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Language, Cognition and Neuroscience
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Entrained theta oscillations guide perception of
subsequent speech: behavioural evidence from
rate normalisation
Hans Rutger Bosker & Oded Ghitza
To cite this article: Hans Rutger Bosker & Oded Ghitza (2018) Entrained theta oscillations guide
perception of subsequent speech: behavioural evidence from rate normalisation, Language,
Cognition and Neuroscience, 33:8, 955-967, DOI: 10.1080/23273798.2018.1439179
To link to this article: https://doi.org/10.1080/23273798.2018.1439179
© 2018 The Author(s). Published by Informa
UK Limited, trading as Taylor & Francis
Group
Published online: 13 Feb 2018.
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REGULAR ARTICLE
Entrained theta oscillations guide perception of subsequent speech: behavioural
evidence from rate normalisation
Hans Rutger Bosker
a,b
and Oded Ghitza
c,d
a
Max Planck Institute for Psycholinguistics, Nijmegen, the Netherlands;
b
Donders Institute for Brain, Cognition and Behaviour, Radboud
University, Nijmegen, The Netherlands;
c
Department of Biomedical Engineering, Hearing Research Center, Boston University, Boston, MA, USA;
d
Neuroscience Department, Max Planck Institute for Empirical Aesthetics, Frankfurt, Germany
ABSTRACT
This psychoacoustic study provides behavioural evidence that neural entrainment in the theta
range (3–9 Hz) causally shapes speech perception. Adopting the “rate normalization”paradigm
(presenting compressed carrier sentences followed by uncompressed target words), we show
that uniform compression of a speech carrier to syllable rates inside the theta range influences
perception of subsequent uncompressed targets, but compression outside theta range does not.
However, the influence of carriers –compressed outside theta range –on target perception is
salvaged when carriers are “repackaged”to have a packet rate inside theta. This suggests that
the brain can only successfully entrain to syllable/packet rates within theta range, with a causal
influence on the perception of subsequent speech, in line with recent neuroimaging data. Thus,
this study points to a central role for sustained theta entrainment in rate normalisation and
contributes to our understanding of the functional role of brain oscillations in speech perception.
ARTICLE HISTORY
Received 11 August 2017
Accepted 2 February 2018
KEYWORDS
Neural entrainment; theta
oscillations; speech rate; rate
normalisation
Introduction
Speech is a communicative signal with inherent slow
amplitude modulations. These amplitude modulations
fluctuate at a rate around 3–9 Hz, in various speech
types and in different languages (Bosker & Cooke, in
press; Ding et al., 2017; Krause & Braida, 2004; Varnet,
Ortiz-Barajas, Erra, Gervain, & Lorenzi, 2017), driven pri-
marily by the syllabic rate of speech. Recently, models
of speech perception have pointed at the remarkable
correspondence between the time scales of phonemic,
syllabic, and phrasal linguistic units, on the one hand,
and the periods of the gamma, theta, and delta oscil-
lations in the brain, on the other (Ghitza, 2011;
Poeppel, 2003). This correspondence has inspired
recent hypotheses on the potential role of neuronal oscil-
lations in speech perception (Ghitza, 2011,2017; Ghitza &
Greenberg, 2009; Giraud & Poeppel, 2012; Peelle & Davis,
2012; Poeppel, 2003). For instance, evidence has accu-
mulated showing that the listening brain follows the syl-
labic speech rhythm by phase-locking endogenous theta
oscillations (3–9 Hz) to the amplitude envelope of
speech (Doelling, Arnal, Ghitza, & Poeppel, 2014; Gross
et al., 2013; Luo & Poeppel, 2007; Peelle & Davis, 2012;
Peelle, Gross, & Davis, 2013). This process, known as
neural entrainment (“speech tracking”), has also been
proposed to explain why low-frequency amplitude
modulations in speech play a crucial role in perception
(e.g. literature on locally time-reversed, interrupted, alter-
nated, and filtered speech; Drullman, Festen, & Plomp,
1994a; Drullman, Festen, & Plomp, 1994b; Elliott & Theu-
nissen, 2009; Ghitza, 2012; Peelle & Davis, 2012; Saberi &
Perrott, 1999; Shannon, Zeng, Kamath, Wygonski, &
Ekelid, 1995; Ueda, Nakajima, Ellermeier, & Kattner,
2017). However, the functional role of this neural entrain-
ment in speech perception remains a topic of debate: is
entrainment causally involved in shaping successful
speech perception (Riecke, Formisano, Sorger, Başkent,
& Gaudrain, 2018; Zoefel, Archer-Boyd, & Davis, 2018)
or is it merely a response-driven epiphenomenon of
speech processing (Obleser, Herrmann, & Henry, 2012)?
The present study will put forward psychoacoustic find-
ings suggesting that, not only does neural entrainment
to a particular syllable rate shape the decoding of concur-
rent speech (Experiment 1),but also that neural entrain-
ment might persist when the entraining rhythm has
ceased, influencing the perception of subsequently pre-
sented words (Experiments 2 and 3).
Neural entrainment shapes perception of
concurrent speech
Evidence that neural entrainment shapes the decoding
of concurrent speech (i.e. the speech presented during
© 2018 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use,
distribution, and reproduction in any medium, provided the original work is properly cited.
CONTACT Hans Rutger Bosker HansRutger.Bosker@mpi.nl
LANGUAGE, COGNITION AND NEUROSCIENCE
2018, VOL. 33, NO. 8, 955–967
https://doi.org/10.1080/23273798.2018.1439179
the time interval in which entrainment occurs) has been
given by both behavioural and neuroimaging exper-
iments. Behavioural evidence has come from studies
using (highly) compressed (i.e. accelerated) speech,
showing that the greater the compression, the more
intelligibility is impaired (e.g. Dupoux & Green, 1997).
Ghitza (2014) demonstrated that the intelligibility of
compressed speech deteriorates particularly sharply
when syllable rates exceed the upper frequency of the
theta range (>9 Hz; cf. Ghitza & Greenberg, 2009). He
postulated that as long as the syllable rate is inside the
theta frequency range, theta oscillations track the
speech input, aligning neural theta cycles to the input
syllabic rhythm.
