Modeling Short-term Noise Dependence of Spike Counts in Macaque Prefrontal Cortex
Correlations between spike counts are often used to analyze neural coding. The noise is typically assumed to be Gaussian. Yet, this assumption is often inappropriate, especially for low spike counts. In this study, we present copulas as an alternative approach. With copulas it is possible to use arbitrary marginal distributions such as Poisson or negative binomial that are better suited for modeling noise distributions of spike counts. Furthermore, copulas place a wide range of dependence structures at the disposal and can be used to analyze higher order interactions. We develop a framework to analyze spike count data by means of copulas. Methods for parameter inference based on maximum likelihood estimates and for computation of mutual information are provided. We apply the method to our data recorded from macaque prefrontal cortex. The data analysis leads to three findings: (1) copula-based distributions provide significantly better fits than discretized multivariate normal distributions; (2) negative binomial margins fit the data significantly better than Poisson margins; and (3) the dependence structure carries 12% of the mutual information between stimuli and responses.