Halliday & Matthiessen (1999: 607):
The neural events that constitute the various interface systems are themselves in the broadest sense semiotic: terms such as "communication", "exchange of information", that are used to characterise the activities of the brain are less abstract variants of the concept of "semiotic systems & processes".
At the same time, the neural networks can be thought of as "realising" the system of language, in the sense that it is in the brain that language materialises as a process of the bio-physical world. In this perspective the relationship between language and the brain is itself a semiotic one, analogous to that between the content plane and the expression plane within language itself; and by the same analogy, there is no necessary or "natural" relationship such that certain parts of the neural network (certain locations within the brain) are dedicated to language or to any particular subsystem within it.
The analogy is relevant here because it is the fact that language and the perceptual systems share a common "realisation" in neural networks and neural processes that enables language to function as a dynamic open system, one that persists in time by constantly being modified through ongoing exchanges with its environment.