Brain electrophysiological processes take place in a variety of spatial and temporal scales. In a microscopic scale down to a single neuron, the electrophysiological activity manifests itself as the neuronal spiking activity and the post-synaptic potential (PSP). The neuronal spiking activity refers to the action potentials that propogate along the axon. The spiking activity reaches and accumulates at the synapse through which one neuron connects to another. When the accumulated spiking activity exceeds a certain threshold, it controls the amount of chemical transmitters released from the synapse. The released neurotransmitters further control the gating of the ion channels at the post-synaptic neuron and hence modulates the PSP. For the spiking activity above the threshold, the synapse effectively behaves as a low-pass filter, owing to its accumulative effect and the delay due to the electrochemical conversion. Under such circumstances, the pre-synaptic spiking activity may be temporally correlated with the PSP. However, for the sub-threshold spiking activity, such correlation does not necessarily hold true.
