Figure 4.

A. Shapes of average response and first 5 principal components. The principal components are mutually orthogonal. All are plotted as probability of spiking as a function of time after stimulus appearance. The principal components are derived from low-pass filtered (3 ms σ Gaussian pulse) and sub-sampled (every 6 ms) spike densities of each trial. These principal components were calculated using all of the responses (~50 per stimulus) elicited by 174 stimuli (~8700 responses). As is standard, the average response was subtracted from each response before the data domain covariance matrix was calculated (thus translating the reference point to the mean of the data) which accounts for the negative values for some parts of some principal components, and for some of the coefficients in panel B. Each stimulus was stepped on for 360 ms. B. Average principal component coefficients for the first 5 principal components for the two stimuli shown in Figure 2. C. The reconstructed peristimulus spike densities (solid and dotted lines, matching those in Fig. 2) using the first 5 principal components (plus the mean). Because the mean response was subtracted from each response, each of the principal components weighted by the coefficient in panel B has been added to the average. High frequency components such as the low-level apparent oscillatory activity in the originals in Figure 2 are reduced or eliminated because the higher principal components have been left out. Thus, this reconstruction with only 5 principal components has the effect of additional low-pass filtering (after the Gaussian convolution described above) the responses.