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Biomedical subjects

S Fairhurst

Publications and source records attributed to S Fairhurst.

At least 19 recordsLinked to original sources
PubMed2003

Temporal landmarks: proximity prevails.

Subjects in conditioning experiments time their conditioned responses relative to the onsets of the conditioned stimuli (CSs). These onsets are temporal landmarks, by reference to which subjects may estimate the location of the unconditioned stimulus (US) in time. In a serial compound conditioning paradigm, a long duration CS comes on first, followed later by a second shorter CS, creating both a long-range and a short-range predictor of the US. We ask whether displacing the short-range predictor relative to the long-range predictor causes subjects to strike a compromise between the different temporal locations predicted by the two CSs. In three experiments with pigeons, we varied the training conditions so as to favor or militate against this outcome. However, in all conditions, there was no compromise; after the onset of the displaced short-range CS, the timing of conditioned responding was governed by it alone. This result contrasts with the compromises that are seen when the feeding time predicted by a CS is put in conflict with the time predicted by the circadian clock, and with the similar compromises sometimes seen when a nearby spatial landmark is displaced relative to a larger spatial context.

Animals↗
PubMed2003

Hazard and risk assessment of industrial chemicals in the occupational context in Europe: some current issues.

This paper is about industrial chemicals, the manner in which their toxicity is assessed and the use of such assessments in regulatory decision-making. It begins with general points concerning toxicological data availability and hazard identification, then moves on to risk assessment and occupational exposure limits, and finally looks briefly at three specific toxicological issues, asthma, chronic toxic encephalopathy, and "low toxicity" dust effects on the lung, where the science is far from resolved. The overall purpose of the paper is to raise, or perhaps to act as a reminder of a number of issues of particular relevance to industrial chemicals and the occupational setting, and hopefully to prompt further thinking and perhaps some new initiatives directed at the areas in question.

Dust↗
PubMed2001

Sharing toxicological information on industrial chemicals.

A clear and comprehensive appreciation of the toxicological hazards of a substance is an essential prerequisite to establishing appropriate, balanced and effective risk management measures in the workplace. For many substances, there are currently numerous problems and issues surrounding: the adequacy of the toxicological information base; its interpretation; the transfer of key messages to, and their understanding by, those who need to take action; and the roles and practices of the various standard-setting bodies operating in this area and the interaction between them. This paper briefly touches on these issues, on a range of activities and initiatives directed at improving the current situation, and on the implications for all those involved, particularly the world occupational hygiene community.

Europe↗
PubMed2000

Generic isolated horizons and their applications

The notion of isolated horizons is extended to allow for distortion and rotation. Space-times containing a black hole, itself in equilibrium but possibly surrounded by radiation, satisfy these conditions. The framework has three types of applications: (i) it provides new tools to extract physics from strong field geometry; (ii) it leads to a generalization of the zeroth and first laws of black hole mechanics and sheds new light on the "origin" of the first law; and (iii) it serves as a point of departure for black hole entropy calculations in nonperturbative quantum gravity.

Journal Article↗
PubMed2000

Phase advance after one or three simulated dawns in humans.

A specially designed apparatus that can simulate the waveform of the dawn or dusk signal at any latitude and any day of the year has been shown to phase shift the circadian pacemaker in rodents and primates at a fraction of the illuminance previously used. Until recently, it was considered that rather high illuminances or rather long exposure episodes to room light were necessary to phase shift human circadian rhythms. This experiment shows that, under controlled conditions of a modified constant routine protocol, a single dawn signal is sufficient to phase advance the timing of the onset of secretion of the pineal hormone melatonin. The significant phase advance of salivary melatonin of 20 minutes, which is enhanced to 34 minutes after three consecutive dawn signals, is small, but appears to be of sufficient magnitude to entrain the human circadian pacemaker, which has an endogenous period of about 24.2h.

Adult↗
PubMed1997

Mnemonics for variability: remembering food delay.

