Functional Limitations of Sense Organs

Our sense organs, however capable, all function within definite limits. The eyes cannot register light that is too dim or too intensely bright to see; in the same way, the ears cannot register sounds that are too faint or too loud to hear, and every other sense organ has its own comparable working range. As human beings, we operate within a genuinely limited range of stimulation. For a sensory receptor to notice a stimulus at all, that stimulus has to reach a certain optimal intensity or magnitude -- and the relationship between stimuli and the sensations they produce is studied by a discipline called psychophysics.

In order to be noticed, a stimulus has to carry at least a minimum value or weight. This minimum value of a stimulus required to activate a given sensory system is called the absolute threshold, or absolute limen (AL). A useful illustration is what happens when sugar is added to a glass of water: adding just one granule of sugar produces no experience of sweetness at all, and adding a second granule may still not make the water taste sweet. But if granules are added one after another, a point is eventually reached where the water is judged to be sweet -- the minimum number of granules needed to reach that point is the AL of sweetness. It is worth noting that the AL is not one single, fixed point; it varies considerably from person to person and from situation to situation, depending on a person's physical condition and motivational state at the time. Because of this, the AL has to be assessed on the basis of a number of separate trials rather than a single test -- specifically, as the stimulus value that produces the target experience (in this case, "sweetness") on 50 per cent of the trials on which it is presented. Adding still more sugar granules beyond this point makes it more likely that the water will be judged sweet on an even greater proportion of occasions.

Just as we cannot notice every stimulus, we also cannot tell every pair of stimuli apart from one another -- for two stimuli to be noticed as different, there has to be some minimum difference between their values. The smallest difference in the value of two stimuli that is necessary for us to notice them as different is called the difference threshold, or difference limen (DL). Continuing the sugar-water illustration: once the water has already reached the point of tasting sweet, further granules have to be added, one at a time, before the water is judged to taste even sweeter than it did before -- the number of additional granules needed to produce this experience of a different (greater) sweetness, on 50 per cent of the trials on which it is tested, defines the DL of sweetness. In short, the difference threshold is the smallest amount of change in a physical stimulus that is capable of producing a noticeably different sensation on half of the trials in which it is tested. …