The role of premotor cortex in speech perception: Evidence from fMRI and rTMS
Ahmanson-Lovelace Brain Mapping Center, Department of Psychiatry and Biobehavioral Sciences, Semel Institute for Neuroscience and Human Behavior,
Brain Research Institute, David Geffen School of Medicine at UCLA, 660 Charles E. Young Drive South, Los Angeles, CA 90095, USA
a r t i c l e i n f o
Motor theory of speech perception
a b s t r a c t
This article discusses recent functional magnetic resonance imaging (fMRI) and repetitive Transcranial
Magnetic Stimulation (rTMS) data that suggest a direct involvement of premotor cortical areas in speech
perception. These new data map well onto psychological theories advocating an active role of motor
structures in the perception of speech sounds. It is proposed that the perception of speech is enabled –
at least in part – by a process that simulates speech production.
? 2008 Elsevier Ltd. All rights reserved.
The premotor cortex is classically considered to be concerned
with the planning, preparation, selection, and initiation of move-
ments (Wise, 1985). Recent data, however, have suggested that
the human premotor cortex may actually be directly involved in
the perception of speech. These data revived a theory that called
for an active role of motor structures in speech perception.
Several years ago, at the Haskins laboratory in Yale, New Haven,
Alvin Liberman and his colleagues were working on reading de-
vices for war veterans that had lost sight. Their idea was to build
devices that would transform text into spoken words so that blind
veterans could read books and newspapers. To their dismay, Liber-
man and colleagues found that the veterans’ perception of the de-
vice’s speech output was unbearably slow. Much slower of even
distorted human speech. This observation inspired Liberman and
colleagues to propose a theory of speech perception according to
which speech sounds are understood not so much as sounds, but
rather as articulatory gestures, as the intended motor plans neces-
sary to speak (Liberman et al., 1957, 1967; Liberman and Matting-
ly, 1985; Liberman and Whalen, 2000). This theory – called the
motor theory of speech perception – basically suggests that the
way our brain perceives other people talking is by simulating that
we are doing the talking ourselves. Although differing in a number
of important aspects, also the direct realist theory of speech percep-
tion emphasizes that the key functional aspect of perceiving speech
sounds has to do more with retrieving the articulatory gestures
that emit those sounds than with the analysis of the acoustic signal
(Fowler and Rosenblum, 1991).
Both the motor theory and the direct realist theory of speech per-
ception were inspired by the observation that the acoustic cues for
each phoneme tend to be highly contextually dependent. How do
phonetic representations become invariant then? The hypothesis
of the motor theory and of the direct realist theory of speech percep-
tion is that the phonetic representations become invariant at the
level of motor control structures.
Some data from brain damaged patients were in support of an
involvement of motor structures in speech perception. Patients
that produce phonemic jargon are more impaired in phonemic dis-
crimination than patients that produce semantic jargon (Alajoua-
nine et al., 1964). In his seminal book Higher Cortical Functions in
Man, Luria also proposed that silent articulation was necessary to
process speech sounds, on the basis of the observation that speech
perception was impaired in patients with articulatory deficits (Lur-
The most important empirical evidence in support of a role of
motor structures in speech perception, however, was considered
the phenomenon called categorical perception. This term refers to
the fact that stop consonants in particular can only be perceived
categorically. Changes in voice onset time (VOT) and formant fre-
quency are typically not perceived by listeners, until they cross a
categorical boundary between two phonemes (Liberman et al.,
1957). Interestingly, stop consonants can also be produced only
in some sort of categorical fashion. It is indeed impossible to emit
a sound that is halfway through two stop consonants. This func-
tional similarity between perception and production suggested
that perception was ultimately an articulatory phenomenon. How-
ever, other empirical evidence challenged this reasoning. Indeed,
categorical perception was demonstrated in infants well before
they began to speak (Eimas et al., 1971). Even more devastatingly
for the role of categorical perception in support of the motor theory
of speech perception, was the demonstration that chinchillas, ani-
mals that do not speak, also have categorical perception (Kuhl and
The discovery of mirror neurons, premotor neurons of the
macaque brain that fire when the monkey performs an action
0928-4257/$ - see front matter ? 2008 Elsevier Ltd. All rights reserved.
