Adonis Diaries

“Psycho-physical methods in Human Factors”

Article #37 in human factors in engineering, (Written in March 29, 2006) 

In the 19^th century, a few physiology and neurophysiology scientists such as Gustav Theodor Fechner (1860), von Helmholtz, and Weber endeavored to study the functional relations between body and mind which resulted in the development of controlled experiments for reaching common reference points that would relate the psychological feelings of the senses to their corresponding physical measurements.

For example, the relationship of the perception of brightness and color versus amplitude and frequency of radiant energy waves or the feelings of loudness and pitch versus amplitude and frequency of mechanical or sound waves.

Fechner demonstrated that the biology of the human does record the variations of the environment by attenuating them to logarithmic values.

The sound intensity of 10 violins in an orchestra is actually heard as one violin. If the conductor wished the audience to hear the sound of ten violins then he will have to increase the number of violinists to one hundred.

If the cord of an instrument vibrates at 80 Hz, then if the cord is pinched in the middle it will vibrate at 160 Hz, at 1/4 it will vibrate at 320 Hz and so on.

Thus, the pitch sensation and identification of octaves in a sound is attenuated logarithmically too.

As the physical stimulant increases geometrically, the biology of the human senses the increase as an arithmetic progression.

Weber discovered that the ratio of the differential threshold to the initial stimulant for the subject to sense a modification in the variation is a constant.

In the case of the pitch the constant is 1.0594; so that for a frequency of 440 Hz (the first La in the octave), then the next frequency to be differentiated would be 440 * 1.0594 = 466 Hz, the next frequency to be sensed would be 494 Hz.

The absolute threshold or the minimal excitation to hear a pitch is 3 Hz.

The psychophysical method have been applied extensively in telephone, TV, radar, illuminating, sound, and geophysical industries who wanted to provide the end users with a satisfactory psychological comfort level without undue expenditure on top notch physical circuitry or infrastructures that would not add to the requisite human performance and capabilities.

Recently, there have been attempts to adapt this method to studying complex cognitive concepts such as attitudes, mental abilities, reliable speech quality, and naturalness of voice in telephone by measuring these concepts.

The measurements of interest are:

1. the absolute thresholds (limens) that mark the boundary between sensation and no sensation,

2. the differential thresholds or the smallest detectable change in sensation corresponding to the amount of changes in the physical stimuli,

3. the equality effects values in sensations produced by two different kinds of physical stimuli like when you filter a green color to make feel as bright as a red color.

Subsequently, whenever stimulus is mentioned it means a physical value and response of a subject refers to his answer for a psychological sensation.

The psychophysical method capitalizes on the capabilities of human:

1. being excellent in simple detection tasks or null indicator or whether a sensation exist or not;

2. being good in differentiating the direction of strength of a sensation such as bigger or smaller than a standard,

3. human is bad is evaluating whether a sensation is twice or three times stronger,

4. he is worst as a meter for exact measurements;

5. human is more accurate in feeling than when relying on his mind and that is why a subject is forced to make a choice between two stimuli rather than responding that the sensation is equally strong.

For example, it is harder to hear speech in a noisy environment than in a quiet one;  and the problem is to investigate how loud should a person raise his voice in order to be heard in a noisy room?

The development of this method generated three basic procedures for measuring the equivalence between psychological sensations and the physical parameters which are:

1. Procedure of adjustment or average error,

2. procedure of limits or just-noticeable difference, and

3. the procedure of constants which is the most accurate and widely applicable.

Consequently, we will focus on the third procedure in the subsequent sections.

The procedures of adjustment and of limits are slightly different but generally the experimenter shows the subject two series of stimuli,

The first series starts with a stimulus well above threshold and the subsequent values diminish by small and equal steps; the second series starts well below threshold and values are increased in equal amounts.  The final responses of the subject are averaged whether the response is an indicator of a sensation or expressing direction of strength such as more than, equal, or less than.  The experimenter either takes control of varying the stimuli or he may allow the subject to control the stimuli until he reaches a definite sensation response.

The procedure of constants starts with the experimenter performing an exploratory study to determine the upper and lowest thresholds and then selects between 4 to 7 stimuli spanning the range of sensations.

These selected stimuli are everytime randomized when presented more than 50 times to the same subject.  If equality or differential thresholds are being measured then a standard value is compared to the subject responses.

Usually, the more intense the stimulus the higher the percentage of correct identification and the choice of threshold criterion vary between 100% correct answers and 50% which basically represents chance happening depending on the behavior of responses or the graph trend and shape; the stimulus value adopted corresponds frequently to a threshold criterion of 75% correct.

An example is in order.

A telephone company is planning to provide TV viewers with signals transmission that give sharp images without the need to invest in expensive top notch circuit infrastructure.

The bandwidth for transmission should be more than adequate for a satisfactory picture; a bandwidth of 4 megacycles transmits blurred images while 10 megacycles is more than excellent.

A cheaper simulator is achieved by using an optical defocusing of movement of the projection lens corresponding for the range of zero to 10 mils.

An observer views a standard scenes compared to an identical but partly blurred scene. The amounts of blurring are set up in steps of 2 mils and the subjects are forced to guess which scene is the sharper.

By selecting a threshold criterion of 75% correctly guessed the company was able to decide on an effective bandwidth of less than 7 megacycles that provide an excellent sharpness psychologically and save on irrelevant costly hardware.

There are many pitfalls for inexperienced investigators conducting psychophysical experiments and many of these problems are the results of systematic biases such as:

1. Observers tend to quickly receive cues from the control devices they are handling and their responses become biased;

2. the space order of the standard and variable stimuli can be a source for error biases, for example, it is well known that a vertical line looks longer than an identical horizontal line;

3. the second of two equal stimuli is judged louder or brighter than the first;

4. a series of “yes” responses for example encourages a subject to persist in the same kind of response, thus the experimenter should vary the direction of responses;

5. observers tend to get cues from the experimenter through his voice, gestures, or instructions.

Most of these errors can be counterbalanced when the order of stimuli presented is randomized at each trial.

The psychophysical method is being used in the study of sensory performance of non-human species as well.