Should You Trust Your Perceptions?

The "privacy problem" is a term used by researchers of human sensation and perception. One person can never really know another person's subjective sensory and perceptual experiences, regardless of how objective an event may be, each individual has a subjective point of view of that event... or do they? A person's perception of the quality of a stimulus is not necessarily reliable, whereas measuring the quantity of a perceived stimulus's intensity is very possible; and this is exactly the topic of study in the field of psychophysics.

Ernst Weber, like so many others, built on Gustav Fechner's break-through technique for quantitatively measuring others' perceptions. Weber, with his Law of Just Noticeable Differences, took perceptual research from measuring to predicting. His Law refers to the question: how much different from itself does a stimulus have to be in order for it to be perceivedas different from itself? For example, how much length must be added to one of two identical lines before a person can perceivably discern the difference between the two? To validly test this issue, a researcher would add tiny increments of length until a person expresses that he or she can discriminate between the two; the point at which this person can tell the difference is appropriately labeled the "difference threshold." By adding length to one line, the researcher is increasing or decreasing the "intensity" of the initial condition.

Ofcourse, for Weber's theory to have the potential to become a law, he had to put things in mathematical terms: DeltaI / I = k is the logarithmic function that enabled Weber to graph and predict the difference threshold for any level of intensity for a stimulus of one sensory modality (sense-of-sight for the line example). Delta is one unit of change in stimulus intensity from that of the original condition of the stimulus (one measured increment of added length to the line). His hypothesis states something like this: The difference threshold (Delta I) is a function of stimulus intensity (I) such that as the stimulus intensity increases or decreases, the difference threshold increases or decreases. This quantified difference threshold value (k or "Weber fraction") should be constant for any degree of magnitude that serves as the standard (original) intensity for that stimulus. This hypothesis has been tested since its first appearance in the 19^th century and it has proved to be nearly 100% accurate time and time again.

The most significant trend drawn by Weber's equation is that it is more difficult to discern between two stimuli when there is only 1 Delta (one unit) of intensity that differentiates the two; it is even harder to distinguish between an initial stimulus of high intensity and that same intensity with minor units of change. What does this mean? Regarding the line example, it is more difficult for people to discern the difference in length between two long lines than it does two short lines. Another example might be that it is easier for people to feel the difference between two light-weight objects of proximal weight than it does two heavy objects; it is easier to discern between two bright lights than between two faint lights; two faint sounds compared to two loud sounds; etc.

How does perceptual analysis apply to everyday life? Why are these crazy psychophysicists expending so much effort and patience on such an abstract topic as perception? Professionals of all fields need to know that a perceivable similarity does not always mean a measurable similarity; this could be a matter of life and death for those involved in architecture, engineering, construction, piloting etc. Realizing patterns in misconception (like that shown by the Weber fraction) can save a lot of time for the professional that must deal with a balance of quantitative accuracy and qualitative aesthetics.