  At the single-neuron level, precisely timed spikes can either constitute
firing-rate codes or spike-pattern codes that utilize the relative timing
between consecutive spikes. There has been little experimental support for the
hypothesis that such temporal patterns contribute substantially to information
transmission. By using grasshopper auditory receptors as a model system, we
show that correlations between spikes can be used to represent behaviorally
relevant stimuli. The correlations reflect the inner structure of the spike
train: a succession of burst-like patterns. We demonstrate that bursts with
different spike counts encode different stimulus features, such that about 20%
of the transmitted information corresponds to discriminating between different
features, and the remaining 80% is used to allocate these features in time. In
this spike-pattern code, the what and the when of the stimuli are encoded in
the duration of each burst and the time of burst onset, respectively. Given the
ubiquity of burst firing, we expect similar findings also for other neural
systems.
