| Mechanisms of plasticity in neurons must serve two goals. They must allow for adaptive modification in the service of learning and memory processes, but they must also allow for homeostatic modification in order to maintain an operating regime that is both stable and useful to its computational ends. These two goals are frequently at odds with one another. While mechanisms of learning might tend to disrupt stable operations, mechanisms of homeostasis might tend to undo valuable learning. Nevertheless, both are essential to the proper function of neurons and neural networks. Synaptic plasticity is no exception. Synapses are subject to both Hebbian learning, which distinguishes between the more and less functionally important synaptic connections, and, as experiments such as those showing distance-dependent synaptic scaling have made clear, homeostatic-type plasticity, which ensures that all synapses have an equal opportunity to participate in the output of a neuron.; This dissertation uses mathematical analysis in addition to computational modeling of multi-compartment neuron models, including equivalent cable models as well as morphologically accurate neuron models, in order to explore these phenomena. Specifically, it is shown that a form of spike-timing dependent plasticity (STDP), called anti-STDP, combined with a form of activity-dependent plasticity provides a powerful homeostatic mechanism that can be used to explain distance-dependent synaptic scaling. It acts to equalize synaptic efficacies and generates location-independent synapses. In doing so, it gives rise to other interesting consequences such as a normalization of dendritic action potential generation. It is proposed that anti-STDP provides a necessary counterpart to conventional Hebbian plasticity in order for synaptic plasticity to achieve its two disparate goals. Without a normalizing force such as anti-STDP, Hebbian plasticity would be driven in large part by location-dependent biases rather than functionally important differences. This dissertation explores some of the attributes and consequences of anti-STDP and makes a theoretical argument for its existence. |