1
When theta is in sync with the
sensory syllable rate (i.e. in the 3–9 Hz range), intelligibil-
ity is high. However, when theta is out of sync, for
instance, for syllable rates above 9 Hz, intelligibility
drops. This account can also explain perceptual learning
of compressed speech (i.e. better comprehension of
compressed speech with greater exposure, even across
talker changes; Adank & Janse, 2009; Dupoux & Green,
1997), since greater exposure would presumably allow
for closer alignment of neural oscillations to the speech
input.
This oscillations-based account of compressed speech
perception predicts that the maximum information
transfer rate of syllables –the auditory channel capacity
–is 9 syllables per second, the maximum speech tempo
where the theta oscillations are still in sync with the
speech input. Support for this prediction has been
given in Ghitza (2014)by“repackaging”compressed
speech: dividing a time-compressed waveform into frag-
ments, called packets, and delivering the packets at a
prescribed packet delivery rate by inserting silent inter-
vals in between packets (cf. Figure 1 below). While com-
pressed speech with syllable rates higher than 9 Hz (i.e.
outside theta range) was largely unintelligible, repacka-
ging these compressed signals such that the packet
delivery rate fell below 9 packets per second (i.e. allow-
ing theta oscillations to be in sync with the input
signal) restored intelligibility to a large extent. Note
that the acoustics inside the packet is the compressed
signal; hence the phonetic material available to the lis-
tener is identical in both (compressed and repackaged)
conditions. Only the packet delivery rate is different
between the compressed and repackaged speech con-
ditions, indicating that auditory channel capacity is
defined by information transfer rate, mostly independent
of speech tempo.
2
Neuroimaging findings support the idea that success-
ful neural entrainment shapes the decoding of concur-
rent speech. For instance, Doelling et al. (2014)
recorded magnetoencephalography (MEG) while partici-
pants listened to speech from which the slow syllabic
amplitude fluctuations had been removed (by filtering;
cf. Ghitza, 2012). The authors observed that neural
entrainment was reduced and intelligibility decreased,
relative to control. However, artificially reinstating the
Figure 1. Waveforms for one example digit string, compressed at different time-compression factors κ, with syllable rate σ. The bottom
waveform shows the repackaged condition, comprised of speech packets 66 ms long (taken from the κ=5 condition with a syllable rate
of about 15 syllables per second), spaced apart by 100 ms, resulting in a 6 Hz packet rate.
956 H. R. BOSKER AND O. GHITZA
temporal fluctuations by mixing in “clicks”at syllabic
intervals restored envelope-tracking activity (Doelling
et al., 2014) and hence speech intelligibility (Ghitza,
2012).
Neural entrainment shapes perception of
subsequent speech
It has recently been suggested that stimulus-induced
entrainment may persist even when the driving stimulus
has ceased (Lakatos et al., 2013; Spaak, de Lange, &
Jensen, 2014). Influencing effects of this persisting
neural rhythm on the perception of subsequent speech
would be strong evidence for the notion of neural
entrainment as a causal factor shaping perception (i.e.
rather than being a response-driven epiphenomenon
of language comprehension). That is, because the
entraining rhythm has already ceased, the possibility of
response-driven effects is excluded. For instance,
Hickok, Farahbod, and Saberi (2015) presented listeners
with entraining white noise modulated at 3 Hz, followed
by stationary noise. Participants’task was to detect near-
threshold pure tones that were embedded in the station-
ary noise signal in half of the trials. The authors observed
a perceptual oscillation in participants’tone detection
performance, matching the period of the entraining
stimulus (i.e. 3 Hz) and persisting over several cycles.
This suggests that neural entrainment may be sustained
after rhythmic stimulation, consequently influencing
subsequent auditory perception.
Most studies on speech perception addressing the
issue of persisting entrainment and ensuing effects
thereof have adopted the “rate normalization”paradigm.
That is, the perception of a target speech sound ambig-
uous between two members of a durational contrast (e.g.
in English, short /b/ vs. long /w/; in Dutch, short /ɑ/ vs.
long /a:/) may be biased towards the longer phoneme
(i.e. /w/ in English; /a:/ in Dutch) if presented after a pre-
ceding sentence (hereafter: carrier) produced at a faster
speech rate (Bosker, Reinisch, & Sjerps, 2017; Kidd, 1989;
Pickett & Decker, 1960; Reinisch & Sjerps, 2013;
Toscano & McMurray, 2015). This process, known as
rate normalisation, has been argued to involve general-
auditory processes, since it occurs in human and non-
human species (Dent, Brittan-Powell, Dooling, & Pierce,
1997), is induced by talker-incongruent contexts
(Bosker, 2017b; Newman & Sawusch, 2009), and even
by non-speech (Bosker, 2017a; Gordon, 1988; Wade &
Holt, 2005); in contrast to other rate-dependent percep-
tual effects, such as the Lexical Rate Effect (Dilley & Pitt,
2010; Pitt, Szostak, & Dilley, 2016). Rate normalisation
has been proposed to be related to the cognitive prin-
ciple of durational contrast (Diehl & Walsh, 1989). That
is, the perception of ambiguous speech segments
should be biased towards longer (shorter) percepts in
the context of other shorter (longer) surrounding seg-
ments (Wade & Holt, 2005). Even though this principle
accounts for the general-auditory nature of rate normal-
isation effects, it is as yet unclear how this cognitive prin-
ciple might be neurally implemented.
One potential neurobiologically plausible mechanism
involves sustained neural entrainment to the preceding
sentence. Bosker (2017a) showed that anisochronous
(“non-rhythmic”) fast and slow carriers do not trigger
rate normalisation, suggesting that rate normalisation
is induced by the periodicity in the carrier. This is in
line with an MEG experiment (Kösem et al., 2017),
where native Dutch participants were presented with
slow and fast carriers (amplitude modulations around 3
and 5.5 Hz, respectively), followed by (uncompressed)
target words containing vowels ambiguous between
short /ɑ/ and long /a:/. Behavioural vowel identification
responses revealed a consistent rate normalisation
effect, with more long /a:/ responses after fast carriers.
