The relative importance of neural temporal and place coding in auditory perception is still a matter of much debate. The current article is a compilation of viewpoints from leading auditory psychophysicists and physiologists regarding the upper frequency limit for the use of neural phase locking to code temporal fine structure in humans. While phase locking is used for binaural processing up to about 1500 Hz, there is disagreement regarding the use of monaural phase-locking information at higher frequencies. Estimates of the general upper limit proposed by the contributors range from 1500 to 10000 Hz. The arguments depend on whether or not phase locking is needed to explain psychophysical discrimination performance at frequencies above 1500 Hz, and whether or not the phase-locked neural representation is sufficiently robust at these frequencies to provide useable information. The contributors suggest key experiments that may help to resolve this issue, and experimental findings that may cause them to change their minds. This issue is of crucial importance to our understanding of the neural basis of auditory perception in general, and of pitch perception in particular.

Frequency tuning and phase-locking are two fundamental properties generated in the cochlea, enabling but also limiting the coding of sounds by the auditory nerve (AN). In humans, these limits are unknown, but high resolution has been postulated for both properties. Electrophysiological recordings from the AN of normal-hearing volunteers indicate that human frequency tuning, but not phase-locking, exceeds the resolution observed in animal models.

In the diverse mechanosensory systems that animals evolved, the waveform of stimuli can be encoded by phase locking in spike trains of primary afferents. Coding of the fine structure of sounds via phase locking is thought to be critical for hearing. The upper frequency limit of phase locking varies across species and is unknown in humans. We applied a method developed previously, which is based on neural adaptation evoked by forward masking, to analyze mass potentials recorded on the cochlea and auditory nerve in the cat. The method allows us to separate neural phase locking from receptor potentials. We find that the frequency limit of neural phase locking obtained from mass potentials was very similar to that reported for individual auditory nerve fibers. The results suggest that this is a promising approach to examine neural phase locking in humans with normal or impaired hearing or in other species for which direct recordings from primary afferents are not feasible.

Frequency selectivity is a fundamental cochlear property. Recent studies using otoacoustic emissions and psychophysical forward masking suggest that frequency selectivity is sharper in human than in common laboratory species. This has been disputed based on reports using compound action potentials (CAPs), which reflect activity in the auditory nerve and can be measured in humans. Comparative data of CAPs, obtained with a variety of simultaneous masking protocols, have been interpreted to indicate similarity of frequency tuning across mammals and even birds. Unfortunately, there are several issues with the available CAP measurements which hamper a straightforward comparison across species. We investigate sharpness of CAP tuning in cat and chinchilla using a forward masking notched-noise paradigm-which is less confounded by cochlear nonlinearities than simultaneous masking paradigms and similar to what was used in the psychophysical study reporting sharper tuning in humans. Our parametric study, using different probe frequencies and notch widths, shows relationships consistent with those of auditory nerve fibers (ANFs). The sharpness of tuning, quantified by Q 10 factors, is negatively correlated with probe level and increases with probe frequency, but the Q 10 values are generally lower than the average trend for ANFs. Like the single fiber data, tuning for CAPs is sharper in cat than in chinchilla, but the two species are similar in the dependence of tuning on probe frequency and in the relationship between tuning in ANFs and CAP. Growthof-maskability functions show slopes G1 indicating that with increasing probe level the probe is more susceptible to cochlear compression than the masker. The results support the use of forward-masked CAPs as an alternative measure to estimate ANF tuning and to compare frequency tuning across species.

The frequency extent over which fine structure is coded in the auditory nerve has been physiologically characterized in laboratory animals but is unknown in humans. Knowledge of the upper frequency limit in humans would inform the debate regarding the role of fine structure in human hearing. Of the presently available techniques, only the recording of mass neural potentials offers the promise to provide a physiological estimate of neural phase locking in humans. A challenge is to disambiguate neural phase locking from the receptor potentials. We studied mass potentials recorded on the cochlea and auditory nerve of cat and used several experimental manipulations to isolate the neural contribution to these potentials. We find a surprisingly large neural contribution in the signal recorded on the cochlear round window, and this contribution is in many aspects similar to the potential measured on the auditory nerve. The results suggest that recording of mass potentials through the middle ear is a promising approach to examine neural phase locking in humans.