title: Concurrent schedule performance of possums: a comparison of two models

authors: Bron, Angela

abstract: These experiments examined the behaviour of brushtail possums (Trichosurus vulpecula) under concurrent variable-interval schedules of reinforcement. In the first experiment the lever pressing of six possums resulted in intermittent access to a barley/carob mixture under four pairs of variable-interval schedules. In most respects, the behaviour of the possums was similar to that observed with other species. However, the degree of undermatching of the response ratios to the obtained reinforcement-rate ratios was greater than is normally observed with these schedules. Both the Generalised Matching Law and the Contingency-Discriminability model described the data well, although, where overmatching was obtained, the parameter values given by the Contingency-Discriminability model did not make sense in terms of the original assumptions of the model. The second experiment examined the possibility that the undermatching related to the length of the changeover delay used. Six possums were exposed to five different pairs of concurrent schedules at each of four different changeover-delay lengths (ranging from 0 to 6 s). Time allocation and post-changeover delay responses more closely approximated matching with a 2-s changeover delay than with a 0-s delay, but no further changes in sensitivity were observed with further increases in changeover-delay length. Overmatching was consistently observed in the post changeover delay data, resulting in un-interpretable parameter values from Contingency-Discriminability analyses. The addition of the punishment parameter (w) to the Contingency-Discriminability model, suggested to deal with such data, did not generally result in more sensible parameter estimates. The third experiment attempted to bias possums responding using qualitatively different reinforcers. Equal variable-interval schedules were used with the barley/carob mixture available on one schedule and Cocopopsᵀᴹ or coconut on the other. Possums’ behaviour was biased by these foods, with a small bias away from the barley/carob mixture resulting from the Cocopops and a large bias towards the barley/carob mixture resulting from the coconut. In the fourth experiment, graded point estimates of bias were obtained by presenting four different concentrations of salted barley/carob mixture (ranging from 0% to 6%) on one schedule and plain barley/carob mixture on the other schedule. Small biases were found with 0% and 2% salt, and large biases were found towards the barley/carob mixture with 4% and 6% salt. To assess whether bias remains constant with changes in the reinforcer-rate ratio, further conditions were conducted with 4% and 6% salt, with four pairs of unequal concurrent variable-interval schedules. Analyses of these data with both the Generalised Matching Law and the Contingency-Discriminability Model revealed an interaction between bias and the reinforcement-rate ratio. This interaction may be due to an improvement in discriminability due to the different reinforcers. The behaviour of the possums was generally well described by both the Generalised Matching Law and the Contingency-Discriminability model. Overall, neither model was better at describing behaviour under the above conditions, although several problems were found with the application of the Contingency-Discriminability model.

text: Concurrent schedule performance of possums: a comparison of two models These experiments examined the behaviour of brushtail possums (Trichosurus vulpecula) under concurrent variable-interval schedules of reinforcement. In the first experiment the lever pressing of six possums resulted in intermittent access to a barley/carob mixture under four pairs of variable-interval schedules. In most respects, the behaviour of the possums was similar to that observed with other species. However, the degree of undermatching of the response ratios to the obtained reinforcement-rate ratios was greater than is normally observed with these schedules. Both the Generalised Matching Law and the Contingency-Discriminability model described the data well, although, where overmatching was obtained, the parameter values given by the Contingency-Discriminability model did not make sense in terms of the original assumptions of the model. The second experiment examined the possibility that the undermatching related to the length of the changeover delay used. Six possums were exposed to five different pairs of concurrent schedules at each of four different changeover-delay lengths (ranging from 0 to 6 s). Time allocation and post-changeover delay responses more closely approximated matching with a 2-s changeover delay than with a 0-s delay, but no further changes in sensitivity were observed with further increases in changeover-delay length. Overmatching was consistently observed in the post changeover delay data, resulting in un-interpretable parameter values from Contingency-Discriminability analyses. The addition of the punishment parameter (w) to the Contingency-Discriminability model, suggested to deal with such data, did not generally result in more sensible parameter estimates. The third experiment attempted to bias possums responding using qualitatively different reinforcers. Equal variable-interval schedules were used with the barley/carob mixture available on one schedule and Cocopopsᵀᴹ or coconut on the other. Possums’ behaviour was biased by these foods, with a small bias away from the barley/carob mixture resulting from the Cocopops and a large bias towards the barley/carob mixture resulting from the coconut. In the fourth experiment, graded point estimates of bias were obtained by presenting four different concentrations of salted barley/carob mixture (ranging from 0% to 6%) on one schedule and plain barley/carob mixture on the other schedule. Small biases were found with 0% and 2% salt, and large biases were found towards the barley/carob mixture with 4% and 6% salt. To assess whether bias remains constant with changes in the reinforcer-rate ratio, further conditions were conducted with 4% and 6% salt, with four pairs of unequal concurrent variable-interval schedules. Analyses of these data with both the Generalised Matching Law and the Contingency-Discriminability Model revealed an interaction between bias and the reinforcement-rate ratio. This interaction may be due to an improvement in discriminability due to the different reinforcers. The behaviour of the possums was generally well described by both the Generalised Matching Law and the Contingency-Discriminability model. Overall, neither model was better at describing behaviour under the above conditions, although several problems were found with the application of the Contingency-Discriminability model.

