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J A Hampton

Publications and source records attributed to J A Hampton.

At least 19 recordsLinked to original sources
PubMed1999

Predicting conjunction typicalities by component typicalities.

In two studies, we investigated to what extent typicalities in conjunctive concepts phrased as relative clauses--such as pets that are also birds--can be predicted from simple functions of constituent typicalities and from extensions of such functions. In a first study, analyses of a large aggregated data set, based on seven different experiments, showed that a calibrated minimum rule model and some extensions of this model accounted for a very large part of the variance in the conjunction typicalities. The same models can also account for the so-called guppy effect. A psychological explanation is presented, which states that typicalities in contrast categories, like pets that are not birds and birds that are not pets, further improve the prediction of conjunction typicalities. This hypothesis is tested in a second study.

Adult↗
PubMed1998

Similarity-based categorization and fuzziness of natural categories.

The adequacy of similarity to prototype as an account of categorization in natural concepts was assessed by analyzing the monotonicity of the relation between typicality of an item in a category and the probability of a positive categorization response using data from McCloskey and Glucksberg (1978). The analysis revealed a strong underlying similarity-based threshold curve, with systematic deviations. Further data collection showed that deviations from the curve could be attributed to the effects of unfamiliarity and non-categorical associations on typicality judgments, as well as differences between the perceptual appearance of an item (which tended to boost typicality) and its underlying nature (which tended to boost categorization). The results are discussed in terms of the different presuppositions and task constraints involved in rating typicality as opposed to performing a categorization.

Adult↗
PubMed1998

Reduction of tumour oxygenation during and after photodynamic therapy in vivo: effects of fluence rate.

It has been proposed that the generation of O2 during photodynamic therapy (PDT) may lead to photochemical depletion of ambient tumour oxygen, thus causing acute hypoxia and limiting treatment effectiveness. We have studied the effects of fluence rate on pO2, in the murine RIF tumour during and after PDT using 5 mg kg(-1) Photofrin and fluence rates of 30, 75 or 150 mW cm(-2). Median pO2 before PDT ranged from 2.9 to 5.2 mmHg in three treatment groups. Within the first minute of illumination, median tumour pO2 decreased with all fluence rates to values between 0.7 and 1.1 mmHg. These effects were rapidly and completely reversible if illumination was interrupted. During prolonged illumination (20-50 J cm(-2)) pO2 recovered at the 30 mW cm(-2) fluence rate to a median value of 7.4 mmHg, but remained low at the 150 mW cm(-2) fluence rate (median pO2 1.7 mmHg). Fluence rate effects were not found after PDT, and at both 30 and 150 mW cm(-2) median tumour pO2 fell from control levels to 1.0-1.8 mmHg within 1-3 h after treatment conclusion. PDT with 100 J cm(-2) at 30 mW cm(-2) caused significantly (P = 0.0004) longer median tumour regrowth times than PDT at 150 mW cm(-2), indicating that lower fluence rate can improve PDT response. Vascular perfusion studies uncovered significant fluence rate-dependent differences in the responses of the normal and tumour vasculature. These data establish a direct relationship between tumour pO2, the fluence rate applied during PDT and treatment outcome. The findings are of immediate clinical relevance.

Animals↗
PubMed1997

Associative and similarity-based processes in categorization decisions.

Two experiments were directed at distinguishing associative and similarity-based accounts of systematic differences in categorization time for different items in natural categories. Experiment 1 investigated the correlation of categorization time with three measures of instance centrality in a category. Production frequency (PF), rated typicality, and familiarity from category norms for British participants (Hampton & Gardiner, 1983) were used to predict mean categorization times for 531 words in 12 semantic categories. PF and typicality (but not familarity) were found to make significant and independent contributions to categorization time. Error rates were related only to typicality (apart from errors made to ambiguous or unknown items). Experiment 2 provided a further dissociation of PF and typicality. Manipulating the difficulty of the task through the relatedness of the false items interacted primarily with the effect of typicality on categorization time, whereas, under conditions of easy discrimination, prior exposure to the category exemplars affected only the contribution of PF to the decision time. The dissociation of typicality and PF measures is interpreted as providing evidence that speeded categorization involves both retrieval of associations indexed by PF and a similarity-based decision process indexed by typicality.

