Frequency selectivity of the middle latency response.

A forward masking paradigm was used to assess the frequency selectivity of the middle latency response (MLR). Tuning curves of the MLR were obtained in unanesthetized gerbils. Changes in the amplitudes of MLR waves A, B, and C with latency values of 10 to 13 ms, 14 to 17 ms, and 20 to 25 ms, respectively, were analyzed as a function of masker frequency and intensity. Tuning curves of the MLR were also compared to tuning curves of the auditory brainstem response (ABR), which was recorded simultaneously with the MLR. The MLR and ABR differed in their response to forward masking. The MLR was reduced in amplitude or eliminated by masker stimuli that had minimal or no effect on the ABR. Forward masking often caused variable and non-monotonic changes in the amplitude of the MLR. Tuning curves of the MLR indicate that the MLR is less frequency selective than the ABR. The MLR is an electrophysiological measure of auditory function central to the auditory brainstem. Therefore, it may provide information concerning central components of normal and pathological auditory function. However, because of the variability of MLR amplitudes with forward masking, tuning curves of the MLR are difficult to obtain and are not efficient for routine measurements of frequency selectivity.