### Figure 11

Left
panels: Volterra kernels from a voxel in the left superior temporal gyrus at
-56, -28, 12mm. These kernel estimates
were based on a single subject study of aural word presentation at different
rates (from 0 to 90 words per minute) using a second order approximation to a
Volterra series expansion modeling the observed hemodynamic response to
stimulus input (a delta function for each word). These kernels can be thought of as a
characterization of the second order hemodynamic response function. The first order kernel _{}(upper panel) represents the (first order) component usually
presented in linear analyses. The second
order kernel (lower panel) is presented in image format. The color scale is arbitrary; white is
positive and black is negative. The
insert on the right represents _{}, the second order kernel that would be predicted by a simple
model that involved linear convolution with _{}followed by some
static nonlinearity.

Right
panel: Integrated responses over a 32-second stimulus train as a function of
SOA. Solid line: Estimates based on the
nonlinear convolution model parameterized by the kernels on the left. Broken
line: The responses expected in the absence of second order effects (*i.e.* in a truly linear system). Dots:
Empirical averages based on the presentation of actual stimulus trains.