Figure 4.

Information-theoretic comparison of spatial representations in the subiculum and in area CA1. A, Relationship between mean firing rate and mutual information conveyed by single neurons in the subiculum. model neurons whose position-phase firing-rate maps were matched to experimental data from putative principal neurons in the subiculum. Symbols are color-coded according to the location of the neuron along the transverse dimension of the subiculum. Neurons with mean firing rates less than 1 spikes/second have low mutual information, whereas higher firing rates are compatible with high mutual information. B, Relationship between mean firing rate and the spatial information conveyed by model neurons whose position-phase firing-rate maps were matched to experimental data from putative principal neurons in the CA1/subiculum transition zone and in distal area CA1. Mean firing rates is strongly correlated with mutual information, and the distribution is skewed so that most neurons are clustered close to zero mean firing rate and convey almost no information. C, Histograms of spatial information conveyed by neuronal ensembles of different sizes in different subregions. Each subpanel is a histogram of mutual information for ensembles of model neurons of a given size. From left to right, columns correspond to single neurons, pairs of neurons, triplets of neurons, etc. Each row represents data from an anatomical subregion. As expected, the distributions shift to the right as the ensemble size increases; however, this shift is larger in the distal and middle subiculum than in the proximal subiculum and in distal area CA1 and the CA1/subiculum transition zone. For single neurons and pairs, the distribution of mutual information is skewed towards zero in the more proximal subregions. As one can also see in B, the mode of the distribution of mutual information among single neurons in area CA1 is near zero. D and E, Medians and inter-quartile ranges of the histograms shown in C. Mutual information increases nearly linearly with the number of neurons in the ensemble. For a given ensemble size, neurons in the proximal subiculum convey as much spatial information as do neurons in distal area CA1 and the CA1/subiculum transition zone, and neurons in the middle and distal subiculum convey more spatial information.