Source: Laboratories of Jonas T. Kaplan and Sarah I. Gimbel—University of Southern California

One principle of brain organization is the topographic mapping of information. Especially in sensory and motor cortices, adjacent regions of the brain tend to represent information from adjacent parts of the body, resulting in maps of the body expressed on the surface of the brain. The primary sensory and motor maps in the brain surround a prominent sulcus known as the central sulcus. The cortex anterior to the central sulcus is known as the precentral gyrus and contains the primary motor cortex, while the cortex posterior to the central sulcus is known as the postcentral gyrus and contains the primary sensory cortex (Figure 1).

Figure 1: Sensory and motor maps around the central sulcus. The primary motor cortex, which contains a motor map of the body's effectors, is anterior to the central sulcus, in the precentral gyrus of the frontal lobe. The primary somesthetic (sensory) cortex, which receives touch, pain, and temperature information from the external parts of the body, is located posterior to the central sulcus, in the postcentral gyrus of the parietal lobe.

In this experiment, functional neuroimaging is used to demonstrate the motor map in the precentral gyrus. This map is often called the motor homunculus, which is Latin for "little man," because it is as if there is a little version of one's self represented in this part of a person's brain. One interesting property of this map is that more cortical space is devoted to body parts requiring finer control, such as the hands and mouth, which results in disproportionate representation of those appendages in the cortex. Also, because of the anatomy of the motor system, the neurons that control the right side of the body are in the left primary motor cortex, and vice versa. Therefore, when a participant in the experiment is asked to move their right hand or foot, an increased activation on their left precentral gyrus is expected.

In this experiment, participants are asked to alternately move their hands and feet, on the left and right sides, while their brain activity is measured with fMRI. Since the fMRI signal relies on changes in blood oxygenation, which are slow in comparison to the movements the participants make, the periods of movement are separated with periods of stillness to ensure that the various conditions can be distinguished from each other and from the resting baseline. To achieve precise timing of the movements, participants are instructed on when to begin and end each movement with a visual cue. The methods in this video are similar to those used by several fMRI studies that have demonstrated somatotopy in primary motor cortex.^1,2