The minimum level of stimulation that can be detected 50 per cent of the time; also called “detection threshold.” What is the finest line the human eye can see, the faintest sound the ear can hear, the slightest taste the tongue can detect? The questions are simple enough, but the answers are surprisingly difficult. The biggest problem is that there is no fixed level of stimulation that always activates a given sense organ. Sensitivity is not static; it changes from moment to moment due to changes in the receptor and the conditions under which it operates. This is why scientists have had to qualify the concept of absolute threshold by defining it as the level of stimulation the individual is able to detect 50 per cent of the time. It is actually a highly relative, statistical concept instead of an absolute quantity as the name might imply. One method of determining the threshold for any sense modality is to present a series of stimuli in the neighborhood of the threshold, asking the subject whether he perceived each of them. The series is usually repeated in several different orders, and the stimulus of least intensity which is detected 50 per cent of the time is taken as the absolute threshold. The logic of this is clear enough, but complicating factors arise as soon as it is put in practice. In vision, the threshold differs for different types of light, different areas of the retina, and especially the state of the eye at the time of the test. An extreme example will illustrate this last point: if a subject has been in darkness for twenty minutes, and is therefore dark-adapted, his eye is 100,000 times more sensitive than it is in bright sun-light. In hearing, the threshold has been found to vary according to the frequency and duration of the test tone, the use of one or two ears and so on. The condition of the organ is important here, too; the threshold is usually high after exposure to continuous loud noises, and the ears are most sensitive after a period of rest.The skin senses show similar variations. The state of the superficial blood vessels affects the thresholds for warmth and cold, and the beat of the pulse radically affects pressure and pain sensitivity. Prior stimulation also has its effect: exposure to warm air lowers the threshold to cold, and exposure to cold lowers the heat threshold. In addition, the chemical senses, smell and taste, are particularly hard to investigate because in this case the stimuli alter the state of the receptors themselves. In spite of these difficulties, threshold experiments have revealed some useful, and often astonishing, facts about human sensitivity. On a dark, clear night the human eye can detect the light of a single match at a distance of thirty miles. A person with normal hearing can detect the tick of a watch at twenty feet in a quiet room. It has been estimated that the auditory nerve cells can respond to vibrations of the bones in the middle ear that are less then 1 per cent of the diameter of a hydrogen molecule. These two senses appear to be very close to the useful limit of operation. If our eyes were any more sensitive, we would probably be able to see light particles (quanta) instead of a steady light; and if our ears were only slightly more sensitive, we would be continually bombarded by the noise of molecules colliding in the air around us! Our other sense organs are also amazingly sensitive. Our sense of pressure (or touch) is so acute that we can feel the weight of a bee’s wing dropped from a height of half an inch. We can smell certain substances, such as mercaptan, in a concentration of one part to 30 billion parts of air; and we can taste quinine sulphate when a single ounce is dissolved in 250 gallons of water. On the other hand our sensitivity to heat and cold is relatively low; it takes a temperature change of a full half degree Fahrenheit before we can feel the difference. Studies of absolute threshold have led to a number of other findings. As we grow older we become progressively less sensitive in taste and smell as well as hearing and sight—but more sensitive to extremes of temperature. Certain illnesses and injuries play havoc with our threshold of sensitivity. The pressure of clothing becomes painful when we are sunburned, and a current of air can be excruciating to a person suffering from neuralgia. In hysteria (conversion reaction) certain types of sensitivity can be either raised or lowered far beyond normal limits. The threshold for pain can be so high that the skin will be completely anesthetic in certain areas, and the threshold for vision and sound can be so low that ordinary light and noise will be practically unbearable. These re-actions can be matched to some degree in normal experience. Think how oversensitive we can be to the dentist’s drill, and how insensitive we become to a cut or bruise received during an exciting basketball game. See SENSITIVITY DISTURBANCES.