Учебники / Hearing - From Sensory Processing to Perception Kollmeier 2007

Time (ms)

Fig. 2 Peristimulus time histograms (PSTHs) of the unit shown in Fig. 1

Frequency (Hz)

Fig. 3 Fourier transforms of the PSTHs shown in Fig. 2. Arrows emphasise F0

arrows). Instead, the response is to 2F0 and the harmonics of 2F0. The magnitude of each Fourier component shows the degree of phase locking to that frequency (indeed if the Fourier transform is normalized by the DC component then the magnitudes are equivalent to the vector strength at that frequency). The Fourier magnitudes at (i.e. synchronization to) F0 and 2F0 are shown in Fig. 1. These plots emphasise that whilst the degree of phase locking to F0 is greater than to 2F0 in the diotic conditions, it is the reverse in the dichotic conditions.

The ratio of synchronization to 2F0 relative to F0 is shown for all units as a function of CF in Fig. 4. It is clear that on average, for diotic presentation, synchronization to 2F0 is lower than, or equal to, synchronization to F0.

units the ipsilateral response is less than half that of the contralateral. There is a slight trend for the responses to be more equal at lower CFs and for the contralateral response to be more dominant at higher CFs.

4Discussion

The data presented here are largely consistent with units responding to the stimulus at a single, dominant, ear. In the diotic conditions the stimuli at each ear share the same spectra and waveforms as each other, and central combination of the signals would not affect this. On the other hand, the dichotic signals have components spaced at twice the fundamental at each of ears, and so have a waveform with an envelope periodicity at twice the rate of the diotic stimulus at each of the ears. Therefore, consideration of either the spectrum, or the waveform, at either ear alone would result in responses characteristic of 2F0. It is only if the spectra, or waveforms, from each ear are somehow combined, that we would expect the dichotic stimulus to have the same response as the diotic.

Our results might, therefore, appear completely unsurprising. However, it remains the case that the pitch cues from each ear are integrated into a single percept, and purely binaural pitches do exist which can be combined with “monaural” pitch cues (Akeroyd and Summerfield 2000). We also need to account for the result that motivated this study, that pairs of harmonics presented to different ears can elicit a pitch corresponding to their separation (Houstma and Goldstein 1972). This result was true for all the pairs of harmonics which were used. However, as the average harmonic number increased (i.e. from harmonics 2 and 3 to 10 and 11), performance declined equally for both monotic and dichotic presentation from perfect performance to near chance. A similar result was reported by Bernstein et al. (2003) (Expt 2), who found that frequency discrimination thresholds of both diotic and dichotic harmonic complexes (with harmonics alternating between ears, like ours) increased significantly as the lowest harmonic in the complex was increased beyond the 10th. Such diotic results have been shown in many experiments (see Bernstein et al. 2003; Shackleton and Carlyon 1994 for reviews), emphasising again that harmonic number is an important variable in pitch perception!

In a further experiment, Bernstein et al. (2003) (Expt 3) compared the perceived pitches of complexes presented diotically and dichotically. If the complexes contained harmonics below the 10th, then the dichotic complex was perceived with a pitch corresponding to F0, whereas if the complexes contained only harmonics above the 15th then the dichotic stimulus had a pitch equal to 2F0 (in between the pitch was uncertain). That is, for low harmonics pitch information is combined across the ears, whereas for high harmonics the pitch corresponds to the periodicity, or harmonic separation, of the stimulus at a single ear. This latter finding is consistent with our results, so we need to consider the harmonic number of our stimuli.

