Brightness contrast: a reinterpretation of compound cue and combined cue experiments with pigeons.
Control relative brightness contrast, not absolute light levels, for stable visual discrimination.
01Research in Context
What this study did
Researchers worked with pigeons in a lab.
They tested how birds tell lines apart from backgrounds.
The team changed how bright the line was and how bright the background was.
They wanted to know if birds cared about the difference between them, not just how bright each part was.
What they found
The birds always pecked most where the line and background had the same brightness difference they learned.
Even when both line and background got darker together, the birds still chose correctly.
This showed the birds tracked the contrast between parts, not the actual light level.
How this fits with other research
Green et al. (1975) and Van Hemel (1973) saw similar response patterns but blamed reinforcement timing.
The new study says brightness contrast, not timing, caused those patterns.
This updates the older work by showing the real controller was visual contrast.
Shah (1992) later added video tracking to the same setup, proving birds weren't just turning away from the key.
Why it matters
When you set up visual discrimination tasks, match the contrast between target and background.
Don't worry about making both items brighter or darker together.
Keep the relative difference the same and learners will stay accurate.
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02At a glance
03Original abstract
A group of three pigeons was trained on a 4-ply multiple schedule: a green color and a vertical line superimposed upon an achromatic background as positive stimuli, and a red color and a horizontal line on an achromatic background as negative stimuli. The pigeons were tested with the vertical line superimposed upon different achromatic background intensities, then with the vertical line superimposed upon different green background intensities, and finally with the vertical line and its training achromatic backgfound attenuated (and unattenuated) by a neutral density filter. The gradients peaked at the luminance of the achromatic background used during training and at the equivalent luminance for the green background when it was substituted for the achromatic background. The brightness contrast, not the background luminance, was the critical variable as the neutral density filter attenuated both the line and the background equally, leaving brightness contrast unchanged; there was no response decrement to this attenuated stimulus. Two other groups of three pigeons showed that they attended to line orientation as well as to brightness contrast. The brightness contrast hypothesis was extended to explain results of attention experiments and combined cue experiments which have used line stimuli in combinations with different backgrounds.
Journal of the experimental analysis of behavior, 1980 · doi:10.1901/jeab.1980.33-87