Three experiments with White Carneaux pigeons (Columba livia) investigated memory and decision processes under fixed and variable reinforcement intervals. Response rate was measured during the unreinforced trials in the discrete-trial peak procedure in which reinforced trials were mixed with long unreinforced trials. Two decision models differing in assumptions about memory constraints are reviewed. In the complete-memory model (J. Gibbon, R.M. Church, S. Fairhurst, & A. Kacelnik, 1988), all interreinforcement intervals were remembered, whereas in the minimax model (D. Brunner, A. Kacelnik, & J. Gibbon, 1996), only estimates of the shortest and longest possible reinforcement times were remembered. Both models accommodated some features of response rate as a function of trial time, but only the second was compatible with the observed cessation of responding.

Animals↗
PubMed1994

Ratio versus difference comparators in choice.

Several theories in the learning literature describe decision rules for performance utilizing ratios and differences. The present paper analyzes rules for choice based on either delays to food, immediacies (the inverse of delays), or rates of food, combined factorially with a ratio or difference comparator. An experiment using the time-left procedure (Gibbon & Church, 1981) is reported with motivational differentials induced by unequal reinforcement durations. The preference results were compatible with a ratio-comparator decision rule, but not with decision rules based on differences. Differential reinforcement amounts were functionally equivalent to changes in delays to food. Under biased reinforcement, overall food rate was increased, but variance in preference was increased or decreased depending on which alternative was favored. This is a Weber law finding that is compatible with multiplicative, scalar sources of variance but incompatible with pacemaker rate changes proportional to food presentation rate.

Animals↗
PubMed1994

Choice between fixed and variable delays with different reward amounts.

Choice between a variable and a fixed food source was studied in pigeons. The variable source yielded different durations of access to food (changed across experimental conditions) uniquely associated with equiprobable delays of either 20 s or 60 s, timed from the choice point. A comparison, elapsing alternative (time-left procedure, J. Gibbon & R.M. Church, 1981), was associated with fixed access to feeder after a 60-s delay, timed from the beginning of the trial. Preference was best characterized as reflecting an average of the 2 local rates of reinforcement in the variable alternative (the average of the ratio of each standard amount and its associated delay). The averaging rule characterized initial choice link data well, but terminal link responding showed that Ss also remembered the 2 delay-amount combinations separately.

Animals↗
PubMed1989

Percutaneous toxicity of ethylene glycol monomethyl ether and of dipropylene glycol monomethyl ether in the rat.

The subacute percutaneous toxicity of dipropylene glycol monomethyl ether (DPM) in male rats dosed 5 days/week for 4 weeks under both occluded and unoccluded conditions has been assessed and compared to the percutaneous toxicity of ethylene glycol monomethyl ether (EGM). DPM caused no significant changes in the clinical chemistry, haematology, or pathology, whereas EGM caused changes in the haematology and clinical chemistry, and both testicular and bone marrow damage at doses of 1000 mg/kg per day.

Administration, Topical↗
PubMed1987

Acute toxicity of T2 toxin in rats, mice, guinea pigs, and pigeons.

The acute intravenous, intragastric, subcutaneous, intraperitoneal and intratracheal toxicity of T2 toxin has been studied in rats, mice, guinea-pigs, and pigeons. The acute LD50 values obtained varied between 1.0 and 14 mg X kg-1, there being little difference between the various routes in any given species. T2 caused vomiting in pigeons at doses of one fifth or less the LD50. In rats doses of 3.0 and 5.0 mg X kg-1 T2 produced lymphopenia, reticulocytosis, and in the highest dose groups normoblastaemia. Additionally, changes in plasma alkaline phosphatase and aspartate aminotransferase activities were seen. Histological changes were observed in lymphoid organs and were most severe in the thymus, lymph nodes, and Peyer's patches. The spleen was less severely affected. Gastrointestinal changes consisting of dead and dying lymphoid cells throughout the lamina propria were seen together with, in some cases, mucosal ulceration. The time course of the development and of the reversal of the changes was followed.

Administration, Oral↗
PubMed1987

Skin effects of trichothecenes and their amelioration by decontamination.