There are two complications in making comparisons with Bernstein and Oxenham (2003). First, all of our stimuli comprised all harmonics below 10 kHz, so they obviously contain the lower harmonics. We assume that the harmonics “seen” by a neuron are only those which are contained within its excitatory response area. To a first approximation, this has the same width as the tuning at the auditory nerve. Second, guinea pig filter bandwidths are approximately twice those of humans (both physiologically and psychophysically) (Evans 2001), so, to the extent that the pitch transition region depends upon the number of harmonics passing through a single filter, we need to compensate for guinea pig peripheral resolvability. We do not have the space here to discuss whether the pitch transition region is determined by peripheral resolvability or harmonic number (see Bernstein and Oxenham 2003 for a discussion), but following Bernstein and Oxenham (2003) we will assume that the critical harmonic numbers are those which would become unresolved if all harmonics in a stimulus were passing through the same peripheral filters. Applying the rule for resolvability derived by Shackleton and Carlyon (1994) to guinea pig filter bandwidths we obtain the transition regions shown in Fig. 4. There is no noticeable difference between the results in the resolved and unresolved regions. However, most of the resolved harmonics were with a fundamental of 400 Hz, which tended to show minimal phase locking to either F0 or 2F0 (Figs. 2 and 3 are typical). It is therefore probable that all of the units which showed significant phase locking were responding to unresolved harmonics. In which case, our results are entirely consistent with the psychophysics.

Throughout this chapter we have mostly been concerned with the temporal properties of IC neurons. However, some pitch theories posit that a place code for periodicity exists in the IC (see Rees and Langner 2005 for a review). About half of our units were preferentially tuned to a single F0 for diotic stimuli and these all were tuned to half-F0 for dichotic stimuli (e.g. Fig. 1). In a population this means that the peak activation for dichotic stimulation would be at 2F0, consistent with the psychophysics for high harmonics.

References

Akeroyd MA, Summerfield AQ (2000) Integration of monaural and binaural evidence of vowel formants. J Acoust Soc Am 107:3394–3406

Bernstein JG, Oxenham AJ (2003) Pitch discrimination of diotic and dichotic tone complexes: harmonic resolvability or harmonic number? J Acoust Soc Am 113:3323–3334

Bilsen FA, van der Meulen AP, Raatgever J (1998) Salience and JND of pitch for dichotic noise stimuli with scattered harmonics: grouping and the central spectrum theory. In: Palmer AR, Rees A, Summerfield AQ, Meddis R (eds) Psychophysical and physiological advances in hearing. Whurr, London, pp 403–411

Bullock DC, Palmer AR, Rees A (1988) Compact and easy-to-use tungsten-in-glass microelectrode manufacturing workstation. Med Biol Eng Comput 26:669–672

Cramer EM, Huggins WH (1958) Creation of pitch through binaural interaction. J Acoust Soc Am 30:413–417

Culling JF, Marshall DH, Summerfield AQ (1998) Dichotic pitches as illusions of binaural unmasking. II. The fourcin pitch and the dichotic repetition pitch. J Acoust Soc Am 103:3527–3539

Evans EF (2001) Latest comparisons between physiological and behavioural frequency selectivity. In: Houtsma AJM, Kohlraush A, Prijs VF, Schoonhoven R (eds) Physiological and psychophysical bases of auditory function. Shaker Publishing BV, Maastricht, pp 382–387

Fourcin AJ (1970) Central pitch and auditory lateralization. In: Plomp R, Smoorenburg GF (eds) Frequency analysis and periodicity detection in hearing. Sijthoff, Leiden, pp 319–328

Goldberg JM, Brown PB (1969). Response of binaural neurons of dog superior olivary complex to dichotic tonal stimuli: some physiological mechanisms of sound localization. J Neurophysiol 32:613–636

Houstma HJM, Goldstein JL (1972) The central origin of the pitch of complex tones: evidence from musical interval recognition. J Acoust Soc Am 44:807–812

Mast TE (1970) Binaural interaction and contralateral inhibition in dorsal cochlear nucleus of the chinchilla. J Neurophysiol 33:108–115

Rees A, Langner G (2005) Temporal coding in the auditory midbrain. In: Winer JA, Schreiner CE (eds) The inferior colliculus. Springer, Berlin Heidelberg New York

Shackleton TM, Carlyon RP (1994) The role of resolved and unresolved harmonics in pitch perception and frequency-modulation discrimination. J Acoust Soc Am 95:3529–3540

Shackleton TM, Skottun BC, Arnott RH, Palmer AR (2003) Interaural time difference discrimination thresholds for single neurons in the inferior colliculus of guinea pigs. J Neurosci 23:716–724

