7 Sensation and Perception
STEP 4 Review the Knowledge You Need to Score High
Just as hearing is sensitivity to pressure on receptors in the cochlea, touch is sensitivity to pressure on the skin. Psychologists often use somatosensation as a general term for four classes of tactile sensations: touch/pressure, warmth, cold, and pain. Other tactile sensations result from simultaneous stimulation of more than one type of receptor. For example, burning results from stimulation of warmth, cold, and pain receptors. Itching results from repeated gentle stimulation of pain receptors, a tickle results from repeated stimulation of touch receptors, and the sensation of wetness results from simultaneous stimulation of adjacent cold and pressure receptors. Transduction of mechanical energy of pressure/touch and heat energy of warmth and cold occurs at sensory receptors distributed all over the body just below the skin’s surface. Neural fibers generally carry the sensory information to your spinal cord. Information about touch usually travels quickly from your spinal cord to your medulla, where nerves crisscross, to the thalamus, arriving at the opposite sides of your somatosensory cortex in your parietal lobes. Weber used a two-point discrimination test to determine that regions such as your lips and fingertips have a greater concentration of sensory receptors than your back. The amount of cortex devoted to each area of the body is related to the sensitivity of that area. Touch is necessary for normal development and promotes a sense of well-being.
Pathways for temperature and pain are slower and less defined. You probably have a harder time localizing where you sense warmth and pain on your skin than where you sense touch or pressure. Pain is often associated with secretion of substance P, and relief from pain is often associated with secretion of endorphins. Because the experience of pain is so variable, pain requires both a biological and psychological explanation. Ronald Melzack and Patrick Wall’s gate-control theory attempts to explain the experience of pain. You experience pain only if the pain messages can pass through a gate in the spinal cord on their route to the brain. The gate is opened by small nerve fibers that carry pain signals. Conditions that keep the gate open are anxiety, depression, and focusing on the pain. The gate is closed by neural activity of larger nerve fibers, which conduct most other sensory signals, or by information coming from the brain. Massage, electrical stimulation, acupuncture, ice, and the natural release of endorphins can influence the closing of the gate. The experience of pain alerts you to injury and often prevents further damage.