The MEG data showed that, during the carrier, neural
theta oscillations efficiently tracked the dynamics of
the slow and fast speech. Moreover, during the target
word time window, theta oscillations in right superior
temporal cortex were observed that corresponded in fre-
quency to the preceding syllable rate, revealing persist-
ing entrainment in the target window. In fact, the
extent to which these theta oscillations persisted into
the target window correlated with the observed behav-
ioural biases: the more evidence for sustained entrain-
ment in the target window, the greater the behavioural
rate normalisation effect (Kösem et al., 2017).
These findings suggest that neural oscillations actively
shape speech perception (Bosker, 2017a; Bosker & Kösem,
2017; Peelle & Davis, 2012). Entrained theta oscillations
would be thought to impose periodic phases of neuronal
excitation and inhibition, thus sampling the input signal at
the appropriate temporal granularity (Bosker, 2017a). In
other words, entrainment at a higher theta frequency
(e.g. 8 Hz) would raise the cortical “sampling frequency”.
If entrained neural rhythms persist after rhythmic stimu-
lation has ended, this higher cortical “sampling frequency”
then also affects the perception of subsequent speech.
That is, if a fast sentence is suddenly followed by an
ambiguous target vowel (e.g. ambiguous between short
/ɑ/ and long /a:/ in Dutch), entrainment to the fast sen-
tence would lead to “oversampling”the target vowel,
inducing overestimation of the target’sduration(i.e.
more long /a:/ responses). Similarly, if the same target
word is presented after a slow carrier, this would induce
cortical “undersampling”, with the target’s duration
being underestimated (i.e. fewer long /a:/ responses).
LANGUAGE, COGNITION AND NEUROSCIENCE 957
Contextual effects of syllable rates inside and
outside theta range
Oscillations-based accounts of rate normalisation
(Bosker, 2017a; Kösem et al., 2017; Peelle & Davis, 2012)
would predict that, whenever theta oscillations are in
sync with the syllable rate of the carrier (i.e. ranging
from 3–9 Hz), they will consistently influence the cortical
“sampling frequency”, with sustained effects influencing
the perceived duration of following target segments.
However, when carriers are compressed to syllable
rates above 9 Hz, theta oscillations would be out of
sync with the syllabic input rate, thus removing contex-
tual influences on subsequent target perception. That
is, these oscillations-based models predict an upper
limit to the syllable rates that induce rate normalisation:
increasing syllable rates to up to nine syllables per
second should bias perception of following ambiguous
targets more and more towards longer percepts;
however, no consistent contextual influences should be
observed for syllable rates exceeding the theta range.
Interestingly, all studies on rate normalisation in the
literature so far have tested the effects of syllable rates
inside the theta range of 3–9 Hz. As far as we could
establish, the fastest syllable rates tested had a syllable
rate of 8 Hz (e.g. Newman & Sawusch, 2009). As such, it
is as yet unknown whether there is an upper limit to
the contextual syllable rates that might induce rate nor-
malisation effects.
In this study, we tested the effect of rate normalisation
with contextual syllable rates beyond capacity. In Exper-
iment 1, Ghitza (2014) was replicated with Dutch
materials. In Experiment 2, Dutch participants were pre-
sented withtime-compressed carriers followed by uncom-
pressed target words containing vowels ambiguous
between Dutch /ɑ/ and /a:/. The carriers were linearly com-
pressed by various compression factors κ, resulting in
some syllable rates within theta range (i.e. below 9 Hz)
and some outside theta range (i.e. above 9 Hz). Instead
of a gradual increase in long /a:/ responses with larger
compression factors, oscillations-based models actually
predict a gradual increase in the percentage of long /a:/
responses for carriers with syllable rates up to 9 Hz.
However, when the carrier has a syllable rate above
9 Hz, cortical theta is assumed to be out of sync with
the syllabic input rate, presumably eliminating any poten-
tial contextual effect on subsequent speech perception.
Restoring rate normalisation through
repackaging
If Experiment 2 would show that rate normalisation
effects are only triggered by syllable rates within theta
range, then this would provide behavioural support for
oscillations-based models of speeded speech compre-
hension. In turn, such a finding would predict that
repackaging compressed speech (Ghitza, 2014; Ghitza
& Greenberg, 2009) might be able to restore rate normal-
isation effects. For instance, compressed speech with a
syllable rate of 15 Hz would not be predicted to bias
the perception of subsequent Dutch target vowels
towards /a:/, because the syllable rate is outside theta.
However, if this compressed speech signal would be
repackaged such that the packet delivery rate would
fall below 9 Hz, rate normalisation effects should be
restored.
Experiment 3 was designed to test the effect of
repackaged speech on the perception of subsequent
speech. Oscillations-based models would predict that
compressed speech with a syllable rate above 9 Hz
would not induce rate normalisation, whereas repacka-
ging this same acoustic signal such that the packet deliv-
ery rate would fall below 9 Hz would induce rate
normalisation (similar to compressed speech with sylla-
ble rates below 9 Hz).
Experiment 1
Experiment 1 served as an attempt to replicate the find-
ings from Ghitza (2014) in Dutch, testing the intelligibility
of Dutch speech recordings compressed with various
compression factors κsuch that resulting syllable rates
would fall either within (i.e. below 9 Hz) or outside
theta range (above 9 Hz). Moreover, it tested whether
repackaging speech such that the resulting packet deliv-
ery rate falls within theta would restore intelligibility of
highly compressed speech, as reported in Ghitza
(2014). If Experiment 1, targeting overall intelligibility,
succeeds in replicating the findings from Ghitza (2014),
this allows us to use the Dutch speech materials for the
subsequent experiments, targeting rate normalisation.
Method
Participants
Native Dutch participants (N= 18) with normal hearing
were recruited from the Max Planck Institute’s participant
pool. They gave informed consent as approved by the
Ethics Committee of the Social Sciences department of
Radboud University (project code: ECSW2014-1003-
196). Data from one participant were excluded for
failing to understand the experimental task. Data from
another participant were excluded because she
requested to stop the experiment, leaving data from 16
participants (12 females, 4 males; M
age
= 21) for analysis.
