STIMULUS GENERALIZATION: THE ORDERING AND SPACING OF TEST STIMULI 1

Twenty‐four pigeons learned a successive discrimination between 500 mμ (S+) and 574 mμ (S—). When tested in extinction, some birds received stimuli around S+, with no S— presentations. These birds showed a positive peak shift, with maximum responding not at 550 mμ, but displaced to 538 mμ and 544 mμ. Other birds were tested with stimuli around S—, with no S+ presentations. These birds showed a negative shift, with least responding not at 574 mμ, but at 586 mμ. Though the first group was tested around S+ and the second around S—, total responding between groups did not differ. When retested on the other half of the continuum, however, birds that had gone from the S+ half to the S— half responded fewer times than those that had gone from the S— half to the S+ half. In a second experiment, reducing stimulus spacing from 6 mμ, to 2 mμ produced flatter gradients and decreased the amount of positive shift. In a third experiment, birds were tested across the whole continuum with stimuli presented in serial order. A sequence from 538 mμ to 586 mμ produced no responding after the first part of the session; a sequence from 586 mμ to 538 mμ produced responding throughout the session.