www.elsevier.com/locate/clinph

Clinical Neurophysiology 118 (2007) 2276–2281

Latency effect of the pitch response due to variationsof frequency and spectral envelope

Steffen Ritter a,*, Hans Gunter Dosch b, Hans-Joachim Specht c,Peter Schneider a, Andre Rupp a

a Section of Biomagnetism, Department of Neurology, University of Heidelberg, Im Neuenheimer Feld 400, 69120 Heidelberg, Germanyb Institute for Theoretical Physics, University of Heidelberg, Im Neuenheimer Feld 400, 69120 Heidelberg, Germany

c Institute of Physics, University of Heidelberg, Im Neuenheimer Feld 400, 69120 Heidelberg, Germany

Accepted 25 June 2007Available online 20 August 2007

Abstract

Objective: A clear definition of pitch and timbre is still an open debate and often both terms are mixed up in investigations of tone height.However, fundamental frequency (f0) and spectral envelope of a sound play a major role in the perception of tone height. Recent elec-trophysiological experiments showed that one sub-component of the complex N100-signal was found to be highly correlated to theperceived tone height.Methods: Tone height was independently varied by both, a change of f0 and spectral envelope in order to disentangle the influence ofboth parameters. Relative tone height was determined psychoacoustically. Neuromagnetic responses were evaluated using source-analysis.Results: Perceived tone height increases with increasing f0 or spectral envelope. Latency of the pitch change response (PCR) reacts oppo-sitely for the two modi of tone height change. For increasing f0 and fixed bandpass condition, tone height increases and the latency ofthe PCR decreases. In contrast, for increasing the center frequency of the bandpass with fixed f0, tone height increases, but the latency ofthe PCR increases.Conclusions: The neuromagnetic pitch response is influenced by both, f0 and spectral envelope.Significance: Further investigations of the influence of pitch and timbre on neurophysiological pitch responses have to take into accountthat both, f0 and spectral envelope, affect tone height and latency of the PCR.� 2007 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.

Keywords: Tone height; Pitch; Timbre; Fundamental frequency; Spectral envelope; Pitch perception; Auditory cortex; Magnetoencephalography; Pitchchange response

1. Introduction

Pitch and timbre are essential properties of sounds.Pitch determines, e.g. a melody, whereas musical instru-ments differ by the timbre of the sounds they produce.According to the American National Standards Institute(ANSI, 1994), ‘‘pitch is that attribute of auditory sensationin terms of which sounds may be ordered in a scale extend-

1388-2457/$32.00 � 2007 International Federation of Clinical Neurophysiolo

doi:10.1016/j.clinph.2007.06.017

* Corresponding author. Tel.: +49 62 21 56 51 89; fax: +49 62 21 56 5258.

E-mail address: steffen.ritter@med.uni-heidelberg.de (S. Ritter).

ing from low to high [...]’’. Timbre is defined only as thoseaspects of a sound’s quality other than pitch (ASA, 1960).The relation between these perceptual qualities and physi-cal quantities was traditionally made through the Fouriertransform of the physical signal: pitch is determined bythe fundamental frequency (f0) and timbre by the spectralenvelope of the sound (von Helmholtz, 1863). These clearrelations were upset by the discovery of the residual pitchby Schouten (1940), who showed that a harmonic complextone can evoke a pitch at f0 even when this component isphysically absent. Since that time, the discussion about agenerally accepted classification of pitch and timbre is

gy. Published by Elsevier Ireland Ltd. All rights reserved.

8

16

32

harm

onic

num

ber

time (ms)0 500 1000

LOW

MID

HIGH

even odd even

500

1000

2000

4000

f (Hz)

4

Fig. 1. A scheme of the stimuli. They consisted of harmonic series with afixed fundamental frequency of 62.5 or 125 Hz and contained either evenor odd multiples of the fundamental frequency only. Three differentbandpass conditions were tested (indicated by different grey shading: aLOW-bandpass, a MID-bandpass and a HIGH-bandpass). Shown is thebeginning of a sequence of even and odd stimuli as used in theneurophysiological investigation.