958 H. R. BOSKER AND O. GHITZA
Design and materials
Sentence materials were generated after Ghitza (2014). A
male native speaker of Dutch (first author) was recorded,
fluently producing 100 digit strings, followed by Dutch
target words. Experiment 1 only made use of the
recorded digit strings; description of the target words
is provided in the method of Experiment 2.
Each digit string was comprised of 7 digits (e.g. twee
een vijf, vier zes vijf drie; “two one five, four six five
three”), approximately 2 s long, with an average syllable
rate around 3 Hz (SD = 0.2). The speaker uttered the
strings as a phone number, using a cluster of 3 digits fol-
lowed by a cluster of 4 digits, without combining digits
(e.g. no “sixteen”,“three times five”, etc.). All digits
were used (0, 1, 2, 3, 4, 5, 6, 7, 9), except acht /ɑxt/
“eight”, because it contained the critical vowel used in
the target words in Experiment 2 and 3.
Digit strings were time-compressed using PSOLA in
Praat (Boersma & Weenink, 2016). This manipulation
alters a signal’s duration while preserving spectral prop-
erties (e.g. original formant patterns and fundamental
frequency contours are maintained; cf. Figure 1). The
time-compression factor κ(i.e. the factor by which the
original duration of an utterance was compressed; e.g.
κ= 5 involves compressing speech to 20% of its original
duration) was chosen such that κ∈{1, 2, 5}, resulting in
three different experimental conditions. Signals with
κ=1 and signals with κ=2 had an average syllable rate
of around 3 and 6 Hz, respectively. As such, their syllable
rates fell below 9 Hz, generally assumed to be the upper
limit of cortical theta and the maximum reliable infor-
mation transfer rate through the auditory channel
(i.e. auditory channel capacity; Ghitza, 2014). Signals
with κ=5 had an average syllable rate of around 15 Hz
(i.e. above 9 Hz), falling outside cortical theta and audi-
tory channel capacity.
A fourth repackaged condition involved repackaging
the κ=5 condition (see Figure 1). Intervals of 66 ms
were excised from the signal with κ=5 and spaced
apart by 100 ms using Praat. This manipulation resulted
in a repackaged condition that contained acoustic-pho-
netic material that was identical to the κ=5 condition,
only with a significantly lower packet delivery rate of
6 Hz (i.e. within the cortical theta range). Finally, low-
level speech-shaped noise was added to all digit
strings (SNR =20 dB; not shown in Figure 1).
Procedure
Stimulus presentation was controlled by Presentation
software (v16.5; Neurobehavioral Systems, Albany, CA,
USA). Each participant was presented with the four
speech conditions (κ=1; κ=2; κ=5; repackaged), with
80 digit strings per condition, chosen at random from
the total set of 100.
Each trial started with a fixation cross presented on
screen. After 500 ms, the auditory stimulus was played.
At stimulus offset, the screen was replaced with a
response screen. Participants were instructed to enter
only the last four digits of the stimulus they had heard
(using digits, e.g. “4653”), and hit Enter to proceed (i.e.
self-paced). Requesting only the last four digits
reduced the bias of memory load on error patterns and
provided an opportunity for the presumed (cortical)
theta oscillator to entrain to the input rhythm of the
first three digits prior to the occurrence of the final
four digits. Participants were instructed to always enter
four digits, in the exact order in which they were
spoken, encouraging participants to guess if they had
missed any digit. After a response had been recorded,
a blank screen was presented for 500 ms, and the next
trial was initiated.
Results
Trials with responses with less than four digits (n=53;
<1%) were excluded from analyses. Proportion correct
scores were calculated as the proportion of digits in a
given string that were correctly registered in the
correct position (i.e. for the digit string “215 4652”, the
response “4652”received a proportion correct value of
1.0; the response “4352”, 0.75; the response “6452”,0.5;
etc.), and derived percentage correct scores are pre-
sented in Figure 2.
Figure 2. Percentage correct intelligibility scores for the four
conditions with different time-compression factors κfrom Exper-
iment 1 (error bars show standard errors). Speech intelligibility is
high for syllable rates within the theta range (i.e. κ=1 and κ=2
with syllable rates below 9 Hz) but deteriorates sharply for sylla-
ble rates outside the theta range (i.e. κ=5 with syllable rates
above 9 Hz). Moreover, when applying “repackaging”such that
the resulting packet delivery rate falls within theta range,
speech intelligibility greatly improves.
LANGUAGE, COGNITION AND NEUROSCIENCE 959
Proportion correct scores were entered into a Gener-
alised Linear Mixed Model (GLMM; Quené & Van den
Bergh, 2008) with a logistic linking function, as
implemented in the lme4 library (Bates, Maechler,
Bolker, & Walker, 2015) in R (R Development Core
Team, 2012), with weights specified as the maximum
number of correct digits per trial (i.e. 4). Condition was
entered as predictor (categorical variable, dummy
coded with the κ=1 condition mapped onto the inter-
cept), with Participant and Digit String entered as
random factors with by-participant and by-digit string
random slopes for Condition (Barr, Levy, Scheepers, &
Tily, 2013).
This model revealed significant differences between
the κ=1 and the κ=5 condition (β=−9.291, SE = 0.688,
z=−13.499, p<0.001; lower accuracy in κ=5vs.κ=1),
between the κ=1 and the repackaged condition (β=
−7.807, SE = 0.699, z=−11.157, p<0.001; lower accuracy
in repackaged vs. κ=1), and between the κ=1 and the
κ=2 condition (β=−3.997, SE = 0.691, z=−5.781, p<
0.001; slightly lower accuracy in κ=2vs.κ=1).
The categorical predictor Condition can only compare
conditions to its intercept (set to κ=1). In order to also
gain insight into comparisons between other conditions,
a mathematically equivalent GLMM was built, including
a re-leveled Condition predictor, this time mapping
the repackaged condition onto the intercept. This
analysis revealed significant differences between the
repackaged and the κ=5 condition (β=−1.484, SE =
0.122, z=−12.147, p<0.001; lower accuracy in κ=5
vs. repackaged), and between the repackaged and
the κ=2 condition (β= 3.811, SE = 0.285, z=−12.147,
p<0.001; higher accuracy in κ=2 vs. repackaged).
