Possible origins of the non-monotonic intensity discrimination function in forward masking.

A non-monotonic intensity discrimination function in forward masking has been recently reported [Zeng et al. (1991) Hear. Res. 55, 223-230; Zeng and Turner (1992) J. Acoust Soc. Am. 92, 782-787] in which just-noticeable-differences (jnds) in intensity are largest for midlevel tones and smaller for soft and loud tones following an intense narrow-band noise. One hypothesis was that this midlevel hump reflects the contribution of low-spontaneous rate (SR) neurons to intensity coding, based on the differential recovery from forward masking of low-SR and high-SR neurons [Relkin and Doucet (1991) Hear. Res. 55, 215-222]. The present study conducted three experiments stimulating different stages of the auditory system in an attempt to determine the peripheral and central origins of the midlevel hump. First, in two cochlear implant (CI) listeners, the forward masker produced a midlevel hump on the intensity discrimination function, suggesting that the synapses between the hair cell and the eighth nerve are probably not responsible for the hump, as they are bypassed and the eighth nerve is stimulated directly. Second, in auditory brainstem implant (ABI) listeners, the forward masker produced no midlevel hump, but the masked jnds were larger than those without a masker. The absence of the midlevel hump in the ABI listeners suggests that the occurrence of the hump requires physiological mechanisms in the auditory nerve transmission, or the intrinsic processing circuits of the cochlear nuclei, or both. Third, in normal-hearing listeners, an ipsilateral, 90 dB SPL, pure-tone forward masker produced a midlevel hump, which is similar to that using a narrow-band noise masker; whereas a contralateral forward masker produced essentially no midlevel hump, suggesting that binaural interactions at superior olivary complex and more central sites are probably not responsible.