Experience-dependent modification of synaptic plasticity in visual cortex

Abstract

In many regions of the cerebral cortex, Ca2+ influx through NMDA (N-methyl-D-aspartate) sensitive glutamate receptors (NMDA receptors) can trigger two forms of synaptic plasticity: long-term depression (LTD) and long-term potentiation (LTP). LTD is induced by low levels of postsynaptic NMDA-receptor activation, for instance in response to low-frequency stimulation, whereas LTP is induced by the stronger activation that occurs following high-frequency stimulation. Theoretical studies have shown that the properties of synaptic LTD and LTP can account for many aspects of experience-dependent plasticity in the developing visual cortex, provided that the LTD-LTP crossover point (the modification threshold, theta(m)) varies as a function of the history of cortical activity. Here we provide direct experimental evidence that the value of theta(m) depends on sensory experience. We find in visual cortex of light-deprived rats that LTP is enhanced and LTD diminished over a range of stimulation frequencies, and that these effects can be reversed by as little as two days of light exposure. Our findings support the idea that a variable synaptic modification threshold allows synaptic weights in neural networks to achieve a stable equilibrium.

Extracellular matrix inhibits structural and functional plasticity of dendritic spines in the adult visual cortex

“…LTP has been studied in the visual cortex in acute cortical slices where it has been found that LTP of layer 2/3 neurons is developmentally regulated and decreases drastically in the adult cortex at the end of the critical period 28 . Here, we asked whether the enhanced spine motility in ChABC-treated adult mice was accompanied by enhanced functional plasticity of L5 neurons.…”

Contextually guided unsupervised learning using local multivariate binary processors

“…An alternative approach, based on simple non-parametric estimation of the conditional averages as functions the integrated elds, may also be computed on-line and involves a storage cost that depends only on the resolution employed in the nonparametric estimators and the numberof units in the processor, but not on the dimensionality of the RF and CF input-spaces hence, the storage cost is linear in the number of output units in the processor The form of the general class of learning rules derived formally from the informationtheoretic objectives in Section 4 (see also Kay & Phillips, 1994 closely resembles the structure of the BCM learning rule which has already been shown to be both computationally powerful and to possess functionality similar to that found in biological forms of learning. See Intrator & Cooper (1995) for a review of the BCM rule, and Kirkwood et al (1996) for further detailed evidence on its relevance to synaptic plasticity in mammalian visual cortex. Both rules have a threshold that depends on a dynamically-computed average of prior activity, t h e main di erence between them being that the Coherent Infomax approach takes contextual inputs into account when computing this average whereas the BCM rule does not.…”

Retinocollicular Synapse Maturation and Plasticity Are Regulated by Correlated Retinal Waves

“…More mature synapses acquire transmission through AMPA receptors (AMPARs) over the course of normal development, and these effects can be hastened by experimentally inducing LTP (Liao et al, 1995;Durand et al, 1996;Wu et al, 1996;Isaac et al, 1997). NMDAR-dependent LTP is also observed during developmental periods in which activity can significantly affect patterns of structural connectivity (Crair and Malenka, 1995;Kirkwood et al, 1996), suggesting that activitydependent increases in synaptic strength may contribute to the stabilization of synaptic connections. Interestingly, both the prevalence of silent synapses and the capacity for LTP induction decline at a similar rate with age in the neocortex (Crair and Malenka, 1995;Kirkwood et al, 1995;Isaac et al, 1997;Rumpel et al, 1998;Franks and Isaacson, 2005).…”