The Brain

Many researchers agree that the brain is poorly understood [Got97, p. 14]. However, numerous studies have been conducted and proved very valuable; more knowledge is acquired by the day. Luckily, complex and often complicated phenomena are found to have underlying principles that are not very complicated by themselves. The brain can be studied from an anatomical or a functional point of view. Anatomically, it must be pointed out that the neo-cortex is the latest evolutionary addition. It is only present in mammals. The cortex in total makes up for more than 80% of the brain's volume. It is the outer layer of the (fore)brain. It is `folded' into gyri and sulci (bumps and cracks) to fit the area of 0.25 m$^2$ (equivalent of four A4 sheets of paper2.1) into our skull. [CGC+98,KW01, p. 56]. The cortex is essential to our higher functioning, because of its high connectedness.

It is an important principle that evolution adds layers and retains the old ones. Our neocortex is the latest addition. For coordinated movement, each layer adds a refinement to the more primitive ones. The more primitive layers provide reflexes and allows you to maintain posture without conscious intervention. For thoughts, each layer allows a higher level of conceptualization. The neocortex consists of an extremely dense set of neurons (circa 10$^4$ neurons/mm$^3$) [RMK06, p. 100]. The brain is organized for information to travel multiple paths even in a specialized subsystem, and lateral connections complement the received signals. Excitation and inhibition are the basic interaction of the 100,000,000,000 neurons2.2. Together with an even larger number of neuroglia2.3 that support the neurons and help direct their growth.

Since our goal is to distil important properties of the brain, we will not discuss neuroanatomy any further. The function of brain and cognition is to enable the organism to attend to, process, and behaviorally respond to the forms of information and conditions that co-varied with survival or reproductive prospects during the species' evolutionary history [Gea05, p. 125].

An important feature that is often forgotten is that the brain is connected to a very versatile and capable body. It allows us to perceive a lot of our environment, and manipulate objects. The brain itself did not evolve by itself, it did so together with this body and, unique to humans, with complex tool use.

Perhaps the most prominent contribution to the understanding of neural dynamics was that of Donald Hebb[RMK06]. In commonly called `Hebbian learning' a synapse between two neurons is strengthened when both the presynaptic (input) and postsynaptic (output) neurons are firing simultaneously. This form of self-organization is, despite its simplicity, now considered a general principle [Nol01, p. 51]. It allows associations of stimuli to arise, and serves to explain phenomena such as conditioning and associative learning2.4.

Erik de Bruijn 2007-10-19