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W Croft

Publications and source records attributed to W Croft.

9 recordsLinked to original sources
PubMed2006

Utterance selection model of language change.

We present a mathematical formulation of a theory of language change. The theory is evolutionary in nature and has close analogies with theories of population genetics. The mathematical structure we construct similarly has correspondences with the Fisher-Wright model of population genetics, but there are significant differences. The continuous time formulation of the model is expressed in terms of a Fokker-Planck equation. This equation is exactly soluble in the case of a single speaker and can be investigated analytically in the case of multiple speakers who communicate equally with all other speakers and give their utterances equal weight. Whilst the stationary properties of this system have much in common with the single-speaker case, time-dependent properties are richer. In the particular case where linguistic forms can become extinct, we find that the presence of many speakers causes a two-stage relaxation, the first being a common marginal distribution that persists for a long time as a consequence of ultimate extinction being due to rare fluctuations.

Journal Article↗
PubMed1998

Why is a pomegranate an apple? The role of shape, taxonomic relatedness, and prior lexical knowledge in children's overextensions of apple and dog.

Children's overextensions (e.g. referring to a pomegranate as apple) raise intriguing questions regarding early word meanings. Specifically, how do object shape, taxonomic relatedness, and prior lexical knowledge influence children's overextensions? The present study sheds new light on this issue by presenting items that disentangle the three factors of shape, taxonomic category, and prior lexical knowledge, and by using a novel comprehension task (the screened-alternative task) in which children can indicate negative exemplars (e.g. which items are NOT apples). 49 subjects in three age groups participated (Ms = 2;0, 2;6, and 4;5). Findings indicate: (1) Error patterns differed by task. In production, errors were overwhelmingly due to selecting items that matched the target word in BOTH shape and taxonomic relatedness. In comprehension, more errors were based on either shape alone or taxonomic relatedness alone, and the nature and frequency of the overextensions interacted with prior lexical knowledge. (2) Error patterns also differed markedly based on the word being tested (apple vs. dog), in both comprehension and production (3) As predicted, errors were more frequent in production than comprehension, though only for children in the two younger age groups. Altogether, the study indicates that overextensions are not simply production errors, and that both taxonomic relatedness and object shape play a powerful role in early naming errors.

Age Factors↗
PubMed1979

Antitumor activity of Listeria monocytogenes on a guinea pig fibrosarcoma.

Listeria monocytogenes-mediated tumor inhibition was studied in strain 13 guinea pigs by using a methylcholanthrene-induced fibrosarcoma (MCA-1). Mixtures of Listeria and tumor cells in ratios of 1:100, 1:200, or 1:400 (Listeria:MCA-1 cells) led to significant suppression of tumor growth. Intralesional injection of tumors on day 6 posttransplantation led to the regression of a highly significant number of tumors. Animals receiving injections of Listeria, either in a mixture with tumor cells or intralesionally, displayed enhanced skin test reactivity to a tumor extract. Tumor regressors were resistant for at least 2 to 3 months after the initial transplant to rechallenge with MCA-1 cells. Thus, with this particular tumor-host system, Listeria was successfully employed as an antitumor agent with no visibly detrimental side effects to the host.

Animals↗
PubMed1979

Mechanisms of Listeria monocytogenes-mediated modulation of tumor growth.

Studies using Listeria monocytogenes as an antitumor agent were initiated to determine the requirements for Listeria-mediated tumor inhibition to occur. When Strain 13 guinea pigs were injected with an admixture of viable Listeria and a methylcholanthrene-induced fibrosarcoma in a ratio of 1 bacterium to 100 tumor cells, Listeria had a marked capacity to inhibit tumor growth. This confirms an earlier study in our laboratory (M. M. Dustoor, A. Fulton, W. Croft, and A. A. Blazkovec, Infect. Immun. 23:54-60, 1979). At no time did animals exhibit overt symptoms of disease as a result of Listeria infection. Animals treated with antilymphocyte serum, which had previously been shown to abrogate T-cell functions, were no longer able to suppress Listeria-tumor cell mixtures. Treatment in vivo with carrageenan, a macrophage-inhibitory agent, also abrogated Listeria-mediated tumor inhibition. These results suggest that Listeria-mediated inhibition requires intact T-lymphocyte and macrophage function. Experiments in which Listeria was given in admixture with the tumor cells or in the opposite flank demonstrated that the antitumor effects require intimate association of the Listeria and tumor cells. Histopathological studies, showing that macrophages and lymphocytes are the predominant inflammatory cells present at sites of tumor destruction, further suggest a role for these cells in Listeria-mediated inhibition. Animals which had rejected prior Listeria-tumor cell inocula were resistant to rechallenge with the homologous tumor for more than 1 year. This work thus confirms in vitro studies demonstrating that both lymphocytes and macrophages are required for Listeria-mediated tumor inhibition to occur. This study demonstrates that viable Listeria can have potent antitumor effects without causing overt disease as a result of Listeria infection.

Animals↗
PubMed1977

Bacteriological and histopathological evaluation of guinea pigs after infection with Listeria monocytogenes.

Randomly bred guinea pigs were infected with Listeria monocytogenes using the intracardial, intravenous and intraperitoneal routes of infection. Doses of Listeria ranged from 5 to 1,000 x the 50% lethal dose based on the 50% lethal dose for intracardially injected Listeria. A complete necropsy was performed on all animals that died after infection. Gross and microscopic examination of tissues revealed major pathological features which include myocarditis, edema and congestion with interstitial pneumonitis present in the lungs, and fatty hepatic changes with focal necrosis. For all or a majority of the animals, large numbers of Listeria were likewise recovered from these organs and from lymph nodes, spleen, kidneys, and adrenal gland tissue. Of the three routes of infection used, guinea pigs were most susceptible to Listeria injected via the intracardial route. The relatively high lethal dose of listeric for the quinea pig, however, suggests that the organism is a low-grade pathogen for this species.

Animals↗