorine PCB compounds can be degraded easily with these cultures and, in favourable circumstances, some compounds containing up to six chlorine atoms per molecule can be degraded. In certain multi-component commercial mixtures some PCB compounds are degraded more readily than if present singly. The significance of this is discussed, particularly the fact that work on single compounds does not enable predictions to be made about the behaviour of commercial mixtures in the enviroment. Some tentative explanations are suggested for the behavious of the mixtures exposed to these cultures.

Biodegradation, Environmental↗