Interim discussion
Experiment 1 replicated the findings from Ghitza (2014)
using Dutch materials. Speech intelligibility was high
for syllable rates within the theta range (i.e. κ=1 and
κ=2 with syllable rates below 9 Hz) but deteriorated
sharply for syllable rates outside the theta range (i.e.
κ=5 with syllable rates above 9 Hz). Moreover, when
maximally compressed speech (i.e. κ=5) was repack-
aged such that the resulting packet delivery rate falls
within theta range, the intelligibility of the speech was
restored to a large degree. Thus, Experiment 1 validated
the use of these materials for subsequent experiments.
Experiment 2
Experiment 2 was designed to test whether there is an
upper limit to the contextual syllable rates that may
induce rate normalisation. Instead of a gradual increase
in rate normalisation effects as the compression factor
(hence the syllable rate) is raised, oscillations-based
models predict that rate normalisation effects should
not be induced by syllable rates above 9 Hz, since corti-
cal theta would then be assumed to be out of sync with
the syllabic input rate.
Method
Participants
Native Dutch participants (N= 21; 17 females, 4 males;
M
age
= 22), that had not participated in Experiment 1,
with normal hearing were recruited from the Max
Planck Institute’s participant pool. They gave informed
consent as approved by the Ethics Committee of the
Social Sciences department of Radboud University
(project code: ECSW2014-1003-196).
Design and materials
A set of 10 digit strings (out of the total set of a hundred)
were adopted for use in Experiment 2. These digit strings
were compressed (PSOLA in Praat) using various time-
compression factors κ,suchthatκ∈{1, 2, 3, 4, 5}, resulting
in five different experimental conditions (see Figure 1).
Signals with κ∈{1, 2, 3} had an average syllable rate
around 3, 6, and 9 Hz, respectively, falling just below the
upper limit of cortical theta and the maximum reliable
information transfer rate through the auditory channel
(i.e. auditory channel capacity; Ghitza, 2014). Signals with
κ>3 had a syllable rate above 9 Hz, falling outside cortical
theta and auditory channel capacity.
The male native Dutch speaker that had been
recorded for Experiment 1 had also produced Dutch
target words following each digit string. These target
words were proper names selected from four different
minimal pairs containing either the short vowel /ɑ/or
the long vowel /a:/: Ad - Aad, /ɑt - a:t/; Bas - Baas, /bɑs
- ba:s/; Dan - Daan, /dɑn - da:n/; Mart - Maart, /mɑrt -
ma:rt/. The /ɑ/-/a:/ vowel contrast in Dutch is cued by
both spectral (lower formant values for /ɑ/, higher
formant values for /a:/) and temporal cues (shorter dur-
ation for /ɑ/, longer duration for /a:/; Escudero,
Benders, & Lipski, 2009). Therefore, one natural /a:/
vowel token was selected, taken from the word Baas,
that had a duration that fell in between the speaker’s
typical /ɑ/ and /a:/ durations (120 ms; i.e. in duration
ambiguous between /ɑ/ and /a:/). This temporally ambig-
uous /a:/ vowel was manipulated to also be spectrally
ambiguous between /ɑ/ and /a:/ using Burg’s LPC
method in Praat. Source and filter models were esti-
mated automatically from the selected vowel. The
formant values in the filter models were inspected and
adjusted (F1 = 820 Hz; F2 = 1150 Hz) to fall in between
960 H. R. BOSKER AND O. GHITZA
the speaker’s/ɑ/ and /a:/ formant values. Recombination
of the source and adjusted filter model resulted in a
vowel that was ambiguous between /ɑ/ and /a:/ in
both its temporal and spectral properties (corroborated
by pretesting). Finally, the vowel was spliced into fixed
consonantal frames (/ʔ_t/; /b_s/; /d_n/; /m_rt/) to
create target words.
Note that only using a single ambiguous vowel would
have made the experiment needlessly difficult for partici-
pants, whose task was to categorise the vowel in the
target word (i.e. negatively affecting participants’motiv-
ation). Therefore, filler trials were included that contained
clear (i.e. unambiguous) /ɑ/ and /a:/ vowels.
Procedure
Each participant was presented with the 10 digit strings
in all 5 compression conditions, followed by any of the 4
ambiguous minimal pairs (n= 200; or followed by any of
the 8 unambiguous target words in filler trials; n=400).
Each trial started with a fixation cross presented on
screen. After 500 ms, a digit string was played, followed
by a 100 ms silent interval and a target stimulus. At
target offset, the fixation cross was replaced by a screen
with two response options, one word on the left,
another on the right (position of /ɑ/-/a:/ words counter-
balanced across participants). Participants entered their
response as to which of the two words they had heard
(Bas or Baas, etc.) by pressing “1”for the option on the
left, or “0”for the option on the right. After their response
(or timeout after 4 s), the screen was replaced by an empty
screen for 500 ms, after which the next trial was initiated.
Results
Trials with missing categorisation responses (n=6; <1%)
were excluded from analyses. Categorisation data, calcu-
lated as the percentage of long /a:/ responses (% /a:/), are
presented in Figure 3, and were analyzed by a GLMM
with a logistic linking function. The dependent variable
was response /a:/ (coded as 1) or /ɑ/ (coded 0). Condition
was entered as predictor (categorical variable, dummy
coded with the κ=1 condition mapped onto the inter-
cept), with Participant and Digit String entered as
random factors with by-participant and by-digit string
random slopes for Condition (Barr et al., 2013).
This model revealed significant differences between
the κ=1 and the κ=2 condition (β= 0.500, SE = 0.121,
z= 4.107, p<0.001; higher percentage of /a:/ responses
in κ=2); and between the κ=1 and the κ=3 condition
(β= 0.353, SE = 0.136, z= 2.601, p=0.009; higher percen-
tage of /a:/ responses in κ=3). However, no differences
were observed between the κ=1, κ=4, and κ=5
conditions.