Adult↗
PubMed1997

Conceptual combination: conjunction and negation of natural concepts.

The operation of negation on combinations of natural categories was examined in two experiments. In the first, category membership ratings of lists of items were obtained for pairs of concepts considered individually and in two logical combinations: conjunctions (e.g., "Tools which are also Weapons") and negated conjunctions--forms of those conjunctions in which the modifier noun category was negated ("Tools which are not Weapons"). For conjunctions, results supported earlier findings of overextension and the geometric averaging of constituent membership values (Hampton, 1988b). Previous findings of concept dominance and noncommutativity within conjunctions were also replicated, both for typicality ratings and for probability of class membership. For negated conjunctions, the pattern of dominance was similar but interacted with order within the conjunction. Negated conjunctions were also overextended. The second experiment explored how the attributes of negated conjunctions were derived from those of the two component concepts. Frequency of generation of attributes expressed positively (has wheels) or negatively (has no wheels) followed rated frequency in the negated category. The distinctiveness of an attribute to distinguish the complement from the head-noun class was associated with the generation of attributes, particularly when there was relatively high overlap between the two categories.

Concept Formation↗
PubMed1997

Predictions of mathematical models of tissue oxygenation and generation of singlet oxygen during photodynamic therapy.

Photodynamic therapy (PDT) is a relatively new protocol for cancer treatment which has recently been approved for limited clinical use. Traditionally, the success of treatment with PDT has been compared on the basis of total light delivery. Using the mathematical model of Henning et al. (Radiat. Res. 142, 221-226, 1995), we have determined that when oxygen is not depleted from the tissue, the concentration of singlet oxygen that is generated is directly proportional to the product of the light fluence rate (phi) and the concentration of the photosensitizer (Cs). Therefore, phiCs is an appropriate parameter for comparing the potential success of PDT protocols under these conditions. For a treatment of time t, the observed photodynamic effect resulting from singlet oxygen exposure should be directly related to phiCst. For high phiCs, the model predicts that oxygen depletion occurs within the tumor tissue. As a result, the photodynamic effect is no longer proportional to phiCst. We have expanded the model of Henning et al. to include the changes in oxygen concentration which occur within the capillary as blood flows through the tissue. Our new predictions with the mathematical model for optimal PDT treatment conditions are significantly different from those predicted by the previous models. Predictions of the model are given using parameters relevant for treatment of solid tumors with Photofrin.

Dihematoporphyrin Ether↗
PubMed1996

Conjuctions of visually based categories: overextension and compensation.

The degree to which overextension effects found with conjunctions of semantic categories, such as sports and games (J. A. Hampton, 1988), would generalize to categories of visual stimuli was tested in 4 experiments. Overextension occurs when participants categorize a stimulus in the conjunction of 2 categories but fail to categorize the same stimulus as belonging to 1 of the 2 constituent categories considered individually. Stimuli for the present experiments were ambiguous colored letter shapes and cartoon faces that could vary along dimensions of happiness and either apparent intelligence or apparent age. Overextension was found with both stimulus sets, thus showing that the phenomenon is not restricted to categorization in superordinate semantic categories. There was also evidence that typicality in 1 category could compensate for borderline membership of the other. More overextension was found for faces than for letters, and there was evidence for asymmetric compensation between category dimensions.

Adult↗
PubMed1996

Transperineal photodynamic ablation of the canine prostate.

