DISCRIMINATION, Sensible, or DIF FERENTIAL SENSIBILITY. A term em ployed in experimental psychology to denote our ability to distinguish likeness and difference among sense impressions. Thus, we are able to say that one tone is the same as another, one light is brighter or duller than another, one scent is stronger or weaker than another, etc. The psychology of sensible discrimination in volves two distinct problems. The first is quan titative; we may compare sense-differences and determine the stimulus-differences which cor respond to sense-differences. The second is qualitative; we may describe the mental proc esses involved in comparison and discrimination. The former is, historically, the older of the two problems; it is intimately related to the broader question of mental measurement (see INTENSITY OF SENSATION), and to the general correlation between stimulus and sensation differences. See WEBER'S LAW.
The quantitative determination is of two kinds. We may seek, first, to find the just no ticeable difference between two impressions. For example, two tones from about the middle of the scale and differing by a single vibration may be readily distinguished as different ; but two tones differing by one-tenth of a vibration will be judged as the same. Somewhere between these two extremes of one and one-tenth a dif ference may be found that is just distinguish able. This difference is known as the just noticeable difference (or j.n.d.). We may seek, secondly, to compare two sense-differ ences; if, in a series of grays ranging from a light to a dark gray we find one that lies mid way between the two extremes, then we, in ef fect, equate the difference between the light and mid-gray to that between the dark and mid gray. Again, we may compare a difference just noticeable when two weights are placed on the resting arm with a difference just noticeable when the weights are lifted. The first kind of difference-comparison, that in which we halve a distance, is known as direct comparison; the second in which stimulus-differences are com pared is called indirect comparison.
The determination of the just noticeable difference is not as simple, however, as we have made it out to be. The judgment of so small a difference is not easy, and the condi tions are so complex as to require a rigorous method if the determination is to be valid. There are, for example, many errors to which the observer is liable; some of these are vari able, such as practice, fatigue, habituation and expectation; others are constant and result from the spatial position or temporal order of the stimuli; still others are accidental in con sequence of slips of attention, the physical con dition of the observer, etc. It was the service
of Fechner to work out the principles of method whereby these errors may be either eliminated or taken account of by mathemati cal treatment of the results. A number of stimuli are chosen, every one of which may be regarded as variable, or one may be constant (standard) and the others variable; these stimuli are arranged in regular order, or at haz ard, in series; the order of presentation of every series is predetermined with regard to the errors; the temporal order and spatial position of standard and variable are reversed and the series repeated to eliminate the constant errors; the observer is carefully instructed as to what he is to do; and finally, a large number of observations are taken and the ultimate result calculated from the data ob tained. The magnitude which represents the just noticeable difference is known as the dif ferential limen, or DL. This may be defined as that difference which is judged less (the form of the judgment depends, of course, upon the nature of the difference) in one-half of a long series of observations, while in the other half the judgment is equal or greater. The form of the magnitude representing the DL may be absolute or relative, i.e., if in the determination of the tonal DL the pitch of the standard is 500 vs., and that of the variable which in 50 per cent of the cases is judged as higher is 500.25 vs., then the absolute DL is 0.25 v., and the relative DL is *7, : = . The differential limen is an ideal value, it represents the most probable value, the mathematical equivalent of the difference which, if all sources of error were eliminated, would in fact be just notice able. The just noticeable difference thus found has proved to be of signal importance for analytic psychology. The differential limen of the quality, intensity, clearness, duration, ex tent or any other attribute of sensation may be obtained and mathematically expressed, it fur nishes the basis for the calculation of the num ber of discriminable qualities in vision and au dition, and it is a significant aid in the analysis of both temporal and spatial perceptions.