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T Elliott

Publications and source records attributed to T Elliott.

130 records · Page 8Linked to original sources
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Axonal processes and neural plasticity.I: Ocular dominance columns.

We present two related computational models of ocular dominance column formation. Both address nervous system plasticity in terms of sprouting and retraction of axonal processes rather than changes in synaptic strength implied by synapse-specific Hebbian models. We employ statistical mechanics to simulate changes in the pattern of network connectivity. Our formalism uses the concept of an energy function, which we interpret as related to the levels of target-generated neurotrophins for which afferents compete. In contrast, synapse-specific Hebbian models impose synaptic normalization, for which there is little experimental evidence, in order to induce competition. Our models make many predictions which require experimental investigation. We suggest that the absence of monocular deprivation effects in the optic tectum may be due to a tendency of amphibian retinal ganglion cells to preserve the complexity of their terminal arbors. One model raises the possibility that boundaries separating columns in the mammalian cortex are poorly innervated if they have been formed by complete but asynchronous retinal activation. Both models exhibit a phase transition, suggesting a discontinuity in the transition from a binocular cortex to one possessing ocular dominance columns. Finally, our other model could account for the perpendicularity of ocular dominance columns to the boundary of the primary visual cortex while admitting of less ordered central patterns.

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Axonal processes and neural plasticity. II: Adult somatosensory maps.

Following our recent presentation of an axonal process sprouting and retraction framework for ocular dominance column formation, we now apply it, unchanged, to address issues of adult somatosensory map plasticity. Specifically, we model the rearrangement of S-I in adult rodents following denervation of a row of vibrissae, and the rearrangement of area 3b in adult monkeys following hyperstimulation of a digit. While we do not attempt to capture the rapid changes which occur as the result of unmasking or potentiating existing connections, we demonstrate that axonal process sprouting and retraction is a possible mechanism mediating many of the long-term changes induced by anomalous peripheral activity. A significant feature of our framework, demonstrated by this study, is that it can account for plasticity in both developing and mature systems, and in different sensory modalities. In contrast, synapse-specific Hebbian models with synaptic normalization, which employ anatomically fixed connections capable of changes in efficacy, may not be able to account for both developmental and adult plasticity without the form of the imposed normalization, which enforces competition between afferents, being changed.

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