By its very nature, brain study has to be both multidisciplinary and interdisciplinary. The neurophysiologists study the brain at the neuronal level at the very bottom, while the psychologists study the brain at the perceptual and behavioral level at the very top. But there are obviously some gaps between these bottom and top levels. How do the microscopic neuronal activities determine our perception and behavior?
Suppose you are trying to understand how a super computer works (without any documents, by the way). To make things worse, suppose the computer was made by the Martians with some foreign technology called FISC for Foreign Intelligent Species Computer instead of CISC or RISC (for ``Complex or Reduced Instruction Set Computer'', in case you are not familiar with computers). You know what the computer can do at the top level, and you can also probe its basic components, say, some kind of transistors, to learn how they behave at the very bottom level. But do you think this is enough for you to thoroughly understand how the computer works?
This is why brain study may need some help from other disciplines such as mathematics, physics, computer science, and engineering. The goal here is to build computational models (which are, very importantly, biologically plausible) to make some educated guesses about how the neuronal activities might determine our perception and behavior. In other words, we hope these computational models can fill the gaps in the brain study between the bottom and top levels. You may be surprised to learn that Fourier transform, one of the most favorite tools used by the engineers, might play an interesting role in visual information processing in the brain!
At the same time, we also have to note that while people like to think our brain as a computer, we should also realize that the brain is not a product by some one-shot optimal design based on modern science and technology. Instead, our brain is a product of millions of years evolution. In this sense, it is indeed a super computer made by some foreign technology that Nature happens to adopt (but it does so many things so much better than the modern computers!). When we do try to apply some mathematical theories to the explanation of certain neural behaviors, keep this in mind: ``Neurons were not evolved for the convenience of mathematicians'' (Francis Crik).