Effects of LTP on response selectivity of simulated cortical neurons.

We report here on specific ways in which synaptic long-term potentiation (LTP) affects the response selectivity of primary sensory cortical cells. LTP increases synaptic efficacy by incremental "steps," up to a "ceiling" at which additional bursts of afferent stimulation cause no further potentiation. Endogenous and exogenous agents have been shown to modulate these two paramenters of LTP, raising the question of the functional implications associated with the sizes of steps and ceiling. We provide an analytical treatment of the effects of these two physiological LTP parameters on the behavior of simulated olfactory (piriform) cortex target cells in response to a range of inputs. A target cell's receptive field, i.e., the set of input patterns to which the cell responds, is broadened with potentiation of the cell's synapses, and is broadened more when the LTP step size is smaller, and when the LTP ceiling is higher. Moreover, the effects of step size and ceiling interact, and their joint relationship to receptive field breadth is nonlinear. Values of step size and ceiling are identified that balance the tradeoff between learning rate and receptive field breadth for particular sensory recognition tasks, and these model values are compared to corresponding known and inferred physiological values.