year: 2001

title: The time machine. Norman Taylor Memorial Lecture 2002

authors: Lowe, David J.

abstract: I've chosen to talk about five topics dealing with longstanding pedological or paleoenvironmental problems involving tephrostratigraphy. I'll begin with the oldest tephras and work 'stratigraphically upwards' to the youngest. Along the way I'll mention or show a few of the people I've worked with, or who were involved historically in the research areas. I've allowed around five or six minutes for each topic (but I'd be grateful if you didn't set your stopwatches!). As promised, the second part of the talk will begin when we reach the year 1952. Among a range of personal observations relating to the birth of the society and to Norman Taylor, I will attempt to explain how I came to be giving this lecture today.

text: The time machine. Norman Taylor Memorial Lecture 2002 I've chosen to talk about five topics dealing with longstanding pedological or paleoenvironmental problems involving tephrostratigraphy. I'll begin with the oldest tephras and work 'stratigraphically upwards' to the youngest. Along the way I'll mention or show a few of the people I've worked with, or who were involved historically in the research areas. I've allowed around five or six minutes for each topic (but I'd be grateful if you didn't set your stopwatches!). As promised, the second part of the talk will begin when we reach the year 1952. Among a range of personal observations relating to the birth of the society and to Norman Taylor, I will attempt to explain how I came to be giving this lecture today.

year: 2002

title: Telomere length in early childhood is associated with sex and ethnicity

authors: Ly, Kien

subjects: Science & Technology

abstract: Telomeres are repetitive DNA sequences at the end of chromosomes that function to protect chromosomes from degradation. Throughout the life course, telomere length decreases with age and is influenced by environmental factors and health conditions. This study aimed to determine the relative telomere lengths in a diverse cohort of about 4000 four-year-old children in New Zealand. Linear regression was used to investigate the relationship between telomere length, child gender, ethnicity, paternal age and deprivation. We observed substantial variation in telomere length according to sex and self-identified ethnicity. Telomere length was longer in females compared to males (coefficient of 0.042, 95% confidence interval (CI) 0.024–0.060). European children had shorter telomere than both the indigenous Māori (coefficient of 0.03, CI 0.007–0.055) and Pacific children (coefficient of 0.15, CI 0.12–0.18). The data suggest that telomere lengths are highly variable and variability between individuals arise from early age, influenced partly by sex and ethnicity. Longer telomeres in indigenous Māori and Pacific children may reflect the heritability of telomere length in genetically less complex populations. This study increases our understanding of telomere dynamics in young children since the majority of telomere studies are conducted in adults.

text: Telomere length in early childhood is associated with sex and ethnicity Telomeres are repetitive DNA sequences at the end of chromosomes that function to protect chromosomes from degradation. Throughout the life course, telomere length decreases with age and is influenced by environmental factors and health conditions. This study aimed to determine the relative telomere lengths in a diverse cohort of about 4000 four-year-old children in New Zealand. Linear regression was used to investigate the relationship between telomere length, child gender, ethnicity, paternal age and deprivation. We observed substantial variation in telomere length according to sex and self-identified ethnicity. Telomere length was longer in females compared to males (coefficient of 0.042, 95% confidence interval (CI) 0.024–0.060). European children had shorter telomere than both the indigenous Māori (coefficient of 0.03, CI 0.007–0.055) and Pacific children (coefficient of 0.15, CI 0.12–0.18). The data suggest that telomere lengths are highly variable and variability between individuals arise from early age, influenced partly by sex and ethnicity. Longer telomeres in indigenous Māori and Pacific children may reflect the heritability of telomere length in genetically less complex populations. This study increases our understanding of telomere dynamics in young children since the majority of telomere studies are conducted in adults.

year: 2019