S. Ritter et al. / Clinical Neurophysiology 118 (2007) 2276–2281 2277

going on and is not only of scientific interest. The composerSchoenberg for example did not accept the strict separationof pitch (Klanghoehe) and timbre (Klangfarbe) butregarded pitch only as one dimension of timbre (Schoen-berg, 1992).

Patterson et al. (1993) emphasized that the musical rep-resentation of pitch is given by a pitch helix (e.g. Ueda andOhgushi, 1987). It is a bi-dimensional mapping of the rep-etition rate of multi-harmonic tones. The circular dimen-sion of the helix is called chroma; the longitudinaldimension is named tone height and rises one octave perrevolution.

By examining the evoked responses of the auditory cor-tex, modern neurophysiological methods allow to investi-gate the neural basis underlying the perception of pitchand timbre.

Today, it is widely accepted that at least in the frequencyregion relevant for speech, the latency of the prominentN100-signal is related to the frequency of a presented puresinusoidal tone (Roberts and Poeppel, 1996). Other MEGexperiments applying sinusoidal tones above 1 kHz didnot find this relation anymore (e.g., Gabriel et al., 2004).The problem is that the N100-deflection arises from differ-ent interfering neural generators and can be evoked by theonset of almost any kind of sound (Naatanen and Picton,1987).

In order to avoid activity induced by the simple onset ofenergy and to isolate pitch specific activity of the N100,Makela et al. (1988) and Krumbholz et al. (2003) appliedtransitions from noise to a tone with a distinct pitch. Theirresults also demonstrated a latency dependence of the so-called pitch onset response to the presented sounds. Weinvestigated in this work activation to a mere pitch changeto minimize the qualitative difference in the transition.

As suggested by Jones et al. (1998) it turns out that thereis a clear distinction between responses to pitch onset andpitch change. Therefore, in the following the resultingprominent deflection that occurs after about 100 ms isreferred to as pitch change response (PCR).

The novelty of the present study is the possibility to dis-sociate the influence of pitch and timbre on the latency ofthe N100-complex. We varied both, fundamental frequencyand spectral envelope, to elucidate the neural representa-tion of pitch and timbre.

2. Materials and methods

Sixteen adult listeners (nine males, seven females) with amean age of 34 years and no reported history of peripheralor central hearing disorder participated in the MEG mea-surements and in the psychoacoustic experiment after giv-ing informed written consent. The experiment conformsto the Code of Ethics of the World Medical Association(Declaration of Helsinki). It is part of a study on temporalprocessing in human auditory cortex which was approvedby the Ethics Committee of the University of Heidelberg,Germany.

The stimuli consisted of harmonic complex sounds, pro-duced at a sampling rate of 48,000 Hz, containing eitherthe odd or even multiples of fundamental frequency f0. Inthe following we shall refer to them as odd or even stimuli.The even stimuli have half the period of the odd tones.There is a distinct pitch difference between these two typesof stimuli (e.g., Ritter et al., 2005). In order to avoid furthercomplications with the perception of chroma (Patterson,1990), we have chosen two fundamental frequencies whichdiffer by a factor of 2 (octave), thus leading to the samechroma. All harmonic complexes were bandpass filteredin the frequency domain using three different bandpassconditions: LOW between 500 and 1000 Hz, MID between1000 and 2000 Hz and HIGH between 2000 and 4000 Hz.The fundamental frequencies were chosen to be 62.5 and125 Hz. Due to the filtering, the component with the funda-mental frequency f0 is missing in all types of stimuli. Alto-gether there were 12 different stimuli determined by f0 (62.5and 125 Hz), filter band (LOW, MID, HIGH) and spectralcomponents (even, odd).

For the neuromagnetic investigations, a sequence of alter-nating even and odd stimuli of the same fundamental fre-quency and the same spectral envelope was formed.Single stimuli of the sequence were balanced in energyand the sound pressure of the sounds was gated on andoff with a 10-ms hanning window. Each stimulus had alength of 510 ms. Each resulting sequence filtered withone fixed bandpass condition, had a total length of10.5 s. Sequences were superimposed with a white back-ground noise (DC–10,000 Hz), attenuated by 20 dB relativeto the peaks in the spectrum, to suppress the perception ofcombination tones. The spectra of the first three stimuli ofa sequence are displayed in Fig. 1. In each session, f0 wasfixed while the bandpass condition was varied randomly.150 sequences were presented resulting in 500 odd/even

1 www.mrc-cbu.cam.ac.uk/Imaging/Common/mnispace.shtml.

