The theory of linear system for both temporal and spatial processes is applied to model both individual and assembly of neurons, as to be discussed in the following sections. If you are not familiar with the theory of linear, take a look at this online notes on linear systems.
The Fourier theory is powerful tool widely used in Engineering and physics. Here it is also used to describe the visual information processing taking place in the visual system, first in the ganglion cells in the retina, and later in the primary cortex. If you are not familiar with the Fourier theory, take a look at this online notes on Fourier theory.