A mathematically equivalent GLMM with a re-leveled
Condition predictor, this time mapping the κ=3 con-
dition onto the intercept, revealed additional significant
differences between the κ=3 and the κ=4 condition
(β=−0.286, SE = 0.112, z=−2.552, p=0.011; lower per-
centage of /a:/ responses in κ=4); and between the
κ=3 and the κ=5condition (β=−0.326, SE = 0.117,
z=−2.778, p=0.005; lower percentage of /a:/ responses
in κ=5).
Interim discussion
Experiment 2 was designed to test whether there is an
upper limit to the syllable rates that elicit rate normalisa-
tion effects on following ambiguous target words. We
observed that compressing a naturally produced carrier
by a factor of 2 induced a higher percentage of /a:/
responses for following ambiguous target words (relative
to the uncompressed signal), replicating earlier findings
in the literature (Bosker, 2017b; Bosker et al., 2017; Rein-
isch, 2016b; Reinisch & Sjerps, 2013).
A novel finding of Experiment 2 is that when carriers
are compressed to have syllable rates outside the theta
range (i.e. the κ=4 and κ=5 conditions), no difference
in target word categorisation is observed compared to
the uncompressed condition (i.e. κ=1). This finding sup-
ports an oscillations-based mechanism underlying rate
normalisation: only when theta oscillations can optimally
track the rate of the carrier do we find effects on the per-
ception of subsequent target words.
Figure 3. Average categorisation data (in % long /a:/ responses)
for the five conditions with different time-compression factors κ
from Experiment 2 (error bars show standard errors). Com-
pression of speech carriers by κ= 2, with syllable rates within
the theta range, leads to an increase in % /a:/ responses.
However, compression of carriers by κ= 4 and κ= 5, with syllable
rates outside the theta range, does not lead to an increase in %
/a:/ responses (comparable target categorisation as in the base-
line κ= 1 condition).
LANGUAGE, COGNITION AND NEUROSCIENCE 961
Note that the categorisation data in the κ=3 con-
dition fall in between the results for the κ=2 and the
κ={1, 4, 5} conditions. This observation may be a conse-
quence of the fact that the average syllable rate in the κ
=3 condition is around 9 Hz, just at the upper border of
the theta range. Recalling the biophysical nature of the
neuronal theta, its frequency range is not precise and
the 9 Hz limit should be considered as an estimated
mean. Hence, a plausible explanation for this finding
may be related to individual differences between partici-
pants in how successful their theta oscillations were in
tracking the speech input at syllable rates near capacity.
Experiment 3
Experiment 3 was designed to test whether repackaged
compressed speech may induce rate normalisation.
Since repackaging of heavily compressed speech can
lower the packet delivery rate to below 9 Hz (i.e. within
theta range), oscillations-based models predict that
repackaging might restore rate normalisation effects.
Method
Participants
Native Dutch participants (N= 29) with normal hearing,
that had not participated in the previous experiments,
were recruited from the Max Planck Institute’s participant
pool. They gave informed consent as approved by the
Ethics Committee of the Social Sciences department of
Radboud University (project code: ECSW2014-1003-
196). Data from 6 participants were excluded for
reasons of technical errors (n= 1), illness (n= 1), fatigue
(n= 2) or non-compliance with the experimental task
(n= 2), leaving data from 23 participants (19 females, 4
males; M
age
= 21) for analysis.
Design and materials
Experiment 3 combined the four speech conditions from
Experiment 1 (κ=1; κ=2; κ=5; repackaged) with the
procedure from Experiment 2. That is, the 10 digit
strings used in Experiment 2 were time-compressed
and repackaged, using the method described in Exper-
iment 1, and presented together with the target words,
described in Experiment 2.
Procedure
The experimental procedure in Experiment 3 mirrored
the procedure in Experiment 2. Each participant was pre-
sented with the 10 digit strings in all 4 conditions, fol-
lowed by any of the 4 ambiguous minimal pairs (n=
160; or by any of the 8 unambiguous target words in
filler trials; n=320). Again, participants’task was to indi-
cate which target word they had heard (Bas or Baas, etc.).
Results
Trials with missing categorisation responses (n=3;
<1%) were excluded from analyses. Categorisation
data, calculated as the percentage of long /a:/
responses (% /a:/), are presented in Figure 4,andwere
analyzed by a GLMM with identical structure as the
one built for Experiment 2.
This GLMM revealed significant differences between
the κ=1 and the κ=2 condition (β= 0.398, SE = 0.101,
z= 3.927, p<0.001; higher percentage of /a:/ responses
in κ=2); and between the κ=1 and the repackaged con-
dition (β= 0.272, SE = 0.100, z= 2.694, p=0.007; higher
percentage of /a:/ responses in repackaged condition).
No difference was found between the κ=1 and κ=5
condition (p>0.7).
A mathematically equivalent GLMM with a re-
leveled Condition predictor, this time mapping the
repackaged condition onto the intercept, revealed an
additional significant difference between the repack-
aged condition and the κ=5condition(β=−0.245,
SE = 0.100, z=−2.440, p=0.015; lower percentage of
/a:/ responses in κ=5). However, no difference was
found between the repackaged condition and the κ
=2 condition (p>0.2).
Figure 4. Average categorisation data (in % long /a:/ responses)
for the four conditions with different time-compression factors κ
from Experiment 3 (error bars show standard errors). Similar to
Experiment 2, compression of speech carriers by κ= 2, with syl-
lable rates within the theta range, leads to an increase in % /a:/
responses. Also, compression of carriers by κ= 5, with syllable
rates outside the theta range, does not lead to an increase in
% /a:/ responses (comparable target categorisation as in the
baseline κ= 1 condition). However, when applying “repacka-
ging”to the κ= 5 condition, rate normalisation is restored: the
repackaged condition induces an increase in % /a:/ responses
comparable to the κ= 2 condition.
962 H. R. BOSKER AND O. GHITZA
Interim discussion
First, results from Experiment 3 replicate the findings for
the κ=1, κ=2, and κ=5 conditions from Experiment
2. That is, only when syllable rates were inside the
theta range, rate normalisation was observed.
