Figure 2.

Shared variability is not detrimental to relative coding schemes. Here we illustrate a hypothetical situation in which most of the trial-to-trial variability is shared. (A) Raster plot of a trial with low spike counts and/or long latencies (for example because the stimulus was presented at a suboptimal phase [source (a)], or because the subject was not attentive [source (b)]. (B) Trial with high spike counts and/or short latencies (for the opposite reasons). If these two kinds of trials are observed, Fano factors and spike time dispersion will be high. However, relative spike counts and/or latencies could be more reproducible (because both sources (a) and (b) could affect spike counts and/or latencies similarly across neurons), and could robustly encode the stimulus. Of course, detecting such cases of “relative coding” requires recording multiple neurons at a time, and looking at stimulus-dependent statistical structure in the cross-correlograms. Conversely, neither a PSTH nor a phase histogram (Figure 1) would help.