The intensity of stimuli in order to be perceived, is:

Background.  Our perception is an approximation of reality.  Our brain attempts to make sense out of the stimuli to which we are exposed.  This works well, for example, when we “see” a friend three hundred feet away at his or her correct height; however, our perception is sometimes “off”—for example, certain shapes of ice cream containers look like they contain more than rectangular ones with the same volume.

Factors in percpetion.  Several sequential factors influence our perception. Exposure involves the extent to which we encounter a stimulus.  For example, we are exposed to numerous commercial messages while driving on the freeway:  bill boards, radio advertisements, bumper-stickers on cars, and signs and banners placed at shopping malls that we pass.  Most of this exposure is random—we don’t plan to seek it out.  However, if we are shopping for a car, we may deliberately seek out advertisements and “tune in” when dealer advertisements come on the radio.

Exposure is not enough to significantly impact the individual—at least not based on a single trial (certain advertisements, or commercial exposures such as the “Swoosh” logo, are based on extensive repetition rather than much conscious attention).  In order for stimuli to be consciously processed, attention is needed.  Attention is actually a matter of degree—our attention may be quite high when we read directions for getting an income tax refund, but low when commercials come on during a television program.  Note, however, that even when attention is low, it may be instantly escalated—for example, if an advertisement for a product in which we are interested comes on.

Interpretation involves making sense out of the stimulus.  For example, when we see a red can, we may categorize it as a CokeÒ.

Weber’s Law suggests that consumers’ ability to detect changes in stimulus intensity appear to be strongly related to the intensity of that stimulus to begin with.  That is, if you hold an object weighing one pound in your hand, you are likely to notice it when that weight is doubled to two pounds.  However, if you are holding twenty pounds, you are unlikely to detect the addition of one pound—a change that you easily detected when the initial weight was one pound.  You may be able to eliminate one ounce from a ten ounce container, but you cannot as easily get away with reducing a three ounce container to two (instead, you must accomplish that gradually—e.g., 3.0  --> 2.7 --> 2.5 --> 2.3 --> 2.15 –> 2.00).

Several factors influence the extent to which stimuli will be noticed.  One obvious issue is relevance.  Consumers, when they have a choice, are also more likely to attend to pleasant stimuli (but when the consumer can’t escape, very unpleasant stimuli are also likely to get attention—thus, many very irritating advertisements are remarkably effective).  One of the most important factors, however, is repetition.  Consumers often do not give much attention to a stimuli—particularly a low priority one such as an advertisement—at any one time, but if it is seen over and over again, the cumulative impact will be greater.

Surprising stimuli are likely to get more attention—survival instinct requires us to give more attention to something unknown that may require action.  A greater contrast (difference between the stimulus and its surroundings) as well as greater prominence (e.g., greater size, center placement) also tend to increase likelihood of processing.

Subliminal stimuli.  Back in the 1960s, it was reported that on selected evenings, movie goers in a theater had been exposed to isolated frames with the words “Drink Coca Cola” and “Eat Popcorn” imbedded into the movie.  These frames went by so fast that people did not consciously notice them, but it was reported that on nights with frames present, Coke and popcorn sales were significantly higher than on days they were left off.  This led Congress to ban the use of subliminal advertising.  First of all, there is a question as to whether this experiment ever took place or whether this information was simply made up.  Secondly, no one has been able to replicate these findings.  There is research to show that people will start to giggle with embarrassment when they are briefly exposed to “dirty” words in an experimental machine.  Here, again, the exposure is so brief that the subjects are not aware of the actual words they saw, but it is evident that something has been recognized by the embarrassment displayed.

Learning and Memory

Weber's Law of
Just Noticeable Differences
USD Internet Sensation & Perception Laboratory

Introduction

The Difference Threshold (or "Just Noticeable Difference") is the minimum amount by which stimulus intensity must be changed in order to produce a noticeable variation in sensory experience.

Ernst Weber (pronouned vay-ber), a 19th century experimental psychologist, observed that the size of the difference threshold appeared to be lawfully related to initial stimulus magnitude.   This relationship, known since as Weber's Law, can be expressed as:

Weber's Law, more simply stated, says that the size of the just noticeable difference (i.e., delta I) is a constant proportion of the original stimulus value.  For example: Suppose that you presented two spots of light each with an intensity of 100 units to an observer.  Then you asked the observer to increase the intensity of one of the spots until it was just noticeably brighter than the other.  If the brightness needed to yield the just noticeable difference was 110 then the observer's difference threshold would be 10 units (i.e., delta I =110 - 100 = 10).  The Weber fraction equivalent for this difference threshold would be 0.1 (delta I/I = 10/100 = 0.1).  Using Weber's Law, one could now predict the size of the observer's difference threshold for a light spot of any other intensity value (so long as it was not extremely dim or extremely bright).  That is, if the Weber fraction for discriminating changes in stimulus brightness is a constant proportion equal to 0.1 then the size of the just noticeable difference for a spot having an intensity of 1000 would be 100 (i.e., delta I = 0.1 X 1000 = 100).

Weber's Law can be applied to variety of sensory modalities (brightness, loudness, mass, line length, etc.).  The size of the Weber fraction varies across modalities but in most cases tends to be a constant within a specific task modality.

Instructions for the Laboratory Experiments

This lab will allow the participant to measure their just noticeable difference thresholds for the discrimination of line length using a psychophysical procedure known as the Method of Constant Stimuli.

Method of Constant Stimuli Experiment

Objective

Choose the longer of the two line segment stimuli presented on the screen (for a given trial).  You will be asked to enter approximately 240 judgments (60 each at four different levels of standard line size).

Sample Method of Constant Stimuli Screen

Explanation of the Stimulus Screen

The bottom of the stimulus screen displays the controls that you will be using for this experiment.  You can indicate which of the lines you judge to be longer by using the mouse and "clicking" the appropriately labeled button.  By doing so, the computer will  lock-in your judgment and automatically display the next pair of line stimuli. [Keyboard Shortcut: You can also indicate which line segment appears longer by using the RIGHT-ARROW and LEFT-ARROW keys.  If these keys appear unresponsive at first, try again after using the mouse once or twice.]

Results

• Once you have completed judging all of the stimulus pairings, the results will be automatically displayed.  These results summarize the percent of the time your judgment was correct as a function of the difference in line length (for each of the four ranges of line length examined).  Save a copy of this data in your lab notebook.

• To find your difference threshold for the four ranges of line length examined, you must first plot the psychometric function obtained for each level of standard stimulus line size (30, 90, 150 and 210 pixels).  This is accomplished by plotting the "percent correct judgment" on the y-axis as a function of the "difference in line size" (delta I) on the x-axis.  Once plotted, interpolate the "difference in line size" value that yields 75% correct performance.  This point is the difference threshold.

• Once each difference threshold (delta I) has been interpolated convert it to the Weber fraction equivalent (delta I/I).

• Plot the Weber fractions obtained at each of the four ranges of line length and determine whether Weber's Law holds true for just noticeable differences in line length.

Notice
Completion of this experiment requires approximately 200 responses (so both patience and effortful attention are required).

Lab Report Format

1. What was the independent variable?
2. What was the dependent variable?
3. Draw the psychometric function for each of the "standard stimulus" lengths explored.
4. Graphically interpolate a difference threshold from each of these psychometric functions.
5. Calculate and plot the Weber fractions as a function of Standard Stimulus line length.
6. Did Weber's Law hold true?  Explain the basis for your conclusion.

End of Weber's Law of Just Noticeable Differences.

What is the intensity of stimuli?

What determines the intensity of a stimulus?

What is intensity in perception?

What is required for a stimulus to be perceived?