2278 S. Ritter et al. / Clinical Neurophysiology 118 (2007) 2276–2281

transitions per session for each bandpass condition andeach fundamental frequency. Stimuli were presented dioti-cally using ER-3 transducers (Etymotic Research, Inc.)equipped with 90 cm plastic tubes and foam ear pieces.The transfer function of the whole setup shows a bandpasscharacteristic with a bandwidth from approximately 500–5000 Hz. A Bruel and Kjær sound level meter was usedto set the overall sound pressure level (SPL) of a sequencewith fixed bandpass condition to about 60 dB. Listenerswatched a silent movie of their own choice and were askednot to pay attention to the auditory stimulation but con-centrate on the movie.

Neuromagnetic responses were DC-recorded with aNeuromag-122 whole head planar gradiometer system ata sampling rate of 1000 Hz. Responses evoked at the tran-sition between even and odd tones were averaged offlinewith BESA 5.1 (MEGIS Software GmbH, Germany).Artefact-contaminated epochs with a gradient higher than800 fT/ms were excluded from averaging. In analogy toearlier experiments investigating the specific activation elic-ited by a pitch transition (Krumbholz et al., 2003 and Rit-ter et al., 2005) a spatio-temporal source model with oneequivalent dipole in each hemisphere was used (Scherget al., 1989). Waveforms were 2–30 Hz bandpass filtered(zero-phase, 12 dB/oct) and used to fit the first prominentnegative deflection (PCR) in a 30-ms interval around thatpeak. In both sessions, the waveforms in response to thetransition of the even tone, and the LOW-bandpass condi-tion provided the best signal-to-noise-ratio and produced astable fit for each single listener.

For the HIGH-bandpass condition, the two-dipolemodel provided a stable fit for f0 of 125 Hz in 12 subjectsand for 62.5 Hz only in 6 subjects. For these subjects, thelocation of the dipoles could differ 3 mm in x-directionand about 5 mm in y-direction. Residual variance of thefit on the HIGH-bandpass condition decreased for thesesubjects slightly at about 5% on average compared to thefit on the LOW-bandpass condition.

Therefore, the fit on the LOW-bandpass condition actedin all cases as a spatial filter to derive the equivalent sourcewaveforms of all other conditions of the subject. No fur-ther constraints concerning dipole location, orientation orsymmetry conditions were applied.

Drift and other low frequency artefacts due to the con-tinuous stimulation were compensated for by computing aprinciple component analysis (PCA) over an interval of400–500 ms after the transition of the unfiltered auditoryevoked responses (Berg and Scherg, 1994). The two dipolesand the PCA component that accounted for the largest var-iance in this interval were used as a spatial filter to derivethe source waveforms for all conditions. For further calcu-lations, the resulting equivalent source waveforms werezero-phase filtered from 1 to 100 Hz (12 dB/oct). An inter-val of 100 ms before the transition was used to define thebaseline. The latency of the PCR could not be deriveddoubtlessly in every condition of every single subject.Therefore, latencies of the PCR were determined with a

bootstrap resampling procedure. One thousand indepen-dent bootstrap resamples were generated by drawing withreplacement from the 16 original waveforms (Efron andTibshirani, 1993).

The averaged neuromagnetic responses to the onsetfrom silence were used in a second model to fit a singleequivalent dipole in each auditory cortex of both hemi-spheres. Each onset of the three bandpass conditions(LOW, MID, HIGH) was filtered and fitted in the sameway as the averaged responses to the transitions.

Magnetic resonance imaging (MRI) was performed inall subjects. One hundred and seventy-six sagittal sliceswere recorded with a Siemens Sonata 1.5T scanner at a rep-etition time of 2.11 s and an echo time of 4.38 ms (voxelsize 0.488 * 0.488 * 1 mm3).

The location of the fitted dipoles of the individual ana-tomical structures of all subjects was rescaled1 accordingto the stereotactic space of Talairach and Tournoux (1988).

