Fig. 1.

Calculation of information density and variance-to-mean ratio for two cells. a,b, Mean ± SD of the responses of a neuron from a 4-week-old infant (a) and from an adult (b) to an optimized, drifting sinusoidal grating stimulus at six different contrasts evenly spaced between 0 and 0.5. The mutual information about selected contrast pairs is indicated.c, d, Mutual information about every possible pair of the six contrasts (15 pairs) in a andb is plotted against the difference in the mean firing rate between each pair of contrasts. These data are fit with a function, the maximal slope of which is a measure of information density (see Materials and Methods). Information density has units of bits per spike, and the computed information densities for each cell are indicated. This measure, unlike total mutual information, does not depend on the specific contrasts tested, which differed somewhat from cell to cell. e, f, Spike count variance at each contrast is plotted against mean spike count for the example cells in a and b. The variance-to-mean ratio (VMR) is taken from the best fitting line with slope = 1; horizontal ticks mark the ratios for each cell. The counting window was 640 msec and contained an integer number of temporal cycles of the drifting stimulus.