Dynamic synaptic modification threshold: computational model of experience-dependent plasticity in adult rat barrel cortex.

Abstract

Previous electrophysiological experiments have documented the response of neurons in the adult rat somatic sensory ("barrel") cortex to whisker movement after normal experience and after periods of experience with all but two whiskers trimmed close to the face (whisker "pairing9). To better understand how the barrel cortex adapts to changes in the flow of sensory activity, we have developed a computational model of a single representative barrel cell based on the Bienenstock, Cooper, and Munro (BCM) theory of synaptic plasticity. The hallmark of the BCM theory is the dynamic synaptic modification threshold, Om, which dictates whether a neuron's activity at any given instant will lead to strengthening or weakening of the synapses impinging on it. The threshold OM is proportional to the neuron's activity averaged over some recent past. Whisker pairing was simulated by setting input activities of the cell to the noise level, except for two inputs that represented untrimmed whiskers. Initially low levels of cell activity, resulting from whisker trimming, led to low values for M. As certain synaptic weights potentiated, due to the activity of the paired inputs, the values of OM increased and after some time their mean reached an asymptotic value. This saturation of OM led to the depression of some inputs that were originally potentiated. The changes in cell response generated by the model replicated those observed in in vivo experiments. Previously, the BCM theory has explained salient features of developmental experience-dependent plasticity in kitten visual cortex. Our results suggest that the idea of a dynamic synaptic modification threshold, OM, is general enough to explain plasticity in different species, in different sensory systems, and at different stages of brain maturity.Although some forms of experience-dependent cortical plasticity disappear at the end of a developmental "critical" period (1, 2), the adult cortex retains a significant capacity to undergo functional changes in response to alterations in sensory input (3,4). We are interested in the rules that determine how the adult rat whisker system adapts to changes in the pattern ofafferent activity. The facial whiskers of rats are aligned in five rows (row A is dorsal and row E is ventral) and the whiskers within a row are numbered from caudal to rostral. Each facial whisker projects via the trigeminal nuclei and the thalamic ventral posterior medial nucleus (VPM) to a separate cluster ofneurons in layer IV ofa cortical barrel (5-8). Recently, we observed that the receptive fields (RFs) of neurons in adult rat barrel cortex were changed by altered whisker use (refs. 9-11; M.E.D., L.B., M. A. Nicolelis, F.F.E., and J. K. Chapin, unpublished work). To alter the pattern of sensory activity, all whiskers except two, D2 and one neighbor in the D row, were cut close to the fur on one side of the face. After 3, 7-10, or 30 days of whisker pairing (whiskers were reclipped regularly), the activity of single neurons in barrel D2 was measured in response...

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“…BCM-like changes in plasticity induction account well for a variety of phenomena, including experience-dependent changes in visual and somatosensory neural activity (Benusková et al, 1994;Kirkwood et al, 1996;Philpot et al, 2003Philpot et al, , 2007 and synaptic plasticity in the dentate gyrus (Benuskova and Abraham, 2007). BCM-like metaplasticity effects over a shorter timescale have also been observed in the hippocampus, providing a tractable experimental preparation for testing the two key postulates noted above.…”

A metaplasticity view of the interaction between homeostatic and Hebbian plasticity

“…It was first examined in the visual system, where sensory experience could be altered and the subsequent effect on synaptic modification rules could be examined. Evidence for the sliding modification threshold was eventually obtained in multiple brain regions beyond the visual system [7][8][9][10][11][12][13].…”