The Formation of Cooperative Cell Assemblies in the Visual Cortex

Abstract

SummaryDuring a critical period of postnatal development of the mammalian visual cortex, synaptic connections are susceptible to use-dependent modifications. Synaptic connections strengthen if pre-and postsynaptic elements are active simultaneously and postsynaptic depolarization is sufficient to allow for the activation of A'-methyl-D-aspartate (NMDA)-receptor-gated conductances. By contrast, synaptic gain decreases if postsynaptic activation exceeds a critical threshold and presynaptic afferents are not capable of activating NMDAreceptor-dependent conductances. These processes lead to selective stabilization of connections between neuronal elements which often exhibit correlated activity and thus modify connectivity according to functional criteria. It is suggested that such experience-dependent selection of circuits serves different purposes at different levels of visual processing. At the input stage to the striate cortex it contributes to optimize the match between the representations of the two eyes. At a later stage of processing it participates in the development of selective connections between cortical columns and thereby serves to establish neuronal representations for frequently occurring constellations of features.Use-dependent changes of synaptic gain can also be induced in the mature visual cortex. These modifications follow the same rules as those occurring during early development and appear to depend on similar molecular mechanisms. However, in the adult the changes of synaptic gain do not seem to be followed by major rearrangements of connectivity. This suggests developmental alterations in mechanisms responsible for growth, removal and stabilization of synaptic connections. Actually, many of the cellular mechanisms thought to be involved in usedependent synaptic plasticity change during development but it is still unclear which of them are responsible for the definitive stabilization of functionally confirmed pathways. Functions requiring use-dependent selection of neuronal connections Fusion and stereopsisHigher mammals and humans, which have frontally positioned eyes with overlapping visual fields, can fuse the images of the two eyes and compute from their differences the distance of objects in space. The basis for this function are J|eurons in the visual cortex which possess two receptive fields, one in each eye,

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“…4). The formation of such assemblies has been related to system function by a number of researchers studying other systems (Singer, 1990;Abeles et al, 1993;Vaadia et al, 1995;Stopfer et al, 1997;Nicolelis et al, 1998;Laurent, 1999;Christensen et al, 2000). Although it is difficult to ascertain whether interactions between neurons provide "more" codingrelevant information than single-neuron firing patterns (as they seem to in olfaction), elimination of this information via random time shifting of individual responses significantly degraded the ability of the ensembles to discriminate between applied tastants (Fig.…”

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“…A correspondência retínica dos dois olhos é similar com relação à orientação, tamanho e resposta à velocidade ou à direção de movimento. Via de regra, o estímulo que vem de um olho predomina e produz resposta neuronal maior (31)(32)(33) . Não foi encontrada nenhuma análise relacionada à dominância ocular e relaxante limbar na literatura.…”

Insights into Visual Cortex Plasticity: Interaction Between Genes and Sensory Experience

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