  Numerical investigations have been made of responses of a Hodgkin-Huxley (HH)
neuron to spike-train inputs whose interspike interval (ISI) is modulated by
deterministic, semi-deterministic (chaotic) and stochastic signals. As
deterministic one, we adopt inputs with the time-independent ISI and with
time-dependent ISI modulated by sinusoidal signal. The R\"{o}ssler and Lorentz
models are adopted for chaotic modulations of ISI. Stochastic ISI inputs with
the Gamma distribution are employed. It is shown that distribution of output
ISI data depends not only on the mean of ISIs of spike-train inputs but also on
their fluctuations. The distinction of responses to the three kinds of inputs
can be made by return maps of input and output ISIs, but not by their
histograms. The relation between the variations of input and output ISIs is
shown to be different from that of the integrate and fire (IF) model because of
the refractory period in the HH neuron.