In the psychoacoustic experiment, tone height was deter-mined with a two-alternative forced-choice (2-AFC) taskfor paired comparisons of the 12 stimuli. As in the neuro-physiological experiment, stimuli were masked with a whitebackground noise, attenuated by 20 dB. Throughout thetest, two different stimuli were presented randomly. Listen-ers had to answer the question, ‘which tone was higher?’.Every combination of paired comparisons was presentedtwice, in reversed order (A–B and B–A). Each listenerjudged all combinations of the 12 different sounds, whichled to a total of 132 trials.

The scale for the relative tone height was derived usingthe BTL method (David, 1988) which is based on theassumption that the perceived tone height of the soundscan be ordered linearly. The result is a relative scale whereonly differences are relevant.

3. Results

The neuromagnetic responses of each listener were fittedon the PCR-complex with a single equivalent dipole in eachhemisphere. Fig. 2 shows the mean coordinates of all listen-ers projected on an axial plane through Heschl’s Gyrus(solid borderlines) and Planum temporale (dotted border-line). A cross-section through an MRI of a single subjectis displayed in the right part of the figure. The locationof Heschl’s Gyrus within the left and right auditory cortexis highlighted. The fitted dipoles to the onset of thesequences from silence (open symbols) are located moreposterior and inside Heschl’s Gyrus compared to the fiton the PCR (filled symbols). A multivariate permutationtest over the three Talairach coordinates of the dipoles fit-ted on the PCR of the tested fundamental frequenciesshowed no significant difference (F(1,15) = 0.48, n.s.; filledcircles vs. filled diamonds).

PCR 125Hz evenPCR 62.5Hz evenOnset 125Hz evenOnset 62.5Hz even

Fig. 2. Left: MRI of a single subject with highlighted left and rightHeschl’s Gyrus. Right: Mean Talairach coordinates of the averaged fittedequivalent dipoles projected onto an axial plane through Heschl’s Gyrus(solid borders) and Planum temporale (dotted borders). Sulcal borderswere obtained from Schneider et al. (2005). Open symbols show thelocation of the equivalent dipoles in response to tone onset from silence,the pitch change response is represented by filled symbols.

S. Ritter et al. / Clinical Neurophysiology 118 (2007) 2276–2281 2279

A multivariate permutation test (Blair and Karniski,1993) for waveform differences, as recommended by Pictonet al. (2000), revealed no significant differences (t = �49.6,n.s.) in the interval of 50 ms around the PCR-deflectionbetween the left and right hemisphere. Therefore, allfurther calculations are based on the mean of bothhemispheres.

Fig. 3 shows exemplarily the equivalent source wave-forms averaged over both hemispheres of one individualsubject. The two-dipole model is fitted on the responsesof the even-condition of the LOW-bandpass filteredsounds. However, the responses of this good subject can

even odd

even odd

even odd

0 200 400 600 800 1000 0 200 400 600 800 1000time (ms)

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10

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(nAm

)

f0 =62.5Hz

LOW-Bandpass

MID-Bandpass

HIGH-Bandpass

even odd

f0=125Hz

even odd

even odd

Fig. 3. Individual equivalent source waveforms of one good subjectaveraged over hemispheres. Responses are evoked by bandpass filteredtones with f0 = 125 Hz and 62.5 Hz (left to right) and filtered with threedifferent bandpass conditions (bottom to top: LOW (500–1000 Hz), MID(1000–2000 Hz) and HIGH (2000–4000 Hz)). As indicated in grey,alternating sounds contained either all even or all odd components withinthe applied bandpass condition.

also be fitted on the HIGH-bandpass filtered sounds.Waveforms of the Figure are underlaid in grey with thestimulus condition.

The grand average of the equivalent source waveformsof 16 subjects is presented in Fig. 4.

As can be seen from Fig. 4, the morphology of the audi-tory evoked signals is very similar for all conditions andagrees well with previous measurements under similar con-ditions (Ritter et al., 2005). All waveforms of the left (blacklines) and right hemisphere (grey lines) exhibit a first posi-tivity between 70 and 80 ms after the transition, followedby the prominent PCR-deflection and the positive P200at about 200–250 ms after the transition. The amplitudesof the three distinctive components clearly decrease withincreasing center frequency (fc) of the bandpass (left toright) and decreasing fundamental frequency f0 (first rowcompared to third and second compared to fourth row,respectively). The latency increase of the PCR is highly sig-nificant when the center frequency fc of the bandpass isincreased (F(2,30) = 49.03, p < 0.0001), but decreases sig-nificantly with increasing f0 (F(1, 15) = 64.46, p < 0.0001).