E-mail address: firstname.lastname@example.org
Journal of Physiology - Paris 102 (2008) 31–34
Contents lists available at ScienceDirect
Journal of Physiology - Paris
journal homepage: www.elsevier.com/locate/jphysparis
and when it observes somebody else performing it (di Pellegrino
et al., 1992; Gallese et al., 1996), revived the hypothesis that motor
structures may be concerned with perceptual processes (Rizzolatti
and Craighero, 2004). Mirror neurons were originally discovered in
area F5 of the ventral premotor cortex of the macaque brain. Indi-
rect evidence of human mirror neuron areas has been provided by
Transcranial Magnetic Stimulation (TMS), (see for instance, among
many others, Fadiga et al., 1995; Strafella and Paus, 2000; Aziz-Za-
deh et al., 2002) and functional magnetic resonance imaging (fMRI)
(see for instance, among many others Iacoboni et al., 1999, 2005;
Grèzes et al., 2003).
One of the appealing aspects of the discovery of mirror neurons
with respect to theories positing a role for motor structures in
speech perception, is the fact that these cells, by firing during both
production and perception of similar actions, embody the concept
of parity between action and perception, a concept that Liberman
proposed as the key functional aspect of successful communication
between the sender and the receiver of a message (Liberman and
Mattingly, 1985). In the motor theory of speech perception, the
parity between articulatory codes and acoustic representations is
considered the common code between the speaker and the lis-
tener. Indeed, immediately after mirror neurons were discovered
in Parma, Giacomo Rizzolatti told Luciano Fadiga that the proper-
ties of those neurons reminded him of the motor theory of speech
perceptionof Alvin Liberman
2. Motor cortices and speech perception: TMS and fMRI
This remark must have inspired Fadiga to use TMS to test the
motor theory of speech perception (Fadiga et al., 2002). In this
experiment, Fadiga and his colleagues stimulated the sector of
the motor cortex that control tongue muscles while recording
the tongue muscle twitches induced by the brain stimulation. Sub-
jects were listening to words through earphones. Fadiga and col-
leagues used two main types of words. One type required strong
tongue movements when produced (double ‘‘r”, such as ‘‘terra”,
which means ground in Italian). The other type required only a
slight tongue movement when produced (double ‘‘f”, such as ‘‘baf-
fo”, which means moustache in Italian). The motor theory of speech
perception predicts that while subjects listen to words that require
strong tongue movements such as ‘‘terra”, the stimulation over the
tongue motor cortex should produce stronger muscular twitches in
the tongue compared to listening to words such as ‘‘baffo”. This
hypothesis was confirmed by the experimental data (Fadiga
et al., 2002). This TMS experiment demonstrates that while listen-
ing to other people talking, the listeners mirror the speaker with
their tongues! Other labs have confirmed the basic phenomenon
(Watkins et al., 2003; Watkins and Paus, 2004).
Following these TMS experiments, an fMRI study looked at
brain activation while subjects speak and while they listen to other
people speaking. In this fMRI experiment, subjects listened to syl-
lables through earphones while they were in the magnetic reso-
nance scanner and also said aloud a series of syllables. In every
subject studied, it was observed that the same speech motor area
that was activated while speaking, was also activated while listen-
ing to other people speaking (Wilson et al., 2004). Notably, this
area was located – according to probabilistic cytoarchitectonic
maps (Geyer et al., 1996; Geyer, 2004) – at the border between
Brodmann area 6 and Brodmann area 4a, well within classical ‘mo-
tor’ territory and well outside classical frontal lobe language areas
for speech production and perception, which tend to be located
more ventrally, between Brodmann area 44 and ventral area 6.
An independent, subsequent study substantially confirmed and
even refined this phenomenon (Pulvermüller et al., 2006).