The ability of trichothecenes, in particular T2 toxin (T2), to cause irritant effects on the skin has been investigated in laboratory rodents and rabbits. Quantitatively, T2 was found to be highly potent in this respect, causing irritant reactions on rat skin at contamination densities of less than 1 microgram.cm-2. The first appearance of skin effects was delayed for approximately 6 h after application, irrespective of the dose applied. Similarly, variation in solvent or injection into or beneath the skin failed to accelerate the onset of the irritant reaction. An aqueous soap solution was largely effective in removing low doses of T2 from the skin, but was ineffective in removing larger doses. However, washing the contaminated skin with polyethylene glycol 300 was very effective at removing even large doses of T2 from the skin. The macrocyclic trichothecene verrucarin A was of comparable irritancy to T2 on rat skin. Diacetoxyscirpenol and nivalenol were less potent.

Animals↗
PubMed1987

Lipid peroxidation and mechanisms of toxicity.

Aerobic organisms by definition require oxygen, and the importance of iron in aerobic respiration has long been recognized, but despite their beneficial roles, these elements can pose a real threat to the organism. During oxygen reduction, reactive species such as O2-. and H2O2 are formed readily. Iron can combine with these species, or with molecular oxygen itself, to generate free radicals which will attack the polyunsaturated fatty acids of membrane lipids. This oxidative deterioration of membrane lipids is known as lipid peroxidation. To protect itself against this form of attack, the organism possesses several types of defense mechanisms. Under normal conditions, these defenses appear to offer adequate protection for cell membranes, but the possibility exists that certain foreign compounds may interfere with or even overwhelm these defenses, and herein could lie a general mechanism of toxicity. This possible cause of toxicity is discussed in relation to other suggested causes.

Animals↗
PubMed1984

Daily meal anticipation: interaction of circadian and interval timing.

Both short-interval and circadian timing systems support anticipatory response accelerations prior to food reinforcement. In the first case, the behavior pattern is determined by a scalar timing process with an arbitrary-reset property. In contrast, under daily cycles of food-availability, behavior reflects a self-sustaining oscillation. With rats as subjects, the concurrent operation of timing of both kinds was studied by addition of premeal auditory cues on the circadian baseline, in the absence of a day-night illumination cycle. Cues within both minute and hour ranges served to lower the level of premeal anticipatory responding, although exponential accelerations were similar to the uncued case. Cues within the minutes range yielded interval-timing functions that reflected approximate superposition. Cues within the hours range suppressed respondings at their outset, in proportion to cue duration. When one of the shorter cues was suddenly lengthened, short-interval accelerations appeared at inappropriate circadian phases. When a premeal cue was extended through mealtime, anticipation rates increased markedly, suggesting that cue termination at the start of mealtime is a potent anchor for premeal anticipation regardless of cue duration. By use of meal-omission probes without external cues, peak rates were located after the onset of expected mealtime, often near its termination. The results suggest interactions between the scalar interval timer and the circadian anticipation timer, as modulated by the circadian free-run timer.

Animals↗
PubMed1983

Development of the cytosolic defence system against microsomal lipid peroxidation in rat liver.

Ascorbate-induced lipid peroxidation in rat liver microsomes reaches the adult level in 2-3 days. NADPH-induced peroxidation develops more gradually, in parallel with the activity of NADPH-cytochrome P-450 reductase, attaining adult levels by 10-12 days. The glutathione-dependent cytosolic enzyme activity which inhibits peroxidation is inhibited by bromosulphophthalein. The development of this system lags behind the development of microsomal lipid peroxidation between the ages of 2 and 20 days, allowing peroxidation to proceed.

Aging↗
PubMed1983

Effect of old age on paracetamol-induced lipid peroxidation in rat liver.

Post-mitochondrial supernatants isolated from the livers of mature rats (3 to 6 months old) 2 h or more after the administration of a single large oral dose of paracetamol (800 mg/kg) showed rapid rates of lipid peroxidation when incubated in vitro. In similar experiments with old rats (27-30 months old) the time between administration of paracetamol and the onset of lipid peroxidation was much longer (6 h or more). In both age groups, lipid peroxidation was dependent on the depletion of glutathione from the liver.

Acetaminophen↗
PubMed1983

Timing the second response in two-response avoidance.

Rats were trained on a free-operant avoidance task requiring two lever presses within R seconds, with the opportunity for each response distinguished by differing stimuli. Response latencies at a variety of response-shock intervals were found to be proportional to the time available for the response. These results are shown to be consonant with a scalar expectancy model of timing behavior.

Animals↗