Human brain is composed of three main parts: the cerebrum, the cerebellum and the brain stem. Cerebrum accounts for two thirds of the total brain weight and is responsible for most of the higher neural activities including conscious mental processes. Most of the vision related activities take place in the cerebrum.
The cerebrum is composed of a highly enfolded or convoluted outer sheet, the cortex of layers of the cell bodies of the neurons called the gray matter and an inner core of axons of the neurons called the gray matter.
As the cerebral cortex is highly convoluted, it is further divided into two types of areas called gyri and sulci. The crest or ridge of a single convolution is called a gyrus, and the fissure or the groove between two gyri is called a sulcus. The cortical areas in the sulci are not visible. That is why sometimes it is necessary to unfold and flatten the cortex, either a physical sample or a computer model, so that both the gyri and sulci on the cortex can be seen.