The forebrain, also known as the prosencephalon, is the largest section of the brain. It is separated into two hemispheres, called the lower diencephalon and the upper telencephalon, that are each split into four lobes: parietal, temporal, occipital and frontal. Most sensory and associative processing occurs in the forebrain, including voluntary and involuntary motor control, emotion, cognition and language.
In the forebrain, the lower diencephalon includes the pretectum, prethalamus, epithalamus, hypothalamus, thalamus and subthalamus. This section of the brain serves as a central sensory processing area and controls the autonomic nervous system, which regulates body temperature, and sleep states, among other things. This area also controls the hormones released to the pituitary gland that regulate metabolism as well as other autonomic functions, such as equilibrium, controlling eye-movement, sensing facial movement or sensation and controlling respiration. Salivating, swallowing and chewing food also are controlled in this area of the brain, as are hearing and speech processing.
The upper telencephalon includes the cerebrum, or cerebral cortex — making the forebrain the most complex part of the central nervous system (CNS). This area of the brain mainly controls personality, memory and cognitive function. It is this part of the brain that affords humans the ability to think abstractly, reason and concentrate. The telencephalon also includes the basal ganglia, which is responsible for controlling motor functions. Defects in this area of the brain are closely connected with such conditions as Parkinson's disease and Huntington's disease.
Other parts that make up the forebrain include the frontal lobe, parietal lobe, temporal lobe, amygdala and hippocampus. The amygdala and hippocampus form the part of the brain called the limbic system. This area is often referred to as the "emotional brain" because it controls such things as the fight-or-flight response, sexual behavior, emotional expression and long-term memory development. This area of the brain may also be linked to depression, but scientists aren't sure if problems in the forebrain cause depression or if depression has an affect on the forebrain.
Other functions of the forebrain are sensory in nature and tie into associative processes. It sends sensory signals throughout the body, for example, and as the information is received, it connects the new data with previously acquired memories. This is why the smell of something baking may produce a sudden memory of mom or grandma in the kitchen, or the smell of flowers might remind one of a past bee sting.