1. Introduction to Psychophysics
Dr. Shailja Sharma
Assistant Professor
Psychology

3. Psychophysics=Psychological events + Physical events
Meaning
Psychophysics=Psycho + Physics
Psycho or Psychology is the science of behaviour
And
Physics studies matter and energy of stimulus
Psychophysics=Psychological events + Physical events

5. Definitions
“An exact science of the functional relations of dependency between body and mind.”
-Fechner (1860)
“ Psychophysics has been regarded as the science that investigates the quantitative relationships between physical events and corresponding psychological events” Guilford (1954)
Psychophysics, study of quantitative relations between psychological events and physical events or, more specifically, between sensations and the stimuli that produce them Encyclopaedia Britannica

7. Basic Concepts of Psychophysics

8. Sensitivity Stimulus-properties such as intensity Sense organ
The capacity of the receptor organs to respond selectively and differentially to physical stimulation.
a) Absolute Sensitivity: Defines the limits of the organism’s capacity to respond to stimulation. It is inversely related to the minimum stimulus which can be detected reliably by a subject.
b) Differential Sensitivity: Defines the organism’s capacity to respond to differences, both qualitative and quantitative, between stimuli. It is inversely related to the minimum difference between stimuli needed for reliable discrimination.

9. Thresholds
The line separating two kind of stimuli – those never yielding responses and those always yielding responses.
Absolute threshold-It is the minimum stimulus necessary for detection, also known as absolute Limen. For purpose of measurements, absolute threshold is considered as that stimulus value which yields a response 50 percent of the time.
Differential threshold-It is the stimulus difference that gives rise to a judgment of “different” 50 percent of the time.

10. Point of Subjective Equality (PSE)
Value of a comparison stimulus that, for a given observer, is equally likely to be judged as higher or lower than that of a standard stimulus

11. Types of Errors
Variable-The degree to which judgments differ from trial to trial provides an index of the amount of variable error. Error here simply refers to the extent of fluctuation in judgments.
Constant-A systematic tendency on the part of the subject toward overestimation or underestimation of the stimuli. Two frequently encountered constant errors:
Time error
Movement/Space error

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13. Problems of Psychophysics

15. Absolute Threshold (AL)
The threshold is usually defined statistically, e.g., as the energy level that is detected as often as not over a series of presentations.
The problem of sensitivity involves the determination of the smallest detectable intensity of a stimulus (called the absolute threshold), often as a function of another stimulus dimension, such as wavelength, frequency, duration, or areal extent. 
For a specific type of sensory input is the minimum stimulus intensity that an organism can detect 50% of the time.

17. Differential Threshold(DL)
Thus the second major concern of psychophysics is to measure the smallest detectable change in a stimulus (the so-called difference threshold) 50% of the time.
The JND increases with the magnitude of the stimulus, e.g. if you add 10 gms to a weight, you will notice the difference; add 10 gms to 1 kg and you will not.

18. Stimulus Equality (Static Invariance of Sensory System)
A third major problem of psychophysics is to determine those arrangements of stimuli that produce responses that are equivalent in some respect.
The goal of this kind of measurement is to specify all the energy configurations in the environment that produce an invariant or equivalent sensory response.
For example, the goal may be to determine the combinations of intensity and duration of a flash target that produce the same apparent brightness.

19. Equality of Ratio (Dynamic Properties of Sensory Systems)
A major problem of psychophysics is to learn how much the magnitude of the sensory response grows when the stimulus intensity increases.
It is one thing to know the stimulus conditions that produce an invariant sensory effect and another thing to know how much larger one sensory effect is than another—e.g., how much brighter one luminance level appears than another or how much two tones seem to differ in pitch.

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