Follow up studies on this premotor/motor area for speech pro-
duction and perception that fulfills at least one prediction of the
motor theory of speech perception1have provided further informa-
tion on its properties. A recent fMRI study has investigated the time-
varying characteristics of this area while subjects were listening to
audio-clips of a narrator that described a series of Warner Bros car-
toons (Wilson et al., 2008). Each subject was scanned during two
fMRI runs of approximately 12 min each, such that subjects could
watch and listen to the narratives of five Warner Bros cartoons. Mod-
el-free inter-subject correlation analyses were used to test whether
this brain area is systematically modulated by the input. The reason-
ing behind this approach is that voxels which tend to respond sim-
ilarly across subjects reveal neural activity that varies in time
following stimulus properties (Hasson et al., 2004), such as dynamic
changes in phonology, syntactic and semantic structure. Inter-sub-
ject correlation analyses during continuous narrative speech com-
prehension demonstrated that this area responds systematically to
the time-varying properties of narrative speech (Wilson et al.,
2008). This is a typical feature of a perceptual area (Hasson et al.,
2004). These recent fMRI data reinforce the notion that this human
brain area at the border between premotor and primary motor cor-
tex is not only concerned with speech production, but also with
The functional properties of this human premotor area that
activates during both speech production and speech perception
were further tested during a fMRI study that investigated the rela-
tionships between producibility of speech sounds and brain activ-
ity (Wilson and Iacoboni, 2006). Subjects first performed a
behavioral task, in which they had to produce non-native pho-
nemes. Subsequently, subjects were studied with fMRI while they
listened to both native and non-native phonemes. In a separate
imaging scan, subjects were also studied while they produced na-
tive phonemes. Both the superior temporal cortex (a classical audi-
tory brain region) and the human premotor area previously
identified as responsive to speech sounds (Wilson et al., 2004)
were activated while subjects were listening to both native and
non-native phonemes. Furthermore, both superior temporal cortex
and premotor cortex discriminated between native and non-native
phonemes, with lower activity for native phonemes. Moreover,
functional connectivity analyses demonstrated that the premotor
area for speech production and perception was functionally con-
nected with the superior temporal cortical regions responsive to
speech sounds. The connectivity analyses demonstrated equivalent
functional connections both when the superior temporal cortex
was used as ‘seed’ area, and when the premotor area for speech
production and perception was the ‘seed’ area (Wilson and Iaco-
boni, 2006). These results suggested that the flow of information
between superior temporal and premotor/motor areas was largely
bidirectional. However, when brain activity while listening to non-
native phonemes was correlated with the behavioral performance
during the task that required subjects to produce non-native pho-
nemes, only the superior temporal activity was found to correlate
with producibility. The activity in the premotor cortical area did
not correlate with producibility of non-native phonemes (Wilson
and Iacoboni, 2006).
The seemingly paradoxical result here is that activity in a sen-
sory area correlates with a production task, whereas the activity
in a premotor area does not. However, the activity in the premotor
area still demonstrated (1) a response to listening to speech
sounds, (2) a discrimination between native and non-native speech
sounds, and (3) functional connectivity with the superior temporal
auditory cortices. Taken together, these fMRI data suggest that
1The motor theory of speech perception also makes assumptions of nativism and
modularity that are irrelevant here and that the studies reviewed here cannot support
M. Iacoboni/Journal of Physiology - Paris 102 (2008) 31–34
while the superior temporal cortex transforms acoustic signal to
phonetic code, the premotor area responsive to both production
and perception of speech sounds may be responsible for the gener-
ation of forward models (Haruno et al., 2001) of phonemic catego-
rization, which would be intrinsically motor. These forward
models, predicting the acoustic consequences of the motor phone-
mic categorization, would then be compared to the acoustic input
in superior temporal cortex. Here, an ‘error’ signal, that is, the dis-
crepancy between the acoustic input and the prediction of the
acoustic consequence of the motor plan, would be generated, pro-
ducing the correlation between activity in an auditory area and
producibility of the speech sounds (Wilson and Iacoboni, 2006).
The error signal, then, would be used to correct the phonemic cat-
egorization in premotor cortex. Thus, if this account is correct,
speech perception is neither a purely sensory nor a purely motor
phenomenon, but rather requires the integration of sensory and
motor information in a recursive sensory-motor process involving
both superior temporal and premotor cortex (Fig. 1).
Obviously, this account implies an active and causal role of the
motor system in speech perception. Since fMRI can only provide
correlative information about activated brain areas, only a TMS
‘virtual lesion’ experiment can demonstrate such active and causal
role. Indeed, a recent repetitive TMS (rTMS) study has provided
such evidence (Meister et al., 2007). In this study subjects per-
formed a phonetic discrimination task, a tone discrimination task,
and a color discrimination task. Task difficulty was equated using
1-up-2-down adaptive staircase procedures. Low frequency rTMS
was applied over the left premotor area for speech production
and perception, and over the left superior temporal cortex. The
model described in Fig. 1 predicts higher TMS-induced deficits in
the tone task when rTMS is applied to the superior temporal cortex
and higher TMS-induced deficits in the speech perception task
when rTMS is applied to the premotor cortex. Indeed, the results
of this recent rTMS study fit quite well this predicted pattern of
activity (Meister et al., 2007). In the tone discrimination task, there
was a significant decrease in correct responses after TMS over the
left superior temporal cortex, but not after TMS over the left pre-
motor area responding to speech sounds. In contrast, there was a
significant decrease of correct responses in the phonetic discrimi-
nation task after TMS over the left premotor area but not after
TMS over the left superior temporal cortex. Finally, the color dis-
crimination task was not affected by TMS over the premotor and
the superior temporal cortex (Meister et al., 2007). These data
clearly support the hypothesis of an interplay between superior
temporal and premotor areas in speech perception, with superior
temporal areas providing an acoustic analysis of the input and pre-
motor areas providing a phonemic categorization of the heard
speech sound. Furthermore, to the best of my knowledge, these
data are the first empirical demonstration of a causal link between
a premotor area and any form of human perception.
