Dynamic Synaptic Modiication Threshold: Computational Model of Experience-dependent Plasticity in Adult Rat Barrel Cortex

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 "pairing"). In order to better understand how the barrel cortex adapts to changes in the ow 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 modiication threshold, M , 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 M 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 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 M increased and after some time their mean reached an asymptotic value. This saturation of M 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 modiication threshold, M , is general enough to explain plasticity in diierent species, in diierent sensory systems and at diierent 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 signiicant 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 of aaerent activity. The facial whiskers of rats are aligned in 5 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 of neurons in layer IV of a cortical barrel …