Psychophysics is a branch of psychology that studies the relationship between physical stimuli and the sensations and perceptions they produce. For many years in early psychological study, it was assumed that reaction to stimuli varied too greatly to be quantified in any definite way, so further investigation was abandoned. However, it was through the work of Ernst Weber that progress in this field was initially made.
Weber created a unit of measurement called the just noticeable difference (jnd), which he identified as being the smallest change between stimuli that a human could detect. Experimenting using differences in weight, Weber found that individuals could only notice a difference in the magnitude of a stimulus if it differed by a factor of 1 in 40. So, two weights would seem equal unless they were 40 grams versus 41 grams or 120 grams versus 123 grams. The difference between each of these weights is, psychophysically speaking, 1 jnd.
Responsiveness to differences in brightness of light has a ratio of 1 in 60. There are several of these ratios and as a whole, they are referred to as Weber fractions.
The experiments noted above all rely on the concept of a threshold, that is, a limit to what an individual can and cannot perceive. An absolute threshold can be defined as the point at which any stimulus can be perceived whereas a differential threshold is the point where two stimuli appear noticeably different. Weber explored the idea of the differential threshold while Gustav Fechner, a fellow German psychologist, did experiments relating to an absolute threshold of perception.
He had a subject sit in a darkened room where there were two glass panels are mounted in a wall with lightbulbs behind them. One panel is the control panel while the other is the comparison panel. Each light is controlled by a dimmer. First, he slowly turned up the light on the comparion panel until the individual said that there was perceptible light emanating from it. That point is where the absolute threshold for visual perception of light is. Next, he set the control panel to a light level equal the comparison panel and began turning the light up slowly until the subject noted a change in light intensity between the two. Here, Fechner begins to define a differential threshold like his contemporary, Weber, had in his experiment.
Now, the concept of absolute and differential thresholds is not fixed; the point where a theshold lies can shift according to varying conditions in the subject's nervous system. For example, neurons in the nervous system can fire even without the introduction of a stimulus. This phenomenon is called "noise." In the presence of "noise," if a stimulus is presented to the subject, it is less certain that they will identify it correctly. If there is no "noise" in the nervous system; if there is little neural activity, than the possibility that the subject will see the stimulus is greater. A theory was created to account for this variance called signal detection theory.
Beyond the problem of "noise" present at the time of stimulus introduction, there is also the issue of response bias. When presented with a test of this type, subjects will respond very differently regardless of whether or not they're detecting anything. If the experiment consists of placing the subject in a dark room and telling them that there will be a warning buzzer indicating that behind the screen in front of them, a light will flick on, or it won't. They have to say "yes" if they see the light, and "no" if there is no light after the buzzer sounds. Early on in such an experiment, the light will be bright and easily noticeable, and so responses are made easily. However, as the test progresses and the light grows dimmer and dimmer, the subject may say "yes" even if they aren't sure they're seeing a light, or they may always say "no," even if there might be a light in front of them, just because they don't want to look stupid.
Signal detection theory takes all of these problems into consideration. Rather than attempting to determine an absolute threshold, researchers realize that individuals are going to respond differently according to their fears about being wrong, or their desire to get everything right. This problem of varying motivations and differences in people's personalities are going to skew test results somewhat.
One way in which researchers have started to combat this problem is through Group Operating Characteristic (GOC) analysis. This is a technique that while introduced in the 1960s, has only been fully refined within the last ten years. Basically, GOC analysis takes the same subject and repeats the stimulus test several times in order to average out error due to neural noise, response bias and incoordination with response manipulanda. When looking at the multiple responses, any "noise" affecting identification that is not common in all responses is averaged out.
Beyond the physiological approach of examining the structure of sensory systems, psychophysics is a branch of science that seeks to understand how sensory systems function; what are their uses, their limits? Psychophysics is making progress towards answering these questions with greater clarity every day.
For more on GOC analysis in detail, please see http://www.psychophysics.org/judi_asa-2000.htm
Be sure to check out their amusing "psychophysical haikus" at http://www.psychophysics.org/vithumour.htm#Psychophysical Haiku and let the hilarity ensue!