Brightness contrast effects elicited by foveally viewed, briefly exposed, point stimuli were measured and related to a psychophysical measure of inducing stimulus output. It was found, even with inducing output held constant, that magnitude of contrast effect depended upon test stimulus illuminance and that contrast effects summated in a nonlinear fashion. Relevance of the findings to more complicated visual phenomena, such as Mach bands, is discussed.
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Test and inducing E designated in log units above 7.5×10−9 ft-c, measured in the plane of the subject’s pupil. The upper figure in each cell represents contrast effect, as defined in the text. The lower figure is the output elicited by the inducing stimulus, measured in the presence of the test stimulus. The units of the output scale have been selected so that the output of either test or inducing stimulus, when presented alone, is approximately numerically equal to log E. The figures in parentheses indicate the number of replications upon which the values are based. Where 8, 12, or 16 replications are specified, results for 2, 3, or 4 different test-inducing permutations have been averaged. Throughout the table, the standard error of contrast effect is about 0.05 and that of inducing output is about 0.10. No observations were made under conditions for which cells are empty. Absolute threshold, in log E, for a test stimulus presented alone is 0.4 for JT, 0.8 for NM. Output elicited by the fixation point, measured without test or inducing stimuli, is 1.1 for JT, 1.2 for NM.
Approximate: variable stimulus could not be made bright enough to obtain a good match.
Results of one replication discarded because test stimulus was not seen on some trials.
Table III
Estimated width of the bright line in a Mach Band pattern at two exposure durations. Conditions I–VIII represent progressively steeper gradients between the light and dark parts of the pattern (sharper focus). Each entry is the number of times the subject gave the particular estimate.
Estimated width (minutes of angle)
I
II
III
IV
V
VI
VII
VIII
4.00 sec
1.5
4
3.0
3
5
5
4
4.5
1
1
5
4
3
3
6.0
7
8
7
3
1
0.08 sec
1.5
1
6
7
3.0
2
5
7
7
8
2
1
4.5
3
7
3
1
6.0
3
1
Tables (3)
Table I
Time sequence for one trial.
Elapsed time (seconds)
Event
00.00
Start of trial
Adaptation light on
07.50
Adaptation light off
12.50
Stimuli on (exposure=0.080 sec)
12.58
Stimuli off
15.00
Start of next trial
Adaptation light on
Table II
Contrast effect as a function of inducing output for various combinations of stimulus illuminances and spatial separations.a
Test and inducing E designated in log units above 7.5×10−9 ft-c, measured in the plane of the subject’s pupil. The upper figure in each cell represents contrast effect, as defined in the text. The lower figure is the output elicited by the inducing stimulus, measured in the presence of the test stimulus. The units of the output scale have been selected so that the output of either test or inducing stimulus, when presented alone, is approximately numerically equal to log E. The figures in parentheses indicate the number of replications upon which the values are based. Where 8, 12, or 16 replications are specified, results for 2, 3, or 4 different test-inducing permutations have been averaged. Throughout the table, the standard error of contrast effect is about 0.05 and that of inducing output is about 0.10. No observations were made under conditions for which cells are empty. Absolute threshold, in log E, for a test stimulus presented alone is 0.4 for JT, 0.8 for NM. Output elicited by the fixation point, measured without test or inducing stimuli, is 1.1 for JT, 1.2 for NM.
Approximate: variable stimulus could not be made bright enough to obtain a good match.
Results of one replication discarded because test stimulus was not seen on some trials.
Table III
Estimated width of the bright line in a Mach Band pattern at two exposure durations. Conditions I–VIII represent progressively steeper gradients between the light and dark parts of the pattern (sharper focus). Each entry is the number of times the subject gave the particular estimate.
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