The novel finding of Experiment 3 is that when the κ
=5 condition was repackaged such that its packet deliv-
ery rate was set to be around 6 Hz (inside theta range),
rate normalisation was restored. In fact, the categoris-
ation responses from the repackaged condition were
comparable to those from the κ=2 condition, with a syl-
lable rate comparable to the packet rate of the repack-
aged condition (around 6 Hz). Note that the acoustics
inside the packets in the compressed and repackaged
condition were identical (i.e. time-compressed by κ=5);
nevertheless, very different contextual effects were
observed on target word perception. Thus, Experiment
3, together with the replicated findings from Experiment
2, supports an oscillations-based account of rate normal-
isation, with a central role for sustained entrainment of
oscillations in the theta range in rate normalisation.
General discussion
The present study contributes psychoacoustic data to
the ongoing debate about the functional role of
entrained neural oscillations in speech comprehension:
does entrainment to a speech rhythm actively shape
the decoding of the spoken input signal (i.e. a causal
factor) or is it merely a manifestation of modulated
evoked responses to the driving auditory stimulus (i.e.
a consequence)? Previous studies have contributed to
this debate by providing empirical evidence that neural
entrainment guides the decoding of concurrent speech:
when the natural speech rhythm is disrupted, entrain-
ment is reduced (Doelling et al., 2014), and intelligibility
suffers (Ghitza, 2012,2014; Ghitza & Greenberg, 2009). A
demonstration that entrainment influences the percep-
tion of subsequent speech (i.e. after the driving stimulus
has ceased) –thus allowing the exclusion of potential
evoked effects from the driving stimulus –could
provide further evidence for the causal influence of
neural entrainment on speech comprehension. Our
study was concerned with providing such evidence by
means of psychoacoustic experimentation.
Experiment 1 showed that compressing Dutch speech
such that syllable rates fall outside the theta range (i.e. >
9 Hz) greatly harms intelligibility, in line with Ahissar et al.
(2001) who showed that MEG neural responses track the
temporal envelope of compressed speech as long as it is
intelligible. This behaviour is explained by the TEMPO
model in Ghitza (2011), suggesting that the decline in
intelligibility with speech speed is dictated by cortical
theta (Ghitza, 2014).
3
If the highly compressed speech
signal is “repackaged”(i.e. delivering compressed
speech packets at a lower rate by inserting silent inter-
vals in between packets; see Figure 1) such that the
packet delivery rate falls within theta range, intelligibility
is much enhanced. Together, these outcomes suggest
that entrainment of theta oscillations supports the
decoding of concurrent speech, extending earlier work
in English (Ghitza, 2014) to a new language: Dutch.
Experiment 2 and 3 targeted potential effects of sus-
tained entrainment on subsequent speech using the
rate normalisation paradigm: participants heard digit
strings compressed by various compression factors, fol-
lowed by (uncompressed) minimal pair target words
ambiguous between containing the short vowel /ɑ/ vs.
the long vowel /a:/ (e.g. Dan - Daan, /dɑn - da:n/). Exper-
iment 2 demonstrated that compression of the digit
strings biased listeners towards reporting more long
/a:/ responses, corroborating previous studies on rate
normalisation (e.g. Bosker, 2017b; Newman & Sawusch,
2009; Reinisch & Sjerps, 2013; Toscano & McMurray,
2015). However, this only applied for those compressed
speech conditions with syllable rates safely within theta
range. In fact, the perception of target words preceded
by compressed digit strings with syllable rates outside
the theta range (i.e. well above 9 Hz) was observed to
be comparable to the baseline uncompressed condition.
Thus, the present study is the first to demonstrate that
there is an upper limit to the syllable rates that induce
rate normalisation, namely a limit around 9 Hz.
One study that may seem to be at odds with this sug-
gestion is Wade and Holt (2005), who reported that tone
sequences with a presentation rate of 25 Hz also elicited
rate normalisation. Note, however, that this study used
non-speech carriers to elicit normalisation effects on
the perception of consonantal target segments (i.e.
short /b/ vs. long /w/ in English). It could be argued
that the perception of vowels (relatively long; syllable
nuclei) is governed by slow theta oscillations (corre-
sponding in modulation rate), whereas the perception
of consonants (relatively short; higher modulation rate;
syllable onsets and codas) would be sensitive to faster
gamma oscillations in the 25–40 Hz range (Giraud &
Poeppel, 2012). This, however, remains speculative and
future studies may examine potentially differential nor-
malisation of vowels and consonants.
Experiment 3 also used the rate normalisation para-
digm but this time participants were presented with
repackaged digit strings, followed by the ambiguous
target words. Oscillations-based models predict that
repackaging highly compressed speech would restore
cortical speech tracking, with consequences for the
LANGUAGE, COGNITION AND NEUROSCIENCE 963
temporal sampling of subsequent speech. Results indeed
showed that, whereas highly compressed speech (with
syllable rates > 9 Hz) did not have an influence on listen-
ers’target vowel perception (replicating Experiment 2),
repackaged compressed speech –with packet delivery
rates inside theta range –did bias listeners to report
more long /a:/ vowels.
Taken together, the present experiments point to a
central role for sustained theta entrainment in rate nor-
malisation. Neural theta oscillations are suggested to
entrain to the rhythmic properties of a driving speech
stimulus (Giraud & Poeppel, 2012; Peelle & Davis, 2012),
imposing an appropriate sampling regime onto the
incoming sensory stimulus (cf. Experiment 1; Ghitza,
2012,2014). These oscillations may persist for several
cycles after stimulation has ceased (Hickok et al., 2015;
Kösem et al., 2017; Lakatos et al., 2013; Spaak et al.,
2014), allowing for the possibility that a preceding
rhythm influences the perception of subsequent
speech by imposing its own cortical “sampling fre-
quency”. For instance, fast rhythms would induce “over-
sampling”of the following speech content, with listeners
overestimating the duration of subsequent spoken seg-
ments (Bosker, 2017a; Bosker & Kösem, 2017). When syl-
lable rates fall outside the theta range (i.e. > 9 Hz), theta
oscillations cannot effectively entrain to the incoming
speech signal, and consequently the sustained effect of
these theta oscillations on subsequent perception is
reduced (cf. Experiment 2). However, when the infor-
mation transfer rate is tuned back to within the theta
range (via repackaging), theta entrainment is restored,
improving the decoding of the concurrent speech (intel-
ligibility enhancement; cf. Experiment 1) and influencing
subsequent perception (rate normalisation; cf. Exper-
iment 3).
