Neuronal similarity between pictures

Similarity between pictures based on the spiking population activity. Each entry (i,j) in the matrix shows the Pearson correlation coefficient between the vectors containing responses of all sites (n=340) to picture i and picture j. The responses consisted of the spike count in the 100 to 300 ms window after stimulus onset (using multi-unit spiking activity). The scale of the correlation coefficients is color coded (see scale at the bottom of the figure). The diagonal is 1 by definition. The dimensions of the matrix are 78 x 78 (picture 1 is a blank image). The dashed lines divide the groups of pictures used in the classification task (see text for details). A representative example from each group is shown above the matrix.