COCHLEAR INITIATION SITES OF THE HUMAN AUDITORY FREQUENCY FOLLOWING POTENTIALS

Abstract

The frequency following potential (FFP) is an auditory evoked response which crudely replicates the waveform of the stimulus. The origins of this far-field potential have been localized to the cochlea and specific auditory nuclei. The initiation site of the FFP along the cochlear partition has been thought to be dependent upon stimulus sound pressure level. Low stimulus levels elicit FFP from the apical turn of the cochlea, while moderate and high stimulus levels may elicit FFP from a large segment of the partition, including the basal turn. Simultaneous masking studies have contributed to the knowledge of this potential, but results from these studies may be confounded by interference from bioelectric potentials generated by distortion products from the masking noise. This investigation used non-simultaneous masking tones set at the subject's pulsation threshold level (PTL). This psychophysical method provides a representation of the frequency-resolving properties of the auditory periphery in a repeated gap masking paradigm. Patterns of the relative masking effect of each masking tone were plotted and compared for several probe levels and in the presence or absence of a continuous, high-pass masking noise. Tuning patterns derived in high-pass making noise effectively represented apical turn FFP. Those patterns without masking noise represented whole-cochlea FFP. The results of this study demonstrarted tuning patterns that reflected diminished FFP amplitudes in the primary 500 Hz region as well as in an extended secondary range of 800 to 2000 Hz, depending on the stimulus-masking parameters. Tuning patterns in high-pass masking noise were consistently flatter in the primary and secondary regions than for the tuning patterns without masking noise. The secondary regions of the tuning patterns extended above 2000 Hz for the patterns obtained without high-pass masking. Tuning patterns were consistently broader for the high stimulus level than for the moderate stimulus level. The multiple cochlear initiation sites for the FFP preclude its use as an audiological tool for the assessment of low-frequency hearing. However, the potential is suggested as a method to assess whole-cochlear response for the difficult-to-test patients with suspected profound hearing loss.