Learning-related shifts in generalization gradients for complex sounds

entially judge novel stimuli as predictive of a learned consequence on the basis of the novel stimuli’s similarity to stimuli experienced during learning (Shepard, 1987). The phenomenon is often studied experimentally by training a subject to respond to one stimulus (S ). Generalization is then measured by removing feedback, presenting novel stimuli, and measuring responses to stimuli that are similar to the S . In a classic example, pigeons trained to respond to a key illuminated with a 580-nm light also responded to other wavelengths when reinforcement for their responses was absent; the number of responses to newly presented wavelengths lessened with decreasing similarity to the conditioned stimulus (Guttman & Kalish, 1956). This phenomenon is seen in animals ranging from invertebrates (Cheng, 1999, 2000, 2002) to humans (reviewed by Thomas, 1993) with various testing strategies and dimensions (Shepard, 1987). Because of its ubiquity, generalization was described by Pavlov (1927) as being a fundamental associative process and by Shepard as psychology’s first law. The universality of generalization likely reflects the fact that an organism’s ability to react appropriately to new environmental conditions often depends on its capacity to predict possible outcomes on the basis of prior experience (Ghirlanda & Enquist, 2003; Shepard, 1987). Despite the pervasiveness and importance of generalization across species, experimental studies of this phenomenon in humans and those in nonhuman animals have proceeded somewhat independently. Processes postulated to underlie generalization by humans, such as identifyg ( , ; in relational cues or rules Ahn & Medin 1992 Doll & Thomas, 1967; Helson, 1964; Imai & Garner, 1965; Shepard, Hovland, & Jenkins, 1961; Thomas, 1993; Thomas & Bistey, 1964; Verbeek, Spetch, Cheng, & Clifford, 2006; Wattenmaker, 1992), using abstract strategies (Gunzelmann, 2008; Rodrigues & Murre, 2007), and language (Fagot, Goldstein, Davidoff, & Pickering, 2006; Purtle, 1973; Shepard et al., 1961), are often difficult or impossible to investigate across species. Studies of nonhuman animals have instead focused on measuring the form of generalization gradients, context effects, dimensional variety, and training procedures (for reviews, see Ghirlanda & Enquist, 2003; Honig & Urcuioli, 1981; Purtle, 1973). There are some comparable data sets from human participants (Cheng, Spetch, & Johnston, 1997; Doll & Thomas, 1967; Spetch & Cheng, 1998; Thomas & Bistey, 1964; Thomas & Mitchell, 1962), but they are relatively rare. Although differences in the processes underlying generalization gradients in humans and nonhumans undoubtedly exist, many fundamental mechanisms are likely shared (Cheng, 2002; Ghirlanda & Enquist, 1999, 2003; Shepard, 1987). A basic process in all animals that strongly impacts generalization is learning. For example, Jenkins and Harrison (1960) found that pigeons reinforced after responses to a tone displayed narrower generalization gradients as f they experienced more training blocks. Another effect o learning on generalization, described as the peak shift effect (Hanson, 1959; Purtle, 1973; Spence, 1937), typically occurs after intradimensional discrimination training in which one stimulus is reinforced (S ), and another stimulus is not (S r ). When generalization tests are given afte such training, subjects mostly respond not to S , as one g p , mi ht ex ect but to one of the unconditioned test stimuli.