Auditory processing of frequency ramps.

Linear frequency ramps of a continuous pure tone have been used as stimuli, and behavioral thresholds for frequency change (DLF) as well as the slow cortical potentials evoked by such ramps have been studied on listeners with normal hearing and with various types of hearing losses. A group with cochlear hearing losses showed elevated DLFs as compared to the normal group by an average factor of 3, independent of ramp duration (10-500 ms). Corresponding to this finding, the slow evoked cortical potentials of the cochlear group had longer N1 latencies at small frequency ramps than those of the normal group. However, at larger ramps, this difference disappeared and turned into the opposite relation at the largest ramps. This result has been verified by a psychoacoustical method, indicating the existence of 'recruitment of pitch change'. In a number of subjects with retrocochlear lesions, the evoked cortical responses to frequency ramps were significantly different from those of the normal group as well as of the group with cochlear lesions.