Final answer:
The sensation of someone scratching your back is processed in the somatosensory cortex within the parietal lobe of the brain. This lobe organizes tactile sensations from different parts of the body, with more complex sensations for areas with more nerves.
Step-by-step explanation:
The ability to recognize when someone is scratching your back is due to the somatosensory cortex located in the parietal lobe of the brain. The parietal lobe, situated immediately behind the frontal lobe, plays a key role in processing sensory information from the body, such as touch, pressure, and temperature. The somatosensory cortex, in particular, is a specialized area within the parietal lobe that is responsible for processing tactile sensations. It is organized in a way that different regions of the somatosensory cortex correspond to sensations from different parts of the body, with a larger sensory area dedicated to body parts that have a greater amount of nerves. Therefore, when someone scratches your back, the neurons in the corresponding area of the somatosensory cortex are activated, enabling you to feel the scratch.
Moreover, other sensations such as pain, itch, and vibration are also processed in the somatosensory cortex, as well as proprioception and kinesthesia, which are the senses of body position and movement. This intricate processing allows for a nuanced perception of various stimuli that the body encounters, including the specific sensation of a back scratch.