Values of the latencies are shown in Fig. 5 as a functionof the relative tone height that was determined in the psy-

choacoustic experiment. As expected, relative tone heightalso increases significantly with the fundamental frequency(F(1,15) = 211.73, p < 0.001) but it also increases with thecenter frequency of the filter band (F(2,30) = 58.04,p < 0.001). It is for the HIGH-bandpass filtered stimuli ofthe fundamental frequency f0 = 62.5 Hz higher than forthe LOW-bandpass filtered stimuli with f0 = 125 Hz. Ifall other parameters are equal, the odd stimulus is judgedto be higher than the even stimulus. This effect is small

Fig. 4. Grand average of equivalent source waveforms of the left (grey)and right (black) auditory cortex based on the fit of the first prominentdeflection after the transition (pitch change response). Left to right: Dipolemoment in response to the transition to LOW (500–1000 Hz), MID (1000–2000 Hz) and HIGH (2000–4000 Hz)-bandpass filtered tones with afundamental frequency of 125 Hz (first and second row) and 62.5 Hz(third and fourth row).

f0

f c

relative tone height

f =62.5Hz0

f =125Hz0

Fig. 5. PCR-latency vs. perceived relative tone height for differentfundamental frequencies f0 and center frequencies fc of the spectralenvelope. Error bars show the bootstrap assessed standard error. For fixedf0, relative tone height as well as latency increases with increasing fc. Forfixed fc, however, relative tone height increases and latency decreases withincreasing f0. This is indicated by the arrows marked f0 and fc.

2280 S. Ritter et al. / Clinical Neurophysiology 118 (2007) 2276–2281

but significant (F(1,15) = 7.21, p < 0.05). No interactionbetween the applied fc of the bandpass and f0

(F(2,30) = 1.84, n.s.) is found. The interaction betweenthe even and the odd conditions and the fundamental fre-quency is as well insignificant (F(1,15) = 0, n.s.) as betweeneven and odd conditions and the applied bandpass(F(2,30) = 1.07, n.s.).

The HIGH-bandpass filter has a center frequency offc = 3000 Hz which is two octaves above the LOW-band-pass (fc = 750 Hz). The transition induces a shift of the rel-ative tone height of about 2.1 rtu (relative tone heightunits) for the even as well as for the odd stimuli and forboth frequencies f0. The increase of f0 by a factor of 2(octave) effects an increase of the relative tone height by1.78 ± 0.14 rtu. The change from even to odd stimuli cor-responds to a decrease of the period by a factor of 2, butthe relative tone height rises by about 0.5 rtu. A detaileddiscussion concerning the tone height of odd tones can befound in Ritter et al. (2005).

Interdependence of the relative tone height and the latency

of the PCR for the different modi. The two modi of toneheight change show an opposite latency effect on thePCR. With increasing fundamental frequency and fixedbandpass condition, latency of the PCR decreases and toneheight increases. For increasing center frequency of thebandpass with fixed fundamental frequency tone heightincreases, too, but the latency of the PCR increases. Inthe first modus relative tone height increase is induced byan increase of the spectral envelope indicated by the arrowfc in Fig. 5. For a fixed f0 of 62.5 Hz the correlationbetween the relative tone height and the PCR-latency isas well very satisfactory (q = 0.9, p < 0.01) as for a funda-mental frequency of 125 Hz (q = 0.94, p < 0.01). The slopeof the linear regression is very similar for both f0, about7.5 ms/rtu.

If the spectral envelope (bandpass condition) is heldfixed and a change of f0 is used to increase relative toneheight, latency of the PCR decreases with increasing toneheight (arrow f0 in Fig. 5). This is the case for all bandpassconditions and both harmonic series (even and odd tones).The relative tone height increase is on average1.78 ± 0.14 rtu for an increase of a factor of 2 in f0, thedecrease of the latency is 14 ± 5.2 ms. It is remarkable thatin contrast to the known latency decrease with increasingfundamental frequency the latency of the PCR increaseswith increasing tone height induced by the spectralenvelope.

