Classical psychophysics

· Classical psychophysics

Psychophysics is commonly defined as the quantitative branch of the study of perception, examining the relations between observed stimuli and responses and the reasons for those relations.

Psychophysics was established by German scientist and philosopher Gustav Theodor Fechner. He coined the word, developed the fundamental methods, conducted elaborate psychophysical experiments, and began a line of investigation that still persists in experimental psychology. Fechner’s classic book Elemente der Psychophysik (1860) may be looked upon as the beginning not only of psychophysics but also of experimental psychology.

Psychophysics quantitatively investigates the relationship between physical stimuli and the sensations and perceptions they effect. Psychophysics has been described as "the scientific study of the relation between stimulus and sensation" or, more completely, as "the analysis of perceptual processes by studying the effect on a subject's experience or behaviour of systematically varying the properties of a stimulus along one or more physical dimensions".

Psychophysics also refers to a general class of methods that can be applied to study a perceptual system. Modern applications rely heavily on threshold measurement, ideal observer analysis, and signal detection theory.

-Absolute threshold

Definition: The amount of stimulation necessary for a stimulus to be detected. In practice, this means that the presence or absence of a stimulus is detected correctly half the time over many trails.

·         The lowest level of stimulation that any one person can detect.

·         A person's ability to sense a stimulus at it lowest intensity.

·         The least amount of stimuli need to be detected or noticed.

·         Eg. If I can hear a very high pitched noise, but someone else cannot, then the sound is above my own absolute threshold and below the other person's absolute threshold. A good example of this would be the "Mosquito" ringtone:

A threshold (or limen), is the point of intensity at which the participant can just detect the presence of a stimulus or presence of a difference between two stimuli. There are two kinds of thresholds: absolute and difference.

An absolute threshold is the level of intensity of a stimulus at which the subject is able to detect the presence of the stimulus some proportion of the time (a p level of 50% is often used).

Absolute threshold is also often referred to as detection threshold.

-Difference threshold

A difference threshold (or just-noticeable difference) is the magnitude of the smallest difference between two stimuli of differing intensities that the participant is able to detect some proportion of the time (again, 50% is often used). To test this threshold, several different methods are used. The subject may be asked to adjust one stimulus until it is perceived as the same as the other, may be asked to describe the magnitude of the difference between two stimuli, or may be asked to detect a stimulus against a background.

-JND

Just noticeable difference (JND) describes the marginal difference in a stimulus (compared to a similar stimulus) needed for that difference to be perceived. Also mathematically represented as:

-Psychophysical methods

The term psychophysical methods is sometimes extended to include certain scaling techniques which are most often usedwith subjective dimensions to which there correspond no simple physical dimensions, for example, food preferences.

In 1860, G. Fechner designed psychophysical methods to measure the absolute threshold, defined as the minimum stimulusenergy that an organism can detect, and the differential threshold, defined as the minimum detectable change in a stimulus.Both quantities had to be defined as statistical averages. To obtain reliable measurements for these averages, Fechner devisedthe method of limits (also called the method of minimal changes) and the method of constant stimuli.

In the method of limits, the experimenter begins with a stimulus which is too weak for the subject to detect. In successivepresentations, the stimulus intensity is increased in small, equal steps, the subject reporting after each presentation whether the  stimulus was perceived until it has been detected. The descending series is then begun, the stimulus intensity beginning at an above-threshold value and decreasing in steps until the subject signals the disappearance of the stimulus. Many such seriesare  given.

In measuring the difference threshold, essentially the same procedure is involved, except that the subject now signals therelation of a comparison stimulus to a standard stimulus. After a large number of such trials, the average of each of these fourthreshold values is computed.

To measure the absolute threshold by the method of constant stimuli, the experimenter selects a small number of stimulusvalues in the neighborhood of the absolute threshold (previously roughly located by informal use of the method of limits) andpresents them to the subject a large number of times each, in an irregular order unknown to the subject. Each time a stimulusis presented, the subject reports the presence or absence of sensation.

In directmatching methods the subject is not required to produce or assess the ratio of one subjective magnitude to another, but only to adjust a comparison stimulus until some attribute appears to match that of a standard stimulus. For example, thesubject might be asked to adjust the physical intensities of tones of various frequencies until their loudness matched that of a 1000-Hz tone of fixed intensity. The result would be an equal loudness contour, showing the intensities to which tones ofvarious frequencies must be set to produce sensations of equal  loudness. These data are of use in acoustics. 

The method of average error, the third of the three methods devised by Fechner, is a special application of direct-matchingmethods to cases in which the point of interest is in discrepancies between perception and stimulation. The subject adjusts acomparison stimulus to match a standard stimulus; the average of a number of such settings gives the point of subjectiveequality, and the difference between this point and the standard stimulus is the average error. Two illustrative uses of  themethod are the measurement of accuracy of distance perception and the measurement of the magnitude of so-called opticalillusions.

-Weber’s law

Weber’s law, also called Weber–Fechner law,  historically important psychological law quantifying the perception of change in a given stimulus. The law states that the change in a stimulus that will be just noticeable is a constant ratio of the original stimulus. It has been shown not to hold for extremes of stimulation.

The law was originally postulated to describe researches on weight lifting by the German physiologist Ernst Heinrich Weber in 1834 and was later applied to the measurement of sensation by Weber’s student Gustav Theodor Fechner, who went on to develop from the law the science of psychophysics. By stating a relationship between the spiritual and physical worlds, the law indicated to Fechner that there is really only one world, the spiritual. To others, the law meant the possibility of a scientific, quantitative psychology. The combined work of Weber and Fechner has been useful, especially in hearing and vision research, and has had an impact on attitude scaling and other testing and theoretical developments.

 

The slope of the line is the Weber fraction (aka Fechner fraction). 

Example 

Let two circles filled with same color and intensity be displayed side by side. Suppose the initial color intensity is 100 for both circles. Now let  us  keep changing the color intensity of any one circle incrementally till the user perceives noticeable change  or  difference  in  their  intensities. We  will stop at this point and record this new color intensity. Let  this new value be 110. Thus the differential   threshold (jnd)  = 10 (ΔI = 110 – 100 ) and the Weber’s constant in this  case  would  be K = 10/100 = 0.1. If the Weber fraction for discriminating changes in stimulus 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.

Some interesting human-sense JNDs (Just Noticeable Differences) are tabulated below : 

Pitch: 1/333               Brightness: 1/60              Lifted Weights: 1/50

Loudness: 1/10          Pressure on skin: 1/7      Taste: 1/5

-Fechner’s law

The magnitude of a stimulus can be estimated by the formula S=klogR, where S = sensation, R = stimulus, and k = a constant that differs for every sensory modality (sight, touch, temperature, etc)

It refers to a psychology-based term formulated by the German psychologist Gustav Theodor Fechner, that focused on the relationship between physical stimulus and psychological experiences. It asserts the proportionality that exists between the intensity of a subjective sensation and the logarithm of the physical stimulus.

-Steven’s power law

A law of magnitude estimation that is more accurate than Fechner's law and covers a wider variety of stimuli. It is represented by the formula S=kl^a, where S = sensation, K= a constant, l = stimulus intensity, and a = a power exponent that depends on the sense being measured.

-Signal detection theory (SDT)

Explains how he detect "signals" consisting of stimulation affectiing our eyes, ears, nose, skin, and other sense organs. Signal detection theory says that sensation is a judgment the sensory system makes about incoming stimulation. Often, it occurs outside of consciousness. In contrast to older theories from psychophysics, signal detection theory takes observer characteristics into account.

-Applications of SDT- Receiver operating curve