Thus, this study should be viewed as providing a
neural implementation of cognitive principles proposed
to underlie rate normalisation, such as the principle of
durational contrast (Diehl & Walsh, 1989; Oller, Eilers,
Miskiel, Burns, & Urbano, 1991; Wade & Holt, 2005).
This principle was formulated to have the psycholinguis-
tic function of biasing the perception of ambiguous
speech segments towards longer (shorter) percepts
when they occur in the context of other shorter
(longer) surrounding segments. The neural implemen-
tation proposed here refines the description of this cog-
nitive principle by showing that rate normalisation is
governed by the syllable/packet rate of the carrier (in
line with Bosker, 2017a). Furthermore, it provides impor-
tant constraints, derived from current oscillations-based
models of speech perception, revealing an upper limit
to rate normalisation effects (i.e. at the upper frequency
of the theta range; 9 Hz).
Note, however, that we do not claim that this is the
only mechanism driving rate normalisation effects in
speech perception. The perception of an ambiguous
target word can also be biased by rate manipulations
in the following, rather than preceding, context
(although typically with smaller effect sizes; J. L. Miller
& Liberman, 1979; Newman & Sawusch, 1996; Sawusch
& Newman, 2000). Moreover, subjective impressions of
fast speech (i.e. without an acoustic increase in syllable
rate) can induce rate normalisation, such as when listen-
ing to speech with segmental reductions (Reinisch,
2016a), habitually fast talkers (Maslowski, Meyer, &
Bosker, in press; Reinisch, 2016b), an unfamiliar language
(Bosker & Reinisch, 2017), or while dual-tasking (Bosker
et al., 2017). We adopt the recently proposed two-stage
model of normalisation processes in speech perception
(Bosker et al., 2017), in which, at an early stage, an auto-
matic general-auditory mechanism is operating indepen-
dent from attentional demands. At a later stage, higher-
level influences, such as subjective impressions, come
into play involving cognitive rather than perceptual
adjustments. The neural mechanism proposed here
would be a candidate mechanism for the first general-
auditory stage of normalisation.
In sum, based on behavioural findings from three
psychoacoustic experiments, the present study (1)
concludes that rate normalisation is governed by the
syllable/packet rate of the carrier; and (2) puts
forward an oscillations-based neurobiological mechan-
ism of rate normalisation. This account is in line with
other behavioural studies (Bosker, 2017a;Bosker&
Kösem, 2017;Ghitza,2012,2014; Ghitza & Greenberg,
2009;Hickoketal.,2015), neuroimaging data (Doel-
ling et al., 2014; Kösem et al., 2017), and previously
formulated cognitive principles, such as durational
contrast (Diehl & Walsh, 1989). Thus, it augments
our understanding of the functional role of entrain-
ment in speech comprehension by proposing that
entrainment not only guides the processing of concur-
rent speech but also shapes the perception of follow-
ing speech content.
Notes
1. In order to be able to track the syllabic irregularities of
spontaneous speech (e.g. a stressed syllable followed
by a non-stressed syllable), the theta oscillator belongs
to a special class of oscillators termed flexible oscillators
(Ghitza, 2011). Such oscillators are different in important
respects from autonomous, rigid oscillators (cf. Ghitza,
2011).
2. It is important to note the distinction between the inter-
ruption of speech via Gating (G. A. Miller & Licklider,
1950), and the insertion of silent gaps via repackaging:
964 H. R. BOSKER AND O. GHITZA
interruption removes part of the speech signal, while
repackaging maintains all speech information with artifi-
cial distribution of information in time, defined by the
packet rate. While phonemic restoration from inter-
rupted speech (Mattys, Brooks, & Cooke, 2009; Warren,
1970) has been attributed to informational masking,
the insertion of gaps provides additional decoding
time: a gradual change in gap duration should be
viewed as tuning the packet rate in a search for a
better synchronisation between the input information
flow and the capacity of the auditory channel; the
optimal range of the packet rate is dictated by the prop-
erties of cortical theta (Ghitza, 2011,2014).
3. It is important to mention here a recent EEG-based
study (Pefkou, Arnal, Fontolan, & Giraud, 2017)
showing that brain-wave oscillations follow the
rhythm of comprehensible fast speech, with speeds
up to 14 syllables/sec. Does this finding contradict
the 9 syllables/sec limit suggested by Ghitza (2014)?
Interpreting their data Pefkou et al. (2017) suggested
two distinct functional processes, a bottom-up theta
driven and a top-down beta driven process, concur-
rently at play during speech perception. And so,
even when the theta oscillator ceases to accurately
track the input, thus preventing the bottom-up
process from extracting important syllabic infor-
mation from the sensory input, a top-down process
helps to obtain the missing information. Hence, the
brain-wave oscillations above 9 Hz –beyond the bio-
physical range of theta –reflect beta feedback. In
relation to our study, note that the effectiveness of
a top-down process is determined by the amount
of contextual information in the speech stream. In
Pefkou et al. (2017), participants listened to short
stories with high semantic context, feeding beta
oscillations and thus the brain-wave activity above
9 Hz. In contrast, our study (as well as Ghitza, 2014)
used random digit strings that do not allow much
opportunity for top-down processing (random
sequences of digits; relatively short; no coherence
between trials), hence involving mainly the bottom-
up theta driven function.
Acknowledgements
We would like to thank Lori Holt and two anonymous reviewers
for helpful comments and suggestions on earlier versions of
this paper, Phillip Alday for advice on statistical analyses, and
Milou Huijsmans for her help in testing participants.
Disclosure statement
No potential conflict of interest was reported by the author(s).
Funding
The first author was supported by a Gravitation grant from the
Dutch Government to the Language in Interaction Consortium.
The second author was supported by a research grant from the
Air Force Office of Scientific Research.
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