Methods of adaptation level

According to Helson 1947, the subject-observer was told that he would be present with two weight I succession. After lifting the second weight he was to give a judgment of it by stating to which of the following categories it belonged;

Very, very heavy

Very heavy

Heavy

Medium heavy

Medium

Medium light

Light

Very light

Very, very light

The subject was further told that after he had judged a number of weights he would be given another set with he would judge in like manner. After a number of observations had been made he was cautioned; “be sure you judge each weight as it is and not as you think it should be”. It will be seen that there was no absolute categorizing of the meanings of the end terms or the medium of the scale. The subject was left free to interpret them as he wished. The results were presented as the average judgment of a group of subject. It should be noted, that although in presenting each stimulus a comparison or standard stimulus was also given as in psychophysical experiments, the judgment made the subject was not, as in psychophysical experiments, a comparison between second and the first stimulus, but was in term of the absolute scale for second stimulus alone, as above shown. The role of the comparison stimulus was provided for in the predictive equation. It was treated as one of the item that makes up the pool from which the adaptation level is established.

      

The mathematical procedure for handling the data of the experiment was developed from another work of the author in the field of brightness vision. That method, as applied to the brightness dimension, involved the taking of a weighted logarithmic mean of all stimuli and background in the field times a constant. For the visual problems the formula found to be adequate was as follows;

AL=K(A3/0A-)1/4

Where Al is adaptation level, K is a factorial constant, A0 is the brightness of the background, and A- is the logarithmic mean of the brightnesses of the sample on the background.

Helson’s experiments were also addressed to the testing of a number of additional theorems deduced from the general theory. These included the following deductions;

1. The predicated equivalence of absolute and relative methods for determining the adaptation level when the comparison stimulus is chosen at account the log mean of the series.
2. the minimum stimulus effective as a standard in shifting the adaptation level
3. the greater effect of a stimulus upon adaptation level when it is used as a standard than when it is used a member of the series
4. the predicted effect of various types of stimulus distribution
5. The effect of practice and past experience.

 Later he derived the formula

∆A = k(X1 + A)/2

Where k is Weber constant (p. 70) and X1 is particular stimulus