Behavioral contrast with multiple positive and negative stimuli on a continuum.

After an initial period of nondifferential training, six pigeons were trained on a go/no-go discrimination involving 12 line tilts from vertical clockwise to horizontal. Responses to the first six tilts (positive stimuli) were reinforced on a variable-interval one-minute schedule, whereas responses to the other six tilts (negative stimuli) were extinguished. During the first several discrimination sessions, the highest response rate was typically to one of the positive stimuli that was relatively close to the negative stimuli or at an intermediate distance, rather than to one of the positive stimuli most distant from the negative stimuli. This effect decreased with extended training up to 50 or 80 sessions.