Psychophysics: threshold, signal detection theory

Psychophysics is the study of the relationship between the physical properties of stimuli and the psychological experiences they evoke. It is a branch of psychology that uses scientific methods to study how humans perceive the world around them.

Threshold

One of the most important concepts in psychophysics is the threshold. A threshold is the minimum intensity of a stimulus that can be detected or discriminated against. There are two main types of thresholds: absolute thresholds and differential thresholds.

Absolute threshold

The absolute threshold is the minimum intensity of a stimulus that can be detected 50% of the time. For example, the absolute threshold for hearing is the faintest sound that can be detected 50% of the time.

Differential threshold

The differential threshold is the minimum difference in intensity between two stimuli that can be detected 50% of the time. For example, the differential threshold for brightness is the smallest difference in brightness between two lights that can be detected 50% of the time.

Signal detection theory

Signal detection theory is a theoretical framework for understanding how people detect stimuli in the presence of noise. Noise is any unwanted signal that interferes with the detection of the desired signal.

Signal detection theory assumes that people make decisions about whether or not a stimulus is present based on a criterion. The criterion is a level of sensory evidence that must be exceeded before the person decides that the stimulus is present.

There are two types of errors that can be made in signal detection tasks: false positives and false negatives. A false positive occurs when the person decides that the stimulus is present when it is actually absent. A false negative occurs when the person decides that the stimulus is absent when it is actually present.

Signal detection theory can be used to explain how a number of factors, such as the intensity of the signal, the intensity of the noise, and the person's motivation to detect the signal, can influence the probability of making false positives and false negatives.

Applications of psychophysics

Psychophysics is a widely used field of research with applications in many different areas, including:

• Sensory systems: Psychophysics is used to study how the different sensory systems, such as vision, hearing, and touch, work.
• Medical diagnosis: Psychophysics is used to develop new and more accurate methods for diagnosing medical conditions, such as hearing loss and vision problems.
• Product design: Psychophysics is used to design products that are more user-friendly and efficient. For example, psychophysics is used to design the optimal brightness and contrast for computer displays and the optimal volume for car stereos.
• Legal system: Psychophysics is used to develop eyewitness identification procedures and to assess the credibility of witness testimony.

Examples of psychophysical experiments

Absolute threshold

One common way to measure the absolute threshold is to use a method called the staircase method. In this method, the intensity of the stimulus is gradually increased until the participant is able to detect it. The absolute threshold is then defined as the average intensity of the stimulus at the point where the participant is able to detect it 50% of the time.

Differential threshold

One common way to measure the differential threshold is to use a method called the constant stimulus method. In this method, the participant is presented with a pair of stimuli that differ in intensity. The participant is asked to indicate which stimulus is brighter, louder, or more intense. The differential threshold is then defined as the smallest difference in intensity between the two stimuli that the participant is able to detect 50% of the time.

Signal detection theory

One common way to measure signal detection theory is to use a method called the four-alternative forced-choice method. In this method, the participant is presented with a series of trials. On some trials, the signal is present (signal trials), and on other trials, the signal is absent (noise trials). The participant is asked to indicate whether or not the signal is present on each trial.

The data from the four-alternative forced-choice method can be used to calculate two important measures: the hit rate and the false alarm rate. The hit rate is the proportion of signal trials on which the participant correctly identifies the presence of the signal. The false alarm rate is the proportion of noise trials on which the participant incorrectly identifies the presence of the signal.

The hit rate and the false alarm rate can be used to calculate a number of other important measures, such as the d' prime (d'), which is a measure of the participant's sensitivity to the signal.

Conclusion

Psychophysics is a powerful tool for understanding how humans perceive the world around them. It has a wide range of applications in many different fields, including sensory systems, medical diagnosis, product design, and the legal system.