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G A Carpenter

Publications and source records attributed to G A Carpenter.

11 recordsLinked to original sources
PubMed2001

S-TREE: self-organizing trees for data clustering and online vector quantization.

This paper introduces S-TREE (Self-Organizing Tree), a family of models that use unsupervised learning to construct hierarchical representations of data and online tree-structured vector quantizers. The S-TREE1 model, which features a new tree-building algorithm, can be implemented with various cost functions. An alternative implementation, S-TREE2, which uses a new double-path search procedure, is also developed. The performance of the S-TREE algorithms is illustrated with data clustering and vector quantization examples, including a Gauss-Markov source benchmark and an image compression application. S-TREE performance on these tasks is compared with the standard tree-structured vector quantizer (TSVQ) and the generalized Lloyd algorithm (GLA). The image reconstruction quality with S-TREE2 approaches that of GLA while taking less than 10% of computer time. S-TREE1 and S-TREE2 also compare favorably with the standard TSVQ in both the time needed to create the codebook and the quality of image reconstruction.

Algorithms↗
PubMed1999

Networks and groups within the genus Neisseria: analysis of argF, recA, rho, and 16S rRNA sequences from human Neisseria species.

To understand the pattern of nucleotide sequence variation among bacteria that frequently exchange chromosomal genes, we analyzed sequences of the recA, argF, and rho genes, as well as part of the small-subunit (16S) rRNA gene, from about 50 isolates of human commensal Neisseria species and the pathogenic N. meningitidis and N. gonorrhoeae. Almost all isolates of these species could be assigned to five phylogenetic groups that are found for all genes examined and generally are supported by high bootstrap values. In contrast, the phylogenetic relationships among groups varied according to the gene analyzed with notable incongruences involving N. cinerea and N. lactamica. Further analysis using split decomposition showed that for each gene, including 16S rRNA, the patterns of sequence divergence within N. meningitidis and closely related species were inconsistent with a bifurcating treelike phylogeny and better represented by an interconnected network. These data indicate that the human commensal Neisseria species can be separated into discrete groups of related species but that the relationships both within and among these groups, including those reconstructed using 16S rRNA, have been distorted by interspecies recombination events.

Base Sequence↗
PubMed1996

Ventricular outflow tract reconstructions with cryopreserved cardiac valve homografts. A single surgeon's 10-year experience.

OBJECTIVE: From January 1, 1985 through December 31, 1994, one surgeon implanted cryopreserved valved homografts into 149 patients--65 since December 1988. This latter series (II) was accomplished in a single hospital, facilitating patient follow-up with biannual echocardiograms. Analysis of these 65 patients is the primary focus of this report; the indications and early surgical results for the two parts of the series (I and II) are compared to assess the evolution of a single surgeon's use of homografts in a mixed pediatric and adult practice. METHODS: Fifty-one variables for each patient (series II) were entered into a computerized database and analyzed (multivariate and univariate) using SPSS 6.1 software (Statistical Products and Service Solutions, Chicago, IL). Cox proportional hazard model was used to identify the independent contribution of each variable for patient mortality and homograft failure. Cumulative survival estimates were made using Kaplan-Meier analysis. Homograft failure was defined as requirement for replacement or death. In series I, there were 41 left ventricular outflow tract (LVOT) reconstructions (31 adult) and 43 right ventricular outflow tract (RVOT) reconstructions (42 pediatric). In series II, there were 55 RVOT reconstructions (52 pediatric) and 10 LVOT reconstructions (7 adult). RESULTS: There were no technical surgical failures. Total surgical mortality rate was 6% (5/84) in series I (3 LVOT, 2 RVOT) and 15% (10/65) in series II (2 LVOT, 8 RVOT) (I vs. II NS; p = 0.11, two-tailed Fisher exact test). By the Cox analysis, only age < 2 years (p < 0.03) and cross-clamp time > 120 minutes (p < 0.05) were significant predictors for death. Age-based survival curves were compared in a sequential bivariate analyses (log rank test) and age < 2 years again was a significant predictor of decreased patient survival (p < 0.006). Actuarial freedom from patient death or reoperation for homograft failure was 82% +/- 7% at 1000 days and 77% +/- 10% at 2000 days. Three patients required re-replacement for homograft failure (5.4%); one of these patients died. The only significant predictor of homograft failure was postoperative endocarditis (p < 0.05). Homograft performance was evaluated by an extensive echocardiography protocol: in surviving patients and homografts, three valved conduits were judged to have severely impaired performance (stenosis or regurgitation), awaiting surgical replacement for a putative total homograft-related structural failures rate of 11% at 5 1/2 years. CONCLUSIONS: Comparisons of series I and II shows, in one surgeon's practice, an evolution away from use of cryopreserved homografts for LVOT reconstructions except when needed for destructive bacterial endocarditis or complex congenital anatomy. Homograft efficacy and durability were similar in RVOT and LVOT positions, with 78.5% of patients surviving at 5 1/2 years; in surviving patients, 89% of homografts have continued to function well. Homografts are not immune to prosthetic bacterial endocarditis, and its occurrence is associated with accelerated deterioration. Cryopreserved homograft valves are an imperfect but satisfactory biological material for specific ventricular outflow reconstructions.

Adolescent↗
PubMed1993

Normal and amnesic learning, recognition and memory by a neural model of cortico-hippocampal interactions.

