Neurological surgeon Walker explains that the brain is composed of neurons, nerve cells that convey impulses to various parts of the body and store memory of impulses. He shows a diagram of the dendrites and axons of the neurons and explains a cross-section model of a neuron. Lynn Poole lists some of the history of the research on brain functions, including that of the early German phrenologist Franz Joseph Gall. In 1817 it was demonstrated that electrical stimulus applied to the brain produced movement on the opposite side of the brain, giving rise to the existence of motor areas of the brain. Using a brain cross-section diagram related to various parts of the body, Dr. Walker shows how the sensory cortex, or homunculus, is closely correlated with the motor cortex. He also discusses what happens when these areas are injured and how they affect vision, hearing, and speech. The association area of the brain is the temporal lobe. Personality and drive may be located in the frontal lobe as lobotomies in that area produce personality change and induce apathy. Additional research is needed to discover if patterns of pathways between nerve cells are responsible for psychological differences.