3. Concluding remarks
This article has discussed recent fMRI and rTMS evidence sug-
gesting an active role of premotor areas in speech perception. This
new evidence fits well previous psychological models of speech
perception that invoked an active role of motor structures in
the perception of speech sounds (Liberman et al., 1957, 1967;
Liberman and Mattingly, 1985; Liberman and Whalen, 2000; Fow-
ler and Rosenblum, 1991). While the data clearly point to func-
tional interactions between classical auditory areas, such as the
superior temporal cortex, and premotor areas in speech percep-
tion, a likely role that premotor areas may have in speech percep-
tion is to provide an internal motor simulation of the perceived
In evolutionary terms, the mechanism of simulation has been
likely selected within the motor system in order to solve motor
control problems due to slow re-afferent feedback (Haruno
et al., 2001). This mechanism has been probably later co-opted
to facilitate other functions, a phenomenon known in biology as
exaptation (Gould and Vrba, 1982). Under this framework, simula-
tion motor mechanisms for action control would have been co-
opted to implement forms of communication between individuals
that eventually became human language (Meister and Iacoboni,
For generous support the author wish to thank the Brain Map-
ping Medical Research Organization, Brain Mapping Support Foun-
dation, Pierson-Lovelace Foundation, The Ahmanson Foundation,
William M. and Linda R. Dietel Philanthropic Fund at the Northern
Piedmont Community Foundation, Tamkin Foundation, Jennifer
Jones-Simon Foundation, Capital Group Companies Charitable
Foundation, Robson Family and Northstar Fund.
Alajouanine, T., Lhermitte, F., Ledoux, M., Renaud, D., Vignolo, L.A., 1964. Les
composantes phonémiques et sémantiques de la jargonaphasie. Revue
Neurologique 110, 5–20.
Aziz-Zadeh, L., Maeda, F., Zaidel, E., Mazziotta, J., Iacoboni, M., 2002. Lateralization
in motor facilitation during action observation: a TMS study. Exp. Brain Res. 144
Eimas, P.D., Siqueland, E.R., Jusczyk, P., Vigorito, J., 1971. Speech perception in
infants. Science 171 (968), 303–306.
Fadiga, L., Fogassi, L., Pavesi, G., Rizzolatti, G., 1995. Motor facilitation during action
observation: a magnetic stimulation study. J. Neurophysiol. 73 (6), 2608–2611.
Fadiga, L., Craighero, L., Buccino, G., Rizzolatti, G., 2002. Speech listening specifically
modulates the excitability of tongue muscles: a TMS study. Eur. J. Neurosci. 15
Fowler, C.A., Rosenblum, L.D., 1991. The perception of phonetic gestures. In:
Mattingly, I.G., Studdert-Kennedy, M. (Eds.), Modularity and the Motor Theory
of Speech Perception. Lawrence Erlbaum, Hillsdale, NJ, pp. 33–59.
Gallese, V., Fadiga, L., Fogassi, L., Rizzolatti, G., 1996. Action recognition in the
premotor cortex. Brain: J. Neurol. 119 (Pt 2), 593–609.
Geyer, S., 2004. The microstructural border between the motor and the cognitive
domain in the human cerebral cortex. Adv. Anat. Embryol. Cell Biol. 174 (I–VIII),
Geyer, S., Ledberg, A., Schleicher, A., Kinomura, S., Schormann, T., Bürgel, U., et al.,
1996. Two different areas within the primary motor cortex of man. Nature 382
Gould, S.J., Vrba, E.S., 1982. Exaptation: a missing term in the science of form.
Paleobiology 8, 4–15.
Grèzes, J., Armony, J.L., Rowe, J., Passingham, R.E., 2003. Activations related to
‘‘mirror” and ‘‘canonical” neurones in the human brain: an fMRI study.
Neuroimage 18 (4), 928–937.
Haruno, M., Wolpert, D.M., Kawato, M., 2001. Mosaic model for sensorimotor
learning and control. Neural Comput. 13 (10), 2201–2220.
