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D J Willshaw

Publications and source records attributed to D J Willshaw.

14 recordsLinked to original sources
PubMed1991

Optimising synaptic learning rules in linear associative memories.

Associative matrix memories with real-valued synapses have been studied in many incarnations. We consider how the signal/noise ratio for associations depends on the form of the learning rule, and we show that a covariance rule is optimal. Two other rules, which have been suggested in the neurobiology literature, are asymptotically optimal in the limit of sparse coding. The results appear to contradict a line of reasoning particularly prevalent in the physics community. It turns out that the apparent conflict is due to the adoption of different underlying models. Ironically, they perform identically at their co-incident optima. We give details of the mathematical results, and discuss some other possible derivations and definitions of the signal/noise ratio.

Animals↗
PubMed1990

An assessment of Marr's theory of the hippocampus as a temporary memory store.

The recent reawakened interest in 'neural' networks begs the question of their relevance to the analysis of real nervous systems. Network models have been criticized for the lack of realism of their individual components, and because the architectures required by some neural-network algorithms do not seem to exist in real nervous systems. In three related papers published in the 1970s, David Marr proposed that the cerebellum, the neocortex and the hippocampus each acts as a memorizing device. These theories were intended to satisfy the biological constraints, but in computational terms they are undetermined. In this paper we reassess Marr's theory of the hippocampus as a temporary memory store. We give a complete computational account of the theory and we show that Marr's computational arguments do not sufficiently constrain his choice of model. We discuss Marr's specific model of temporary memory with reference to the neurophysiology and neuroanatomy of the mammalian hippocampus. Our analysis is supported by simulation studies done on various memory models built according to the principles advocated by Marr.

Animals↗
PubMed1986

The discontinuous visual projections on the Xenopus optic tectum following regeneration after unilateral nerve section.

The establishment of retinotectal projections following transection of one optic nerve in developing Xenopus has been investigated. Between 3 weeks and 11 months after the operation, the nerve fibre tracer horseradish peroxidase (HRP) was applied to either the operated or the unoperated nerve, and the brains were prepared for examination as whole mounts. In most cases fibres from the operated nerve innervated both tecta, with the result that one tectum was doubly innervated and one tectum singly innervated. Two months after transection of the optic nerve in tadpole life, between stages 50 and 54, this nerve usually made a uniform projection on the contralateral tectum and a striped projection on the ipsilateral, doubly innervated, tectum. The projection made by the unoperated nerve on this tectum was a similar pattern of stripes, which ran generally rostrocaudally. Two months after transection of the optic nerve of newly metamorphosed animals, the projection formed by the operated nerve on the doubly innervated tectum was usually a pattern of spots or spots mixed together with stripes in no particular orientation superimposed on a roughly uniform background. In a small number of cases the projections made by the same nerve on the two tecta were approximately complementary; that is, the presence of label on one tectum corresponded with its absence on the other tectum. The results are examined in the context of the development of the retina and of the tectum. It is suggested that the consistently oriented stripes which result from nerve transection at a stage at which only a small proportion of the retinal fibres had reached the tectum are formed by the interaction of two equally matched sets of developing fibres, stripe orientation being determined by the mode of growth of the optic tectum. The formation of patterns of spots or spots mixed together with stripes following nerve transection after the end of the main phase of tectal histogenesis, and when 50% of the optic fibres had already reached the tectum, is attributed to an unequal competition between the two sets of fibres.

Animals↗
PubMed1985

The distribution of fibres in the optic tract after contralateral translocation of an eye in Xenopus.

In Xenopus embryos of stage 30 the right eye was translocated, without rotation, to a left host orbit. Shortly after metamorphosis the visuotectal projection through the operated eye was mapped electrophysiologically and shown to be normal dorsoventrally but reversed nasotemporally. Labelling of small groups of retinal axons with HRP showed that the fibre trajectories from dorsal and ventral retina were normal, whereas fibres from nasally placed retina had diencephalic pathways and tectal terminations typical and temporal fibres, and fibres from temporally placed retina had diencephalic pathways and tectal terminations typical of nasal fibres. Thus from just beyond the chiasma the fibres had already achieved the major uniaxial rearrangement necessary to establish a normal tract distribution despite the eye translocation. The fibre rearrangement required to permit the formation of a nasotemporally inverted visuotectal projection appears, therefore, to occur not on the tectum or in the optic tract, but either within the nerve or at the chiasma.

Animals↗
PubMed1977

How to label nerve cells so that they can interconnect in an ordered fashion.

We present a method for setting up topographically ordered mappings between two sheets of nerve cells. A set of chemical markers that express the neighborhood relationships within the presynaptic sheet is induced by the fibers into the postsynaptic sheet. The markers are used to guide the fibers to their terminal sites. A case for which this idea may be relevant is the retinotectal projection; our model exhibits types of plasticity found experimentally. The fact that the postsynaptic markers remain after removal of the projecting fibers suggests an important difference between development and regeneration. This paper concentrates on explaining the basic idea, and in addition presents a set of preliminary computer simulations.

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