Inter-Spike Interval

The inter-spike interval (ISI) is the time between two consecutive spikes in a spike train, measured in milliseconds.

Why it matters: ISI distributions reveal fundamental properties of neuronal firing: refractory periods (minimum ISI ~1–2 ms confirms single-unit isolation), bursting behavior (clusters of short ISIs), and firing regularity.

ISI histogram: Plotting the distribution of ISIs from a spike train produces an ISI histogram — a standard quality metric for spike sorting and a tool for classifying neuron types.

Key metrics: Mean ISI (inverse of firing rate), coefficient of variation (regularity), burst index (fraction of ISIs <10 ms), and refractory period violations (ISIs <1 ms suggesting sorting errors).