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Biomedical subjects

C I Howarth

Publications and source records attributed to C I Howarth.

At least 19 recordsLinked to original sources
PubMed1998

Axonal processes and neural plasticity: a reply.

We examine the claim that a class of sprouting-and-retraction models is mathematically equivalent to a fixed-anatomy model. We accept, subject to important caveats, a narrow mathematical equivalence of the energy functions in both classes of model. We argue that this narrow equivalence of energy functions does not, however, entail equivalence of the models. Indeed, the claim of complete model equivalence hides significant dynamical differences between the approaches, which we discuss. We also disagree that our work demonstrates that subtractive constraint enforcement is natural in fixed-anatomy models.

Animals↗
PubMed1997

Axonal processes and neural plasticity. III. Competition for dendrites.

In previous work we have developed a computational framework for topographic map formation and plasticity based on axonal process sprouting and retraction, in which sprouting and retraction are governed by competition for neurotrophic support. Here we show that such an approach can account for certain aspects of the dendritic morphology of cortical maps. In particular, we model the development of ocular dominance columns in the primary visual cortex and show that cortical cells near to column boundaries prefer to elaborate dendritic fields which avoid crossing the boundaries. This emerges as different functional inputs are spatially separated. We predict that afferent segregation occurs before or simultaneously with, but not after, the emergence of dendritic bias. We predict that animals reared with complete but asynchronous stimulation of the optic nerves do not develop a dendritic bias. We suggest that the emergence of a dendritic bias might provide a partial account for the critical period for a response to monocular deprivation. In particular, we predict that animals reared with asynchronous optic nerve stimulation might exhibit an extended critical period. Our results also indicate that the number of synapses supported by cortical cells depends on the intra-ocular image correlations used in our simulations. This suggests that inter-ocular image correlations, and thus strabismic rearing of kittens, may also affect the innervation density.

Afferent Pathways↗
PubMed1996

Neural competition and statistical mechanics.

Computational models of activity-dependent competitive neural plasticity typically impose competition in networks in which plasticity is accommodated by permitting changes in the efficacies of synapses in an anatomically fixed network. This is despite the fact that much evidence suggests that neurons compete for neurotrophins, during both target innervation and activity-dependent synaptic re-arrangement, which influence the sprouting and retraction of axonal processes. We therefore present a new approach to the computational modelling of competitive neural plasticity which permits neurons to compete explicitly for neurotrophins. This competition is associated with the sprouting and retraction of axonal processes. Because there is much uncertainty regarding the basic mechanisms, we adopt the powerful machinery of statistical mechanics to avoid the need to address these issues. We show that such an approach can readily account for a wide range of plasticity phenomena in a range of systems, including the results of various pharmacological manipulations.

Animals↗
PubMed1996

The exposure of young children to accident risk as pedestrians.

Pedestrian road accidents show a marked peak for children aged 5, 6 and 7 years with boys twice as involved as girls at these ages. Howarth et al (1974) described a framework in which measures of exposure were defined and related to the accident statistics to obtain estimates of absolute levels of risk for different categories of pedestrian in different traffic situations. The present paper describes a survey of children's exposure carried out to provide suitable data for this quantitative analysis. We interviewed a representative sample of Nottingham schoolchildren about their journeys in the previous 24 hours and recorded the number of roads crossed and the traffic densities of these roads. The measures of exposure obtained are presented in relation to the accompaniment of children on their journeys, the type of area in which they live, and time of day. Risk was assessed by relating exposure measures both to the national and local accident statistics. The analysis provides estimates of the risk to children of different ages and sex in their normal pattern of road crossing and in crossing roads of different traffic density and indicates that the accident statistics alone considerably underestimate the degree of risk to children under the age of eight. Interviews with a sample of the parents of the children suggest that children may provide a more accurate measure of their exposure than do their parents.

Accidents, Traffic↗
PubMed1993

Modelling of safety measure effects and risk compensation.

A model is proposed for the description of effects of safety measures introduced into a road transport system. The model explains the motives of road users' risk compensation towards the measures, and indicates when behavioural adaptation is likely to take place and its effects on road safety programmes. At its core the model has the notion that when a transport system is changed by engineering safety measures, road users do not respond only in the direction towards safety improvement but respond, in general, with three possible ways of behavioural adaptations, one of them towards more risk taking. Supported by illustrative examples, it also suggests that due to risk compensation, engineering safety measures alone are usually not sufficient, and, hopefully, carefully designed motivational safety measures can give us chances to modify road users' behaviour to make traffic safer.

Accidents, Traffic↗
PubMed1982

The structure of conversations with 6- to 10-year-old deaf children.

In stage 1 classroom conversations between sixteen teachers and their pre-lingually deaf children were videotaped and analysed to examine both the styles used by teachers in controlling conversation and the functions pursued in dialogue. In stage 2 a sub-sample of four teachers with twenty children of known hearing losses and non-verbal intelligence was analysed in greater detail to examine relationships between these factors, teaching styles and the child's performance in dialogue. The analyses show that deaf children respond in a similar fashion to young hearing children in the way they react to different styles of teacher talk; that teachers differ in the functions they pursue in conversation; and that functions change as a consequence of the child's hearing loss but not mental age. The implications of the findings for linguistic development in pre-lingually deaf children are explored.

Child↗