Fig. 1. According to a recent model (Wilson and Iacoboni, 2006), the superior te-
mporal cortex (STC) would implement acoustic analysis while the premotor cortex
would implement a simulation (forward model) of phoneme production. This for-
ward model would allow the prediction of the acoustic consequences of phoneme
production that would be compared in the superior temporal cortex with the ac-
oustic analysis of the heard speech sounds. This comparison would generate an
error signal to be sent back to premotor cortex, which would generate a corrected
phoneme production simulation to be used for phoneme categorization.
M. Iacoboni/Journal of Physiology - Paris 102 (2008) 31–34
Hasson, U., Nir, Y., Levy, I., Fuhrmann, G., Malach, R., 2004. Intersubject
synchronization of cortical activity during natural vision. Science 303 (5664),
Iacoboni, M., Woods, R.P., Brass, M., Bekkering, H., Mazziotta, J.C., Rizzolatti, G.,
1999. Cortical mechanisms of human imitation. Science 286 (5449), 2526–
Iacoboni, M., Molnar-Szakacs, I., Gallese, V., Buccino, G., Mazziotta, J.C., Rizzolatti, G.,
2005. Grasping the intentions of others with one’s own mirror neuron system.
Plos. Biol. 3 (3), e79.
Kuhl, P.K., Miller, J.D., 1975. Speech perception by the chinchilla: voiced-voiceless
distinction in alveolar plosive consonants. Science 190 (4209), 69–72.
Liberman, A.M., Mattingly, I.G., 1985. The motor theory of speech perception
revised. Cognition 21, 1–36.
Liberman, A.M., Whalen, D.H., 2000. On the relation of speech to language. Trends
Cogn. Sci. 4, 187–196.
Liberman, A.M., Harris, K.S., Hoffman, H.S., Griffith, B.C., 1957. The discrimination of
speech sounds within and across phoneme boundaries. J. Exp. Psychol. 54, 358–
Liberman, A.M., Cooper, F.S., Shankweiler, D.P., Studdert-Kennedy, M., 1967.
Perception of the speech code. Psychol. Rev. 74, 431–461.
Luria, A.R., 1966. Higher Cortical Functions in Man. Basic Books, New York.
Meister, I.G., Iacoboni, M., 2007. No language-specific activation during linguistic
processing of observed actions. Plos ONE 2 (9), e891.
Meister, I.G., Wilson, S.M., Deblieck, C., Wu, A.D., Iacoboni, M., 2007. The essential
role of premotor cortex in speech perception. Curr. Biol. 17 (19), 1692–1696.
di Pellegrino, G., Fadiga, L., Fogassi, L., Gallese, V., Rizzolatti, G., 1992. Understanding
motor events: a neurophysiological study. Exp. Brain Res. 91 (1), 176–180.
Pulvermüller, F., Huss, M., Kherif, F., Moscoso del Prado Martin, F., Hauk, O., Shtyrov,
Y., 2006. Motor cortex maps articulatory features of speech sounds. Proc. Natl.
Acad. Sci. USA 103 (20), 7865–7870.
Rizzolatti, G., Craighero, L., 2004. The mirror-neuron system. Annu. Rev. Neurosci.
Strafella, A.P., Paus, T., 2000. Modulation of cortical excitability during action
observation: a transcranial magnetic stimulation study. Neuroreport 11 (10),
Watkins, K., Paus, T., 2004. Modulation of motor excitability during speech
perception: the role of broca’s area. J. Cogn. Neurosci. 16 (6), 978–987.
Watkins, K.E., Strafella, A.P., Paus, T., 2003. Seeing and hearing speech excites the
motor systeminvolved inspeechproduction.Neuropsychologia 41(8),989–994.
Wilson, S.M., Iacoboni, M., 2006. Neural responses to non-native phonemes varying
in producibility: evidence for the sensorimotor nature of speech perception.
Neuroimage 33 (1), 316–325.
Wilson, S.M., Saygin, A.P., Sereno, M.I., Iacoboni, M., 2004. Listening to speech
activates motor areas involved in speech production. Nat. Neurosci. 7 (7), 701–
Wilson, S.M., Molnar-Szakacs, I., Iacoboni, M., 2008. Beyond superior temporal
cortex: intersubject correlations in narrative speech comprehension. Cereb.
Cortex 18 (1), 230–242.
Wise, S.P., 1985. The primate premotor cortex: past, present, and preparatory. Annu.
Rev. Neurosci. 8, 1–19.
M. Iacoboni/Journal of Physiology - Paris 102 (2008) 31–34