PURPOSE: Experiments were undertaken to determine the effects of transperineal interstitial photodynamic therapy on the canine prostate. MATERIALS AND METHODS: Mongrel dogs were injected intravenously with the photosensitizer, tin (II) ethyl etiopurpurin dichloride. Twenty-four hours later, 2 optical fibers were implanted in 1 hemisphere of the prostate, which was then treated with red light (660 nm.). RESULTS: Acutely, the treated areas showed extensive hemorrhagic necrosis. At 3 and 6 weeks, the treated lobes were largely replaced by fibrous connective tissue. CONCLUSION: Transperineal photodynamic therapy of the canine prostate is feasible. Further preclinical investigation is warranted to determine the applicability of this approach to the treatment of localized prostate cancer.

Animals↗
PubMed1995

A transient mathematical model of oxygen depletion during photodynamic therapy.

A transient one-dimensional mathematical model is presented to help visualize the qualitative and quantitative effects on inter-capillary tissue undergoing photodynamic therapy (PDT). The model is solved by a Crank-Nicholson finite difference formulation to provide time-dependent concentrations of the Type II mechanism's photo-oxidation species in the tissue surrounding a capillary. The time-dependent solution allows educated decisions to be made as to the optimum timing of light fractionation (on/off) cycles. Qualitative and quantitative optimization of the PDT process is considered along with a case study of data in the literature, the main goal being to provide optimized light therapy regimens for eventual clinical use.

Animals↗
PubMed1994

Exposure to alkyllysophospholipids inhibits in vitro invasion of transitional cell carcinoma.

Alkyllysophospholipids (ALP) are a group of synthetic analogs of a naturally occurring 2-lysophosphocholine. They are directly cytotoxic to a variety of neoplastic cell lines and can modulate the activation of macrophages against tumor cells. Moreover, recent reports have demonstrated the ability of racemic 1-octadecyl-2-methyl-sn-glycero-3-phosphocholine (ET-18-O-CH3) to prevent tumor cell invasion when given in noncytotoxic concentrations. Using an in vitro model, we studied the ability of ET-18-O-CH3 to prevent transitional cell carcinoma invasion. Cytostatic activity was determined by clonal growth assay (25,000 cells per plate). Suppression of colony growth was found at concentrations greater than 4 micrograms/ml. of ET-18-O-CH3 in rat and mouse transitional cell carcinoma (TCC) cell lines and greater than 2 micrograms/ml. in a human TCC line. Inhibition of tumor cell invasion was assessed by the effects on cell migration through Matrigel-coated 8 microns-pore polycarbonate filters (using 1 x 10(5) cells per chamber). Invasion was reduced to 50 to 70% of controls in both the mouse and rat TCC lines at the highest concentration (4.0 micrograms/ml.). In the human TCC line, invasion was reduced to less than 30% of controls at concentrations as low as 0.5 micrograms/ml. Motility (without invasion) of the human TCC line, as measured by cell migration through the micropore filter without the Matrigel coating, was inhibited at the same concentration of ET-18-O-CH3 found to inhibit invasion. The anti-invasive effect seen with noncytotoxic concentrations of ALP may prove useful in the treatment of transitional cell carcinoma.

Animals↗
PubMed1994

Iminium salt benzochlorins: structure-activity relationship studies.

An iminium salt of copper(II) octaethylbenzochlorin (CDS1) is an effective new photosensitizer despite the fact that it does not produce singlet oxygen, does not fluoresce and the triplet state lifetime can only be less than 20 ns. A number of octaethylbenzochlorin derivatives were synthesized in order to determine the structural component(s) that is(are) responsible for the photodynamic action of these new photosensitizers. Studies utilizing the N-(4-[5-nitro-2-furyl]-2-thiazolyl)formamide-induced urothelial tumor revealed that the coexistence of the copper inside the aromatic ring and the iminium group at the meso position are required for the photodynamic effect.

Animals↗
PubMed1993

Copper benzochlorin, a novel photosensitizer for photodynamic therapy: effects on a transplantable urothelial tumor.

An iminium salt of octaethylbenzochlorin with copper in the aromatic ring, CDS1, was tested for its tumoricidal effects on the AY-27 N-[4-(5-nitro-2-furyl)-2-thiazolyl] formamide tumor line. CDS1 was found to be an effective photosensitizer in vivo when used in combination with either a xenon arc lamp or a pulsed alexandrite laser. Hemodynamically, CDS1 and light caused a rapid decrease in tumor blood flow. Skin photosensitization was found to be minimal when drug-injected mice were illuminated in a solar simulator.

