Peak Shift Effect

The peak shift effect is a phenomenon in discrimination learning in which, after an animal or person has learned to respond to one stimulus, the positive stimulus, and not to a second, similar stimulus, the negative stimulus, along the same physical dimension, the point of strongest responding shifts away from the original positive stimulus toward a new stimulus that lies even further from the negative stimulus. In a classic example, a pigeon trained to peck at a disc showing a 550 nanometer light and not to peck at a 540 nanometer light will often peck most vigorously not at 550 nanometers but at 560 nanometers, a wavelength further still from the negative stimulus. Researchers have proposed that the effect has evolutionary significance in areas such as warning coloration in animals.