  We describe general characteristics of the Hodgkin-Huxley neuron's response
to a periodic train of short current pulses with Gaussian noise. The
deterministic neuron is bistable for antiresonant frequencies. When the stimuli
arrive at the resonant frequency the firing rate is a continuous function of
the current amplitude $I_0$ and scales as $(I_0-I_{th})^{1/2}$, where $I_{th}$
is an approximate threshold. Intervals of continuous irregular response
alternate with integer mode-locked regions with bistable excitation edge. There
is an even-all multimodal transition between the 2:1 and 3:1 states in the
vicinity of the main resonance, which is analogous to the odd-all transition
discovered earlier in the high-frequency regime. For $I_0<I_{th}$ and small
noise the firing rate has a maximum at the resonant frequency. For larger noise
and subthreshold stimulation the maximum firing rate initially shifts towards
lower frequencies, then returns to higher frequencies in the limit of large
noise. The stochastic coherence antiresonance, defined as the maximum of the
coefficient of variation as a function of noise intensity, occurs over a wide
range of parameter values, including monostable regions.