Winer JA, Schreiner CE (2005) The inferior colliculus. Springer, Berlin Heidelberg New York Zurek PM (1979) Measurements of binaural echo suppression. J Acoust Soc Am 66:1750–1757

Comment by Langner

Pitch is not defined by single neurons in the midbrain. The neuron you show responds very nicely to the periodicity of 200 Hz in the stimulus. This would also correspond to the perceived pitch provided the stimulus would contain only harmonics of 200 Hz. Under your special dichotic conditions the perceived pitch is 100 Hz because the orthogonal map of frequency and periodicity in the midbrain contains also lower, resolved, harmonics – all multiples of 100 Hz – and this lower pitch obviously dominates the pitch. In conclusion: I believe that your results are in line with a periodicity map in the midbrain.

Reply

We are not in disagreement. During the presentation we were at pains to stress that we were looking for representations of pitch cues which could underpin the extraction of a pitch percept. Further, in the last paragraph of the paper we point out that if a periodicity map exists, the results presented are consistent with the psychophysics for unresolved harmonics. A periodicity map containing units behaving like ours would predict a doubled pitch for dichotically alternating harmonic stimuli. To explain the pitch of stimuli containing resolved harmonics your argument requires units which are only

responding to individual harmonics, however our data do not show this, although it is possible that our method was biased against finding them.

Comment by Greenberg

You may not have used the appropriate stimuli in searching for binaural integration underlying dichotic pitch. In humans, dichotic pitch is restricted to fundamental frequencies lower than 330 Hz (Bilsen and Goldstein 1974) and extremely low sound pressure levels (generally within 10–20 dB of the listener’s threshold). Even under optimal listening conditions, dichotically generated pitch is extremely weak; many listeners have difficulty hearing it at all. However, with trained listeners, as those used in the studies reported by Bilsen and Goldstein (1974) and Houtsma and Goldstein (1972), reliable pitch matching and discrimination data can be obtained. Therefore, it would be useful to use signals known to generate a reliable sensation of dichotic pitch and whose acoustic properties are sufficiently distinct in the monotic and dichotic cases that there would be no possibility of confusing responses to the two sets of stimulus conditions. Both dichotic repetition noise (Bilsen and Goldstein 1974) and multiple-phase-shift noise (Bilsen 1977) have such properties and therefore could be used for investigating the neural correlates of dichotic pitch in central brainstem nuclei.

References

Bilsen FA (1977) Pitch of noise signals: evidence for a central spectrum. J Acoust Soc Am 61:150–161

Bilsen FA, Goldstein JL (1974) Pitch of dichotically delayed noise and its possible spectral basis. J Acoust Soc Am 55:292–296

Houtsma AJM, Goldstein JL (1972) The central origin of the pitch of complex tones: evidence from musical interval recognition. J Acoust Soc Am 51:520–529

Reply

We agree with your sentiment that we should use stimuli which distinguish between monaural and binaural processing. However, we question whether dichotic pitches are the relevant stimulus. We chose the alternating harmonics stimulus because it produces different pitches depending upon whether monaural or binaural cues are being used. Dichotic pitches, however, are necessarily generated by binaural processing, so do not, on their own, indicate whether fusion is taking place in, or before, the inferior colliculus. Indeed Hancock and Delgutte (2002) have already demonstrated a representation of Huggins pitch consistent with coincidence detection in superior olive in the IC. We were, however, planning to extend our studies

by comparing the responses to diotic and alternating harmonic stimuli, and “pseudo” harmonic stimuli created by binaural interaction like multiplephase shift noise.

References

Hancock KE, Delgutte B (2002) Neural correlates of Huggins dichotic pitch. Assoc Res Otolaryngol Abstr 25:40

Comment by Hartmann

Your introduction begins by noting the fusion of binaural information in the Houtsma-Goldstein experiment. This experiment has an interesting history. When it was first announced in the early 1970s, many psychoacousticians attempted to hear the effect and failed. Eventually it emerged that listeners who were not specially trained needed to hear the stimuli at quite low levels, and it was also helpful if the region of the low-pitch was cued by previous trials that pointed unambiguously to that region. Subsequently, Houtgast (1976) showed that, with appropriate cuing and given a low signal to noise ratio, a low pitch, or subharmonic, could be evoked by only a single sine component!

