Separating the effects of salience and disparity on the rate of observing.

The team worked with pigeons in a small lab chamber. Birds could peck a key to turn on one of two colors for a few seconds.

The colors differed in two ways. Salience: how much each color stood out from the dark chamber walls. Disparity: how different the two colors were from each other.

By changing these two properties across tests, the researchers could see which one made the birds peck the "observing" key more often.

Pigeons pecked most when the colors were bright against the dark wall, even if the two colors were almost the same.

Making the colors very different from each other helped a little, but brightness against the background mattered far more.

Newman et al. (1991) later showed that disparity does matter when you also change the rate of food for each color. Their pigeons shifted choice as payoff changed, something A et al. did not test. The two studies fit together: salience drives looking; disparity plus payoff drives choosing.

Roane et al. (2001) moved the idea to humans. They found that both stimulus contrast and response options (how different the keys were) shaped accuracy. Again, the basic message holds across species: clearer signals give better control.

Varley et al. (1980) had already shown pigeons track brightness contrast, not raw light level. Bowe et al. (1983) add that this contrast is more powerful than the difference between the two stimuli themselves.

When you set up discrimination tasks, make the teaching stimuli pop against the background before you worry about how different they are from each other. Use high-contrast pictures, bold fonts, or bright icons on a plain screen. Once the learner reliably looks, you can fine-tune the difference between S⁺ and S⁻. This order saves prep time and can speed acquisition.