Schematic depicting the causal relationship between the outputs and
the recent history of the inputs to a nonlinear dynamical system, in this
instance a brain region or voxel.
This relationship can be expressed as a Volterra series, which expresses
the response or output y(t) as a nonlinear convolution of the
inputs u(t), critically without reference to any [hidden] state
variables. This series is simply a
functional