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Biomedical subjects

D R Williams

Publications and source records attributed to D R Williams.

At least 307 records · Page 17Linked to original sources
PubMed1975

The chemical principles of chronotherapy as established from an in vitro model of circadian concentration rhythms.

A model reaction for investigating the effects of chemical parameters on circadian rhythms is suggested. Its main advantage is that oscillations occur on a minute scale rather than daily and so accelerate studies. Some factors were found to increase the period of amplitude of the oscillations, others decreased them. The chemical principles prevailing during such oscillations are itemized. The effects of adding poisons or drugs are also noted and a computer calculation has been used to show that desferrioxamine B treatment for serum iron removal could be 65 percent more effective if administered at the correct point in the circadian cycle.

Circadian Rhythm↗
PubMed1974

The prevalence of salmonellas in mink.

The mean isolation rates of salmonellas from the mesenteric lymph nodes and faeces of healthy mink were 16.7% of 618 animals from three farms and 3.9% of 772 animals from four farms respectively. Salmonella senftenberg was the most commonly isolated serotype. S. typhimurium, S. dublin, S. livingstone, S. menston, S. enteritidis, S. bredeney and S. infantis were also seen.The prevalence of salmonellosis in 316 dead mink from 12 farms was 0.6%.The epidemiological aspects are discussed.

Animal Feed↗
PubMed1974

Choice behavior on discrete trials: a demonstration of the occurrence of a response strategy.

Three pairs of pigeons were trained to peck at two keys presented simultaneoulsy in discrete trials with intertrial intervals of 1, 22, or 120 sec. Left-key responses incremented the probability of reinforcement for the first right-key response and, conversely, right-key responses incremented the probability of reinforcement for the first left-key response. In terms of relative response rates, it was found that all birds' choices were described by a momentary maximizing strategy, but this fact was not reflected in the detailed sequential statistics for birds with the longer (22 or 120 sec) intertrial intervals. It was hypothesized that choice behavior, in general, may be accurately described by a momentary maximizing sequence, but that prior failures to demonstrate this were due to "errors" in executing the momentary maximizing sequence. These misappropriated responses, which are hypothesized to be randomly distributed among the responses defining the momentary maximizing sequence, caused successive choices to appear to be statistically independent when, in fact, they were not.

Journal Article↗
PubMed1973

Associative factors underlying the pigeon's key pecking in auto-shaping procedures.

Key pecking in pigeons can be engendered by associating response-independent food presentations with illumination of a key. Specific pairings of key and food are not necessary for this phenomenon. Differential positive association between key and food (defined in terms of relative densities of reinforcement), however, is necessary and sufficient to produce and maintain key pecking. Thus, the occurrence of key pecking in auto-shaping can be considered to depend on associative processes similar to classical conditioning. Consequently, auto-shaped pecking can be virtually eliminated by the addition of food presentations in the intertrial interval, thus removing the association between key and food. Initial exposure to random reinforcement, or reinforcement only in the absence of the key, results in lower rates of pecking in subsequent auto-shaping procedures.

Journal Article↗
PubMed1972

The role of the response-reinforcer contingency in negative automaintenance.

When a response key is briefly illuminated before a grain reinforcer is presented, key pecking is reliably developed and maintained in pigeons, even if pecking prevents reinforcement (negative automaintenance). This experiment demonstrated that pigeons are sensitive to a negative response-reinforcer contingency, even though it does not eliminate responding. Within individual pigeons, two kinds of trials were compared: red key trials, in which reinforcement was negatively contingent on responding, and white key trials, in which reinforcement was unrelated to responding. Reinforcement frequency in non-contingent trials was yoked to the obtained reinforcement frequency in negatively contingent trials. All eight pigeons pecked substantially more on the non-contingent key than on the negative key, and preferred the non-contingent key to the negative key on occasional "choice" trials where both were presented together. When the stimuli correlated with the two conditions were reversed, the pigeons' behavior also shifted. These response differences are taken as evidence that pigeons are sensitive to the negative response-reinforcer contingency.

Journal Article↗
PubMed1972

Two different kinds of key peck in the pigeon: some properties of responses maintained by negative and positive response-reinforcer contingencies.

Pigeons emitted almost exclusively short-duration key pecks (shorter than 20 msec) when on negative automaintenance procedures, in which pecks prevented reinforcement. Peck durations under fixed-interval and fixed-ratio reinforcement schedules were generally two to five times longer than pecks under a negative automaintenance schedule. However, initial key pecks were of short duration, independent of procedure. The frequency of short-duration pecks was insensitive to differential reinforcement, while the frequency of long-duration pecks was sensitive to differential reinforcement. It is proposed that short-duration pecks arise from the pigeon's normal feeding pattern and are directly enhanced by food presentation, while long-duration pecks are controlled by the contingent effects of food presentation. The implications of the existence of two classes of pecks for the functional definition of operants and the separation of phylogenetic and ontogenetic sources of control of key pecking are discussed.

Journal Article↗