Holding the fundamental frequency and the bandpasscondition fixed, latency of the PCR increases between theodd–even and even–odd transitions. This effect is smallcompared to the f0 – and bandpass effect but reaches signif-icance level (F(1, 15) = 8.76, p < 0.01).

4. Discussion

The transition between two tones evoked activity cen-tered in the lateral aspect of Heschl’s Gyrus. This is inaccordance with Krumbholz et al. (2003) and Ritter et al.(2005) who reported the center of pitch processing to bein Heschl’s Gyrus. In line with these results, the PCR ismore anterior compared to the equivalent dipoles fittedon the onset of the tone from silence.

With regard to the variation of the fundamental fre-quency, the resulting latencies of the neuromagneticresponses are consistent with previous observations. Alatency decrease has been observed after the transitionfrom silence to sound (Ragot and Lepaul-Ercole, 1996;Roberts and Poeppel, 1996) as well as from noise to sound(Krumbholz et al., 2003) whenever the fundamental fre-quency increases. Fujioka et al. (2002) and Gabriel et al.(2004) investigated the N100m-signal in response to thetransition from silence to pure sinusoidal tones up to fre-quencies above 10,000 Hz and did not find this relationanymore.

The latency of the PCR of the present experiment can beseen from the projection of the results on the ordinate ofFig. 5. The variation of the fundamental frequency is indi-cated by the arrow marked f0. The causal relation betweenpitch and latency of the neurophysiological responses is notclear, since the frequency dependence and the order ofmagnitude cannot be explained simply by the differencein travelling time of the wave on the basilar membrane(e.g., de Boer, 1996; Rupp et al., 2002). Furthermore, stim-uli are bandpass filtered in this work and the fundamentalfrequency is even missing. Thus, the observed latency dif-ferences with a change of f0 or a change of the spectralenvelope cannot be explained by time differences in theauditory pathway occurring at the level of the basilarmembrane.

The latency of the PCR increases however, if tone heightincreases due to a shift of the spectral envelope to higherfrequencies (see Fig. 5). This clear result seems to

S. Ritter et al. / Clinical Neurophysiology 118 (2007) 2276–2281 2281

contradict the findings of Ragot and Lepaul-Ercole (1996).However, the N100-signal consists of several, spatially aswell as temporally overlapping components (Lutkenhoneret al., 2001). In the investigation of Ragot and Lepaul-Ercole, the transition was from silence to tone and theresulting pitch-dependent responses interfered with activitysimply evoked by the onset of any auditory event. A con-tinuous stimulation, as applied in our experiment, allowsto study the PCR without any interfering activation.

With an increase of the spectral envelope to higher fre-quencies, the observed amplitudes of the PCR alsodecreased. Gabriel et al. (2004) hypothesized that a possi-ble increased hearing threshold of higher frequencies couldbias this effect. However, additional measurements with avarying SPL of 12 dB showed no relevant or systematicchange of the evoked waveforms. Thus, an increasedthreshold of higher frequencies does not seem to play a rolein the observed amplitude decrease.

Our results show that the well-known and clear relationbetween the perceived tone height and the latency of thepitch sensitive component of the N100m-complex has tobe considered in view of fundamental frequency and spec-tral envelope of a presented harmonic complex tone. Fur-ther experiments, investigating the neurophysiologicalunderpinnings of the perception of tone height, need totake into account that fundamental frequency and centerfrequency of the spectral envelope have an opposite influ-ence on the latency of the pitch responses.

With the transition from the LOW to the HIGH spectralenvelope one passes from the domain of resolved over tounresolved harmonics (Houtsma and Smurzynski, 1990).Psychoacoustic experiments (Carlyon and Shackleton,1994) demonstrated that the ability to distinguish betweentwo sounds drops steadily with an increase of the lowestharmonic number present. It is conceivable that the transi-tion between tone height extraction from resolved harmon-ics to that of unresolved harmonics is responsible for theincrease of the PCR-latency.

Acknowledgement

This work was supported by the Deutsche Forschungs-gemeinschaft (Ru 652/3-1 and Ri 1229/2-1).

Thanks to Stefan Uppenkamp, University of Olden-burg, who kindly provided the MR images.

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