The Bernstein-Oxenham work is similar to Houtsma-Goldstein in that the S/N was low – only 10 dB SL for each component. It also seems likely that the method, with its varying lowest harmonic number but f_0 always in the same range, helped to cue the periodicity pitch.

More research needs to be done on the Houtsma-Goldstein effect with the goal of determining how “real” it actually is. Should it stand together with monaural periodicity pitch as motivation for physiological investigations? By contrast, the dichotic noise pitches, also noted in your introduction, don’t seem to require any special conditions for audibility.

References

Houtgast T (1976) Subharmonic pitches of a pure tone at low S/N ratio. J Acoust Soc Am 60:405–409

Reply

We thank you for your clarification of the original Houtsma-Goldstein effect. Whilst it is true that this effect was an initial motivator, the experiment we conducted is one which has intrinsic merit just from a physiological point of view. The question of how, when, and where the information from the two, independent, ears is integrated into a single, fused percept is fundamentally important.

Comment on Shackleton et al. and Greenberg by Frans A. Bilsen

In response to Greenberg, I want to comment that in order to have a listener perceive the (strong) low pitch, the binaural two-tone stimulus (Houtsma and Goldstein 1972; see also Bilsen 1973) has indeed to be presented at a rather low sensation level. This might be related to the fact that, due to the optimal separation of harmonics, optimal rivalry exists between the synthetic mode of perception (low pitch) and the analytic mode of perception (harmonics perceived separately). On the other hand, the (relatively weak) low pitch evoked by dichotic stimuli based on white noise at either ear does not call for a low sensation level. In the past, low sensation levels were applied only to insure the absence of acoustic cross talk.

In response to Shackleton and colleagues, as to the place of generation of the low pitch in the auditory pathway, one has to make a clear distinction between the binaural two-tone stimulus and dichotic-pitch stimuli. The twotone stimulus is expected to fuse and stimulate the central pitch processor directly at a ‘central’ level, while dichotic-pitch stimuli require binaural processing at a ‘peripheral’ level beforehand (compare Bilsen 1977, Fig. 6 therein). The search for low-pitch coding with stimuli related to the two-com- ponent stimulus at a rather ‘peripheral’ level like the inferior colliculus, therefore seems not to have a firm basis.

But more importantly, ample psychophysical evidence exists that optimal low pitch is derived from the lower (resolved) harmonics. However, given the response areas of inferior colliculus units investigated and the diotic vs dichotic stimuli used, the present experiments (as argued by Shackleton and colleagues) dealt with the rate (PSTH) response of units to unresolved harmonics mainly. Those might indeed not be expected to show either the proper low-pitch coding or the binaural integration as hypothesized by the psychophysics of the two-tone stimulus.

References

Bilsen FA (1973) On the influence of the number and phase of harmonics on the perceptibility of the pitch of complex signals. Acustica 28:59–65.

Bilsen FA (1977) Pitch of noise signals: evidence for a “central spectrum”. J Acoust Soc Am 61:150–161

Houtsma AJM, Goldstein JL (1972) The central origin of the pitch of complex tones: evidence from musical interval recognition. J Acoust Soc Am 51:520–529

Reply

We agree with Bilsen that dichotic-pitch stimuli and binaural two-tone stimuli are likely to have different origins; see our reply to Greenberg. However, they need to be integrated somewhere and to set arbitrary labels for what is central and what peripheral does not seem helpful. The inferior

colliculus is the first major relay station in the auditory system where monaural and binaural information converges, therefore it is a possible place for pitch (or at least periodicity) integration. Indeed, Langner (above) argues that there is a periodicity map in the IC.

In response to the second point, as we argued in the paper, since most of the neurons we record from are responding to unresolved harmonics we have not tested the integration hypothesis as rigorously as we would have liked. However, there are responses from neurons which are in the resolved region which are no different to the responses to unresolved harmonics.