  Sensory stimuli are usually composed of different features (the what)
appearing at irregular times (the when). Neural responses often use spike
patterns to represent sensory information. The what is hypothesized to be
encoded in the identity of the elicited patterns (the pattern categories), and
the when, in the time positions of patterns (the pattern timing). However, this
standard view is oversimplified. In the real world, the what and the when might
not be separable concepts, for instance, if they are correlated in the
stimulus. In addition, neuronal dynamics can condition the pattern timing to be
correlated with the pattern categories. Hence, timing and categories of
patterns may not constitute independent channels of information. In this paper,
we assess the role of spike patterns in the neural code, irrespective of the
nature of the patterns. We first define information-theoretical quantities that
allow us to quantify the information encoded by different aspects of the neural
response. We also introduce the notion of synergy/redundancy between time
positions and categories of patterns. We subsequently establish the relation
between the what and the when in the stimulus with the timing and the
categories of patterns. To that aim, we quantify the mutual information between
different aspects of the stimulus and different aspects of the response. This
formal framework allows us to determine the precise conditions under which the
standard view holds, as well as the departures from this simple case. Finally,
we study the capability of different response aspects to represent the what and
the when in the neural response.
