Georgia Clark of Green Island School asks :-

How do scientists know what part of the brain controls memory, touch, taste, etc?

Richard Watts, a medical physicist at the University of Canterbury and who uses magnetic resonance imaging (MRI) to study the brain, responded.

Almost all the early work on human brain mapping of function was based on brain injuries. So, for example, if a patient had a stroke that produced a particular deficit then after the patient died, an autopsy could show where the stroke was, and the function could be matched with a particular location.

One example of this was "Tan", so named because he could only speak the word "tan" because of damage to the area of the brain used to generate language (now called Broka's area). Another famous case was that of Phineas Gage, who had an railroad injury that removed a large part of his frontal lobe (this changed his personality).

Wartime injuries were also a source of brain injuries which could be readily identified.

More recently, we can map out brain areas in several ways. Directly by, during surgery, we can either stimulate areas of the brain, or detect electrical signals on the surface of the brain. Indirectly by measuring changes in blood flow caused by neurons firing. This can be done by injecting a radioactive version of sugar (18-FDG) into a vein, and seeing where it is concentrated using positron emission tomography (PET).

Even more recently, we can do functional MRI, where we make the MRI scan sensitive to blood flow and oxygenation. This has the advantage of not involving ionizing radiation.