Animals↗
PubMed1993

Iminium salt of copper benzochlorin (CDS1), a novel photosensitizer for photodynamic therapy: mechanism of cell killing.

The mechanism of cell killing by CDS1, an iminium salt of octaethylbenzochlorin with copper in the aromatic ring, in combination with light from a noncoherent light source was investigated. Using a standard clonogenic assay and the AY-27 FANFT tumor line, photoactivation of CDS1 was shown to be cytotoxic. The photodynamic cell killing ability of CDS1 required the presence of molecular oxygen. The reactive species generated by light activation of CDS1 were effectively quenched by N,N'-diphenyl-p-phenylenediamine. Additionally, the photodynamic effect of CDS1 was not enhanced by deuterium oxide. To characterize the reactive oxygen species generated by the photoactivation of CDS1 the well-characterized erythrocyte ghost model was used. Superoxide dismutase and catalase were potent inhibitors of CDS1-induced lipid peroxidation of erythrocyte membranes. Sodium azide only partially inhibited lipid peroxidation. These findings differed from the known singlet oxygen generator, tin (II) etiopurpurin dichloride (SnET2). Sodium azide was a potent inhibitor of SnET2-induced lipid peroxidation, whereas superoxide dismutase and catalase were totally ineffective. Based on these results, we conclude that CDS1 requires the presence of molecular oxygen for cell killing to occur but appears to act primarily through a non-singlet oxygen mechanism.

Animals↗
PubMed1992

Mechanisms of cell killing in photodynamic therapy using a novel in vivo drug/in vitro light culture system.

Photodynamic therapy of certain neoplasms has emerged as a promising form of cancer treatment. This type of therapy involves the exogenous administration of a photosensitizer with subsequent exposure to light. The ensuing photochemical reaction results in destruction of the tumor. Whether tumor cells are destroyed directly by the photodynamic treatment or indirectly as a result of destruction of the tumor microvascular bed is unknown. To address this question, methods were adapted to test whether combinations of a photosensitizer and light resulted in direct cell killing of precision cut tissue slices placed in culture. The major advantages of this culture system are that photosensitizers are administered in vivo, tissue slices produced in minutes, placed in culture medium, and irradiated in vitro. Any resulting cellular destruction occurs in the absence of a functioning vascular system and indicates that photodynamic therapy acts through a direct cell killing mechanism. Tissue slice viability was monitored by two standard methods: assay for intracellular potassium and morphological examination at the electron microscopic level. The effects of hematoporphyrin derivative and light were examined on tissue slices produced from a prostate adenocarcinoma transplanted into male Copenhagen rats. The data indicate that direct killing of tumor slices occurs and is dependent on the irradiation protocol used.

Animals↗
PubMed1991

Photodynamic therapy: effect on the endothelial cell of the rat aorta.

Previous studies in our laboratory have demonstrated that photodynamic therapy (PDT) of experimental bladder tumors leads to rapid destruction of the endothelial lining within the tumor microvasculature. Endothelial cell death during PDT may be a consequence of direct cell injury resulting from retention of photosensitizer within the endothelial cell or, alternatively, result from intravascular activation of circulating photosensitizer with subsequent indirect endothelial damage. In the experiments described here, we investigated the possibility that photosensitizer retained within the endothelial cell was sufficient to cause endothelial cell injury in the absence of circulating drug. The experimental model was rat aorta photosensitized in vivo via the intravenous injection of tin(II) etiopurpurin dichloride (SnET2), and subsequent in situ or in vitro (in explant culture) light (670 nm) treatment from an argon pumped dye laser. Damage to the lining of the aorta was assessed morphometrically by determining the areal density of silver stained endothelial cells. Results indicate that purpurin SnET2-PDT directly damages the endothelial lining.

Animals↗