A plausible neural circuit for classical conditioning without synaptic plasticity.

The cellular bases of learning are currently under active investigation by both experimental and theoretical means. In this paper, a simple neuronal wiring diagram is proposed that can reproduce both simple and higher-order behavioral paradigms seen in invertebrate classical conditioning experiments. Learning in this model does not take place by modification of synaptic strength values. Instead, the model uses a layer of interneurons with modifiable thresholds for spike initiation, as suggested by the plasticity mechanisms thought to operate in Hermissenda [Alkon, D. L. (1983) Sci. Am. 249, 70-84]. The model therefore has an advantage in plausibility compared with more standard models using Hebb synapses or their functional equivalents, which have not yet been demonstrated in any invertebrate organism.