Big brains

Perception happens after the sense organs convert the stimuli into neural impulses. According to latest estimates [15], human bodies contain around 86 billion neurons. Around 20 billion are devoted to the part of the brain called the cerebral cortex, which handles perception and many other high-level functions such as attention, memory, language, and consciousness. It is a large sheet of neurons around three millimeters thick and is heavily folded so that it fits into our skulls. In case you are wondering where we lie among other animals, a roundworm, fruit fly, and rat have 302, 100 thousand, and 200 million neurons, respectively. An elephant has over 250 billion neurons, which is more than us!

Only mammals have a cerebral cortex. The cerebral cortex of a rat has around 20 million neurons. Cats and dogs are at 300 and 160 million, respectively. A gorilla has around 4 billion. A type of dolphin called the long-finned pilot whale has an estimated 37 billion neurons in its cerebral cortex, making it roughly twice as many as in the human cerebral cortex; however, scientists claim this does not imply superior cognitive abilities [224,277].

Figure 2.18: A typical neuron receives signals through dendrites, which interface to other neurons. It outputs a signal to other neurons through axons.

Another important factor in perception and overall cognitive ability is the interconnection between neurons. Imagine an enormous directed graph, with the usual nodes and directed edges. The nucleus or cell body of each neuron is a node that does some kind of ``processing''. Figure 2.18 shows a neuron. The dendrites are essentially input edges to the neuron, whereas the axons are output edges. Through a network of dendrites, the neuron can aggregate information from numerous other neurons, which themselves may have aggregated information from others. The result is sent to one or more neurons through the axon. For a connected axon-dendrite pair, communication occurs in a gap called the synapse, where electrical or chemical signals are passed along. Each neuron in the human brain has on average about 7000 synaptic connections to other neurons, which results in about $10^{15}$ edges in our enormous brain graph!