Sensation is an animal's, including humans', detection of external or internal stimulation (e.g., eyes detecting light waves, ears detecting sound waves). It is different from perception, which is about making sense of, or describing, the stimulation (e.g., seeing a chair, hearing a guitar).
Sensation involves three steps:
For example, when touched by a soft feather, mechanoreceptors - which are sensory receptors in the skin - register that the skin has been touched. That sensory information is then turned into neural information through a process called transduction. Next, the neural information travels down neural pathways to the appropriate part of the brain, wherein the sensations are perceived as the touch of a feather.
Children are often taught five basic senses: seeing (i.e., vision), hearing (i.e., audition), tasting (i.e., gustation), smelling (i.e., olfaction), and touching. However, there are actually many more senses including vestibular sense, kinesthetic sense, sense of thirst, sense of hunger, and cutaneous sense.
Psychologists who specialize in psychophysics measure sensory sensitivity by identifying:
The wavelength, intensity and complexity of Light are detected by visual receptors in the retina of the eye. There are two types of visual receptors: rods and cones. Rods are sensitive to dim light, which makes them useful for seeing at night. Cones are more sensitive to color and bright light, which makes them more useful in daylight. Signals from rods and cones are transduced into useful neural information via the optic nerve. Blindness is the complete or nearly complete inability to see.
The frequency, intensity, and complexity of sounds waves in the external world are detected by auditory receptors (cilia or hair cell receptors) in the ear. Different patterns of cilia movement lead to different neural codes, which ultimately lead to hearing different loudness, pitch, and timbre of sounds. Deafness or hearing loss may occur in one or both ears.
Taste receptors (i.e., taste buds or papillae) are activated by the presence of food or another object on the tongue. Four basic tastes include sweet, salty, sour, and bitter. There is some debate on whether umami, or meatiness, is a fifth basic flavor. Aging is associated with loss of intensity in taste. Complete inability to taste is called ageusia.
Smells in the external world activate hair receptors in nostrils. These receptors then send signals to the olfactory bulb, which is located at the base of the brain. Anosmia is the inability to smell.
Somatosensory sensations occur when receptors detect changes on one's skin or within one's body.
Sensations on the skin are detected by cutaneous receptors. These receptors may feel sensations such as pain, tickle, cold, hot, soft, and rough. Mechanoreceptors detect light pressure (e.g., caress), vibration, and texture, nociceptors detect strong pressure (e.g., pain), and thermoreceptors detect temperature.
For example, if your dog lightly presses its nose on your leg, mechanoreceptors in your skin will sense the smooth texture of your dog's nose whereas thermoreceptors will detect its coldness. When a dog bites someone, nociceptors detect the sharp pressure. Astereognosis is the inability to identify an object by touch.
Osmoreception is the body's sensation of thirst. When the amount of water in one's body falls below a certain threshold, the concentration of osmolytes (e.g. salt) increase in one's blood.Osmoreceptors, or sensory receptors in the hypothalamus, detect these changes in osmotic concentration. These signals are then transferred to neural signals of thirst.
Many types of sensory loss occur due to a dysfunctional sensation process, whether it be ineffective receptors, nerve damage, or cerebral impairment. Unlike agnosia, these impairments are due to damages prior to the perception process. Conditions do exist where the patient experiences sensory loss, but experimental evidence shows that the effect is perception based.