  The manner in which groups of neurons represent events in the external world
is fundamental to neuroscience. Here, we analyze the population code of the
retina during naturalistic stimulation and show that the information conveyed
by specific multi-neuronal firing patterns can be very different from the sum
of the pattern's parts. Synchronous spikes convey more information than either
of the participating cells, but almost always less than their sum, making them
redundant coding symbols. Surprisingly, patterns of spiking and silence are
mostly synergistic - carrying information that is accessible only by observing
the whole pattern of activity, rather than its components, and signifying
unique features in the stimulus. These results demonstrate that the retina uses
a combinatorial code and that the brain can benefit significantly from
recognizing multi-neuronal firing patterns.
