Preyer reflex in jaundiced rats: central auditory effects.

Normal control rats (N:45) and 27 genetically hyperbilirubinemic rats from an NIH colony were tested for the Preyer reflex (Pr) threshold using pure tones. About half of all animals (N:39) were tested at 4, 6, and 8 kc/s only, while 39 were also tested at 10, 15, 20, 25, and 30 kc/s. Data were not included from 3 jaundiced rats who died during the study, which began when Ss were 3 wks old. Some jaundiced rats differed from control rats in their elevated Pr thresholds for mid-frequency tones (approximately 10 kc/s) (group mean threshold difference of 6.4 db (p less than .05). Since jaundiced rats have been shown to have extensive damage in the first brain-stem synapse, neural dysfunction can be inferred. The Pr of jaundiced rats did not differ from normals at frequencies below 10 kc/s. Normal low-frequency sensitivity in jaundiced rats likely represents a tonotopic vulnerability gradient in the central auditory pathway.