POSSIBLE ORIGINS OF THE NONMONOTONIC INTENSITY DISCRIMINATION FUNCTION IN FORWARD MASKING

A non-monotonic intensity discrimination function in forward masking h as been recently reported [Zeng et al. (1991) Hear. Res. 55, 223-230; Zeng and Turner (1992) J. Acoust Sec. Am. 92, 782-787] in which just-n oticeable-differences (jnds) in intensity are largest for midlevel ton es and smaller for soft and loud tones following an intense narrow-ban d noise, One hypothesis was that this midlevel hump reflects the contr ibution of low-spontaneous rate (SR) neurons to intensity coding, base d on the differential recovery from forward masking of low-SR and high -SR neurons [Relkin and Doucet (1991) Hear. Res. 55, 215-222]. The pre sent study conducted three experiments stimulating different stages of the auditory system in an attempt to determine the peripheral and cen tral origins of the midlevel hump. First, in two cochlear implant (CI) listeners, the forward masker produced a midlevel hump on the intensi ty discrimination function, suggesting that the synapses between the h air cell and the eighth nerve are probably not responsible for the hum p, as they are bypassed and the eighth nerve is stimulated directly. S econd, in auditory brainstem implant (ABI) listeners, the forward mask er produced no midlevel hump, but the masked jnds were larger than tho se without a masker. The absence of the midlevel hump in the ABI liste ners suggests that the occurrence of the hump requires physiological m echanisms in the auditory nerve transmission, or the intrinsic process ing circuits of the cochlear nuclei, or both. Third, in normal-hearing listeners, an ipsilateral, 90 dB SPL, pure-tone forward masker produc ed a midlevel hump, which is similar to that using a narrow-band noise masker; whereas a contralateral forward masker produced essentially n o midlevel hump, suggesting that binaural interactions at superior oli vary complex and more central sites are probably not responsible.