Psychophysics and its Method: Method Of Limits and Methods of constant stimulus

Psychophysics and its Method:

Psychophysics is a combination of two terminologies. Psycho is related to mind and physics is related to physical quantity. In psycho physics, physics refers to the measurement of some mental experience. We can measure physical quantities like length, height, weight, etc. of a particular physical object. Similarly, in psychophysics, we measure a quantity called precept. This is the perception of an individual corresponding to a physical quantity of some physical object. For example, length can be perceived as size, weight can be perceived as heaviness, intensity can be perceived as brightness volume can be perceived as loudness and so on. The perception of an individual makes difference in analyzing the physical objects in everyday life. For example, the same loudness of music can be low for one person, but for other it can be high. Another example can be the force of a push. One person can perceive it as a gentle push and other can perceive it as a harsh push.

1 Method of Limits

To measure absolute threshold with the method of limits, a researcher would present the stimulus in either ascending or descending order to determine the smallest amount detectable. For example, you would be given a large amount of light and asked if you could see it. On the next trial, the amount of light would be decreased. This would occur until you report that you can no longer see the light. This would be the method of descending limits because we start with something big and work down. In the method of ascending limits, we would start with a small amount of light and work up.

 In this method, experiments are conducted with random values. For example, an experiment is conducted with different volume levels for sound. Different volume levels are presented randomly to the participants and they are asked whether the sound is loud or not. Pros of this method are that we only have to conduct the experiments and plot the curve. Cons of this method are that we must have some idea about the threshold and we have to conduct lots of experiments. 

2 Method of Adjustment

In the method of adjustment, the observer or the researcher continuously adjusts the stimulus until they can or cannot perceive it. This differs from the method of limits, because the individual is not giving a yes or no response on each trial. Instead, they are responsible for adjusting the stimulus themselves. For example, if you were asked to identify the smell of gym socks, you might have a knob that you can turn until you can just smell the scent. Or, you might turn the knob until you can no longer smell the scent.

: In this method, the experiment is started from a specific level. Let us assume that the experiment is to detect the threshold of loudness of sound. The experiment will start from a specific sound level, let us say 10. Let us assume that the threshold intensity is 50. 10 is not loud enough, so the frequency is increased to 60. 60 is loud, so the frequency is decreased to 20 and so on. In this, the graph converges in the end. A question was asked whether the intensity goes back to zero, then changes. If the intensity is high, the test value is reduced. If the intensity is low, the test value is raised.

3 Method of Constant Stimuli

The method of constant stimuli differs from the method of limits and the method of adjustment in that it presents the stimuli in a random order. For example, if I'm looking to see what is the lowest amount of sound that you can detect, I would randomly present several different tones at different sound levels. Some would be loud, while others would be quiet. As the observer, your job would be to let me know when you can or cannot hear the sounds. 

: In this case, the subjects adjusts themselves till they think they have detected the stimulus. The participants can increase or decrease the value of the intensity. The participants are given an initial test value. They can use buttons or dial to increase or decrease the value of intensity. In these experiments, trials will be of two types, ascending trial and descending trial. Ascending trial is where the participant increase the level of intensity. Descending trial is the where the participant decreases the level of intensity. For example, participant is given a test intensity of 10. He increases it to 50. This will be an ascending trial. If he is given 70 as test intensity, he will reduce it to 50. This will be a descending trial. After experiments, we will take mean of ascending trials and mean of descending trials. These means should not be too different. In the end, we will take a grand mean. This mean will be Point of Subjective equality.