Perception is a general term for the body's recognition of external signals from the environment. Haptic perception refers to the way a person can gain information about his or her environment through touch. It involves skin sensors and receptors in other parts of the body like muscles, which recognize sensations such as pressure. These work together to send signals to the brain, which interprets them to form a representation of the environment for the person to understand.
The types of differences in sensation that the body can recognize through touch are primarily mechanical, which is why many of the sensors important in haptic perception are called mechanoreceptors. These register alterations in pressure and vibration. As well as sensing mechanical changes, haptic perception also uses data from thermoreceptors, which register changes in temperature.
Skin is an organ which protects the interior of the body from damage and infection, and also provides structure. In addition to these roles, skin is home to both mechanoreceptors and thermoreceptors, because the outside of a person is the part that most often interacts with the environment. Receptors register sensations and send the data through nerves to the brain. The brain then collects all of the data from the touch sensations, takes into account previous knowledge of similar objects, and forms a concept of the item being touched.
Haptic perception does not only involve sensations from the skin. Mechanoreceptors are also present in the inside of the body, at places that respond to the environment. Basically these are muscles, joints and tendons, which undergo changes in position when a person touches an object or surface. For example, when a person presses a finger into a cake to see if it is cooked, the position of the muscles, tendons and joints of that finger vary with the springiness of the cake.
Generally, when a person is touched by something else, he or she primarily concentrates on the resultant sensations on the body and neglects the physical characteristics of the touching item. In contrast, when a person touches something in the environment, he or she forms a perception of the physical environment. Haptic perception is generally used for the second situation, where it is the characteristics of the environment that are important.
Roughness of a surface, oiliness of a liquid, or heaviness of an object, are all measures that the brain arrives at after interpreting signals from receptors. Sometimes accurate haptic perception requires the person to move his or her skin across the surface, in order to judge differences in sensation such as roughness. Receptors that are at the base of hairs can recognize when a spider is crawling on a hand, and in what direction it scuttles. The size and shape of objects are also recognizable through haptic perception. An example of this is a child digging blindly in a Halloween sack of candy to find his or her favorite lollipop, which may be round, flat and large, as opposed to small and spherical.