Stimulus - Pigeons

Attention is a limited resource and is not an all-or-nothing response: the more attention devoted to one aspect of the environment, the less is available for others. A number of experiments have studied this in animals. In one experiment, a tone and a light are presented simultaneously to pigeons. The pigeons gain a reward only by choosing the correct combination of the two stimuli (e.g., a high-frequency tone together with a yellow light). The birds perform well at this task presumably by dividing attention between the two stimuli. When only one of the stimulus variations is presented at its rewarded value, discrimination improves on the variable stimulus, but Discrimination on the alternative stimulus worsens. These outcomes are consistent with the notion that attention is a limited resource that can be more or less focused among incoming stimuli. What did the experiment involving the pigeons indicate?