Dual-site stimulation responses show sublinear summation and response normalization. Signal amplitude (ΔI/I) measured in the SGS from one slice following dual-site stimulation is plotted along the mediolateral extent of the SC. A, Symmetrical full current (100 μA) and (B) Symmetrical half current (50 μA) for single-site (blue and yellow) and dual-site (red) stimulation. Gaussians are fitted to the single- and dual-site data. The black lines are the normalization model fits to the dual stimulation data. C, Mediolateral distribution of signal amplitude (ΔI/I) throughout the SGS following full current stimulation (single, blue and yellow; dual-site, red) for five slices with similar interstimulus distances (mean interstimulus distance ± SD = 607 ± 67 μm). Colored lines show the Gaussian fits to the data. Black lines show normalization model fits to the dual-site data. D, Same as in C for the half current stimulation. E, Comparison of the Gaussian amplitude parameter (a1) following dual-site stimulation (abscissa) to the Gaussian amplitude parameter of the linear summation of responses to stimulation of site 1 and site 2 (ordinate). Colors indicate data pairs; filled circles, full stimulation; filled triangles, half stimulation. Dashed line is unity. Points falling below the line indicate that the actual, measured response amplitude was reduced compared with that predicted by a linear sum model.