  Various classes of neurons alternate between high-frequency discharges and
silent intervals. This phenomenon is called burst firing. To analyze burst
activity in an insect system, grasshopper auditory receptor neurons were
recorded in vivo for several distinct stimulus types. The experimental data
show that both burst probability and burst characteristics are strongly
influenced by temporal modulations of the acoustic stimulus. The tendency to
burst, hence, is not only determined by cell-intrinsic processes, but also by
their interaction with the stimulus time course. We study this interaction
quantitatively and observe that bursts containing a certain number of spikes
occur shortly after stimulus deflections of specific intensity and duration.
Our findings suggest a sparse neural code where information about the stimulus
is represented by the number of spikes per burst, irrespective of the detailed
interspike-interval structure within a burst. This compact representation
cannot be interpreted as a firing-rate code. An information-theoretical
analysis reveals that the number of spikes per burst reliably conveys
information about the amplitude and duration of sound transients, whereas their
time of occurrence is reflected by the burst onset time. The investigated
neurons encode almost half of the total transmitted information in burst
activity.
