Final answer:
Penfield and Rasmussen mapped muscle areas on the cerebrum by probing the cortex during surgery and observing patient responses. They discovered the motor homunculus in the primary motor cortex, which has an exaggerated cortical space for muscles involved in fine, agile movements such as those of the fingers and face.
Step-by-step explanation:
Wilders Penfield and his colleague Theodore Rasmussen mapped the muscle areas by stimulating the cerebrum. They performed this mapping by probing the surface of the cortex while patients were under local anesthesia, allowing them to observe the patients' responses to the stimulation. This was to gain an understanding of the precentral gyrus's role in muscle movement, which they believed directly stimulated muscle movement. Furthermore, it has been noted that the primary motor cortex is organized into a motor homunculus, with a topographical map of the body mapped onto the cortical area representing different muscle groups.
Studies have shown that areas such as the hand and face, which perform fine, agile movements, occupy greater space in the motor cortex, contrasting with power muscles like the back muscles, which occupy less space and are involved in coarser movements. Advanced imaging techniques like functional magnetic resonance imaging (fMRI) can provide detailed maps of brain activity and demonstrate that during tasks, much more than 10 percent of the brain is in use, debunking the myth that we use only a small portion of our brain.