The processes by which humans and other primates learn to recognize objects have been the subject of many models. Processes such as learning, categorization, attention, memory search, expectation and novelty detection work together at different stages to realize object recognition. In this article, Gail Carpenter and Stephen Grossberg describe one such class of model (Adaptive Resonance Theory, ART) and discuss how its structure and function might relate to known neurological learning and memory processes, such as how infero-temporal cortex can recognize both specialized and abstract information, and how medial temporal amnesia might be caused by lesions in the hippocampal formation. This model also suggests how hippocampal and inferotemporal processing might be linked during recognition learning.

Amnesia↗
PubMed1991

Air hygiene in a pullet house: effects of air filtration on aerial pollutants measured in vivo and in vitro.

1. The natural history of air hygiene in a pullet house was assessed at three-weekly intervals using a combination of in vitro and in vivo assays. The performance of an internal air filter was also examined as an experimental technique for providing clean air. 2. Overall, air hygiene was poor by comparison with occupational standards for human health. The mass concentrations of respirable and inspirable dust were 1.4 and 11.3 mg/m3 compared to human exposure limits of 5 and 10 mg/m3 respectively. The concentration of ammonia was typically about 20 ppm. 3. The majority (greater than 99%) of airborne particles were non-viable. Commensal bacteria from the skin were the most numerous airborne bacteria. Scopulariopsis and Aspergillus spp. were the most prevalent fungi recovered from the air and bird's lungs respectively. The concentrations of airborne and lung fungi were positively correlated with ammonia concentrations. 4. The differences between in vivo and in vitro assays of airborne microorganisms suggest that an aerosol sampler should be devised which better mimics the physical and biochemical environment of the respiratory tract.

Air Microbiology↗
PubMed1985

A neural theory of circadian rhythms: split rhythms, after-effects and motivational interactions.

A neural theory of the circadian pacemaker within the hypothalamic suprachiasmatic nuclei (SCN) is used to explain parametric data about mammalian operant behavior. The intensity, duration, and patterning of ultradian activity-rest cycles and the duration of circadian periods due to parametric (LL) and nonparametric (LD) lighting regimes are simulated. Paradoxical data about split rhythms and after-effects are explained using homeostatic and nonhomeostatic neural mechanisms that modulate pacemaker activity. These modulatory mechanisms enable the pacemaker to adjust to pervasive changes in its lighting regime, as during the passage of seasons, and to ultradian changes in internal metabolic conditions. The model circadian mechanisms are homologous to mechanisms that model hypothalamically mediated appetitive behaviors, such as eating. The theory thus suggests that both circadian and appetitive hypothalamic circuits are constructed from similar neural components. Mechanisms of transmitter habituation, opponent feedback interactions between on-cells and off-cells, homeostatic negative feedback, and conditioning are used in both the circadian and the appetitive circuits. Output from the SCN circadian pacemaker is assumed to modulate the sensitivity of the appetitive circuits to external and internal signals by controlling their level of arousal. Both underarousal and overarousal can cause abnormal behavioral syndromes whose properties have been found in clinical data. A model pacemaker can also be realized as an intracellular system.

Animals↗
PubMed1984

A neural theory of circadian rhythms: Aschoff's rule in diurnal and nocturnal mammals.

A neural model of the suprachiasmatic nuclei suggests how behavioral activity, rest, and circadian period depend on light intensity in diurnal and nocturnal mammals. These properties are traced to the action of light input (external zeitgeber) and an activity-mediated fatigue signal (internal zeitgeber) on the circadian pacemaker. Light enhances activity of the diurnal model and suppresses activity of the nocturnal model. Fatigue suppresses activity in both diurnal and nocturnal models. The asymmetrical action of light and fatigue in diurnal vs. nocturnal models explains the more consistent adherence of nocturnal mammals to Aschoff's rule, the consistent adherence of both diurnal and nocturnal mammals to the circadian rule, and the tendency of nocturnal mammals to lose circadian rhythmicity at lower light levels than diurnal mammals. The fatigue signal is related to the sleep process S of Borbély (Hum. Neurobiol. 1: 195-204, 1982.) and contributes to the stability of circadian period. Two predictions follow: diurnal mammals obey Aschoff's rule less consistently during a self-selected light-dark cycle than in constant light, and if light level is increased enough during sleep in diurnal mammals to compensate for eye closure, then Aschoff's rule will hold more consistently. The results are compared with those of Enright's model.

Animals↗
PubMed1983

Which behavior does the lamprey central motor program mediate?

The isolated lamprey spinal cord, when bathed in 2 millimolar D-glutamic acid, will generate a pattern of motor neuron discharge that has generally been assumed to represent the central motor program for swimming. Motion pictures of behaving lampreys were analyzed by a computer algorithm to estimate undulatory movement parameters that could be directly compared with those generated during D-glutamate--induced undulations. The D-glutamate--induced movement parameters were significantly different from those observed during normal behaviors, including swimming, but accurately predicted the undulations produced by spinally transected adult lampreys.

Animals↗
PubMed1978

An analysis of the mammalian ventricular action potential.

A Hodgkin-Huxley model for ventricular excitation is abstracted from electrophysiological data. A singular perturbation analysis of the 8-dimensional phase portrait of the model characterizes the role of calcium during the plateau phase of the ventricular action potential and demonstrates how the calcium refractory period prevents tetanization.

Action Potentials↗