In contrast to the results of sucrose consumption and taste reactivity studies, however, some evidence of an adolescent-related attenuation in the hedonic response to a rewarding social stimulus has been observed when using emissions of 50 KHz ultrasonic vocalizations (USVs) as an index of positive affect (Blanchard et al., 1993; Fu & Brudzynksi, 1994). In previous research, rats have been shown to emit USVs in the range of 22 KHz under a variety of aversive circumstances (see Brudzynski, 2001), including foot shock (Tonoue et al., 1986) and the presence of predator odor (Blanchard et al., 1991). Expression of USVs in the 50–55 KHz range, however, was associated with circumstances inducing a positive affective state, such as play fighting (Knutson et al., 1998), experimenter “tickling” (Panksepp & Burgdorf, 2000), and electrical stimulation of the reward pathway (Burgdorf et al., 2000). When production of these 50 KHz USVs was assessed during a 10-min period of social interaction with an age- and sex-matched conspecific, adolescents were found to produce significantly fewer positive calls than adults during this “consummatory” period, even though the adolescents
