Computational consequences of temporally asymmetric learning rules: II. Sensory image cancellation

The electrosensory lateral line lobe (ELL) of mormyrid electric fish is a c erebellum-like structure that receives primary afferent input from electror eceptors in the skin. Purkinje-like cells in ELL store and retrieve a tempo rally precise negative image of prior sensory input. The stored image is de rived from the association of centrally originating predictive signals with peripherally originating sensory input. The predictive signals are probabl y conveyed by parallel fibers. Recent in vitro experiments have demonstrate d that pairing parallel fiber-evoked excitatory postsynaptic potentials (ep sps) with postsynaptic spikes in Purkinje-like cells depresses the strength of these synapses. The depression has a tight dependence on the temporal o rder of pre- and postsynaptic events. The postsynaptic spike must follow th e onset of the epsp within a window of about 60 msec for the depression to occur and pairings at other delays yield a nonassociative enhancement of th e epsp. Mathematical analyses and computer simulations are used here to tes t the hypothesis that synaptic plasticity of the type established in vitro could be responsible for the storage of temporal patterns that is observed in vivo. This hypothesis is confirmed. The temporally asymmetric learning r ule established in vitro results in the storage of activity patterns as obs erved in vivo and does so with significantly greater fidelity than other ty pes of learning rules. The results demonstrate the importance of precise ti ming in pre- and postsynaptic activity for accurate storage of temporal inf ormation.