  In sensory neurons the presence of noise can facilitate the detection of weak
information-carrying signals, which are encoded and transmitted via correlated
sequences of spikes. Here we investigate relative temporal order in spike
sequences induced by a subthreshold periodic input, in the presence of white
Gaussian noise. To simulate the spikes, we use the FitzHugh-Nagumo model, and
to investigate the output sequence of inter-spike intervals (ISIs), we use the
symbolic method of ordinal analysis. We find different types of relative
temporal order, in the form of preferred ordinal patterns which depend on both,
the strength of the noise and the period of the input signal. We also
demonstrate a resonance-like behavior, as certain periods and noise levels
enhance temporal ordering in the ISI sequence, maximizing the probability of
the preferred patterns. Our findings could be relevant for understanding the
mechanisms underlying temporal coding, by which single sensory neurons
represent in spike sequences the information about weak periodic stimuli.
