EFFECTS OF STIMULUS RATE ON THE FELINE BRAIN-STEM AUDITORY-EVOKED RESPONSE DURING DEVELOPMENT .2. PEAK AMPLITUDES

The effects of click rate on brain-stem auditory evoked response (BAER ) peak amplitudes were investigated in adult cats and kittens in four age groups: 10, 15, 20, and 30 post-natal days. Conventional BAERs wer e obtained at rates of 5, 10, 30, 60, and 90 Hz. Maximum length sequen ces (MLSs) were used to obtain responses at rates of 83, 125, 250, 500 , and 1000 Hz. For both techniques, clicks were presented at 90 dB pSP L, and at 20 dB above threshold (20 dB SL). BAERs were observed for al l age groups, at all click rates. However, not all peaks could be iden tified in all animals, with peak absence more common for the youngest kittens at the higher rates. BAER waves i thfough iv decreased in ampl itude with increasing click rate for all age groups. In general, the s mallest peak amplitudes were observed for the youngest animals (10- to 15-day-old) studied, while the largest amplitudes were found for the adult, 20- or 30-day-old animals, regardless of rate. When amplitudes were normalized to eliminate the influences of differences in absolute values due to developmental stage (i.e., made proportional to the amp litude observed at 5 Hz), the younger age groups exhibited enhanced se nsitivity to the effects of stimulus rate. However, exceptions to this trend were observed. In general, amplitudes of BAER peaks obtained wi th the MLS procedure were smaller than those obtained using convention al averaging with a similar average click rate, regardless of age. Fin ally, the most efficient method for obtaining a response at a criterio n signal-to-noise ratio was typically by the use of conventional avera ging at a rate of 60 or 90 I-It, for all peaks and age groups. Overall , these data suggest that higher stimulus rates produce greater neural adaptation resulting in reduced BAER peak amplitudes. The mechanism r esponsible for the acquisition of adult-like adaptation properties app ears to develop during the early post-natal period in the cat. (C) 199 6 Acoustical Society of America.