Sensation and perception are two separate processes that are very closely related. Sensation is input about the physical world obtained by our sensory receptors, and perception is the process by which the brain selects, organizes, and interprets these sensations. In other words, senses are the physiological basis of perception. Perception of the same senses may vary from one person to another because each person's brain interprets stimuli differently based on that individual's learning, memory, emotions, and expectations.

The Senses

There are five classical human senses: sight, sound, taste, smell, and touch. Two other senses, kinesthesia and the vestibular senses, have become widely recognized by scientists. Kinesthesia is the perception of the positioning of the parts of the body, commonly known as "body awareness." Vestibular senses detect gravity, linear acceleration (such as speeding up or slowing down on a straight road), and rotary acceleration (such as speeding up or slowing down around a curve). Both kinesthesia and the vestibular senses help us to  balance.

Sensory information (such as taste, light, odor, pressure, vibration, heat, and pain) is perceived through the body's sensory receptors. These sensory receptors include the eyes, ears, mouth, nose, hands, and feet (and the skin as a whole). Rod and cone receptors in the retina of the eye perceive light; cilia in the ear perceive sound; chemical receptors in the nasal cavities and mouth perceive smell and taste; and muscle spindles, as well as pressure, vibration, heat and pain receptors in the skin, perceive the many sensations of touch.

Specialized cells in the sensory receptors convert the incoming energy (e.g., light) into neural impulses. These neural impulses enter the cerebral cortex of the brain, which is made up of layers of neurons with many inputs. These layers of neurons in the function like mini microprocessors, and it is their job to organize the sensations and interpret them in the process of perception.

fMRI and the senses

This fMRI chart shows some of the neural activation that takes place during sensation. The occipital lobe is activated during visual stimulation, for example.

Motor Homunculus

The "motor homunculus" is a theoretical physical representation of the human body within the brain. It is a neurological "map" of the anatomical divisions of the body. Within the primary motor cortex, motor neurons are arranged in an orderly manner—parallel to the structure of the physical body, but inverted. The toes are represented at the top of the cerebral hemisphere, while the mouth is represented at the bottom of the hemisphere, closer to the part of the brain known as the lateral sulcus. These representations lie along a fold in the cortex called the central sulcus. The homunculus is split in half across the brain, with motor representation for each side of the body represented on the the opposite side of the brain.

The amount of cortex devoted to any given body region is proportional to how many nerves are in that region, not to the region's physical size. Areas of the body with greater or more complex sensory or motor connections are represented as larger in the homunculus. Those with fewer or less complex connections are represented as smaller. The resulting image is that of a distorted human body with disproportionately huge hands, lips, and face (because those regions have huge numbers of nerve endings).

Motor Homunculus

The motor homunculus is a theoretical visualization of the locations in the cortex that correspond to motor and sensory function in the body.