B R I E F C O M M U N I C AT I O N S

Listening to speech activatesmotor areas involved in speech
production© 2004 Nature Publishing Group http://www.nature.com/natureneuroscience

Stephen M Wilson1,2, Ayşe Pınar Saygın4, Martin I Sereno4 &Marco Iacoboni1,3
To examine the role of motor areas in speech perception, wecarried out a functional magnetic resonance imaging (fMRI)
study in which subjects listened passively to monosyllables andproduced the same speech sounds. Listening to speech
activated bilaterally a superior portion of ventral premotor
cortex that largely overlapped a speech production motor area
centered just posteriorly on the border of Brodmann areas 4aand 6, which we distinguished from a more ventral speech
production area centered in area 4p. Our findings support theview that the motor system is recruited in mapping acoustic
inputs to a phonetic code.
Language depends on the maintenance of parity between acoustic andarticulatory representations: there must be a common phonetic code1.
Phonemes are organized in terms of distinct features that are primarily
defined by articulatory properties; for instance, /p/ is a voiceless bilabialstop consonant. If the common phonetic code has motor properties, thenthe motor system could be involved in perceiving speech, as a transformation must be made from the acoustic signal to a phonetic representation. Recent work on mirror neurons has revitalized interest in the ideathat motor areas are involved in perceptual processes2. Most neuroimaging studies of speech perception, however, have focused on characterizing
the strong responses consistently observed in the superior temporal
lobe3,4. Frontal areas have sometimes been reported to be activated bypassive listening to speech3,5 and are often responsive during audiovisual
speech perception6, but the potential motor properties of the areas foundin these studies have not been investigated. Although Broca’s area is frequently implicated in studies involving phonological tasks or syntactic
comprehension, it is presumed to be important for higher levels of linguistic processing7. Two studies using transcranial magnetic stimulation(TMS) have reported facilitation of tongue8 and lip9 muscles when subjects listened to speech; however, the precise brain areas involved are notknown owing to limitations of spatial localization with TMS.
We used fMRI to examine whether passive listening to meaninglessmonosyllables activates motor areas involved in producing speech. Ten
subjects listened to 16-s blocks, each containing 23 repetitions of ameaningless monosyllable. During the same scanning sessions, subjects were cued to produce the same syllables. The motor tasks lasted
only 3 s; to avoid movement artifacts, we discarded volumes acquired
during actual motor activity and analyzed subsequent volumes inwhich the delayed hemodynamic response occurred. Eight of the ten
subjects also listened to blocks of control nonspeech stimuli: a burst ofwhite noise or a bell, and carried out a bimanual motor task. Details
are provided in Supplementary Methods online.
In all ten subjects, regions in the precentral gyrus, extending into theanterior bank of the central sulcus, were significantly activated by listening to speech, as compared to rest (Fig. 1). These activations wereFigure 1 Areas activated by passive listening to meaningless monosyllablesin three representative subjects. Individual activation maps werethresholded at P < 10–4 (uncorrected) for listening conditions or P < 10–12
for motor conditions, with a minimum cluster size of 300 mm3. Mean
Montreal Neurological Institute (MNI) coordinates for centers of mass ofareas activated by listening to speech were (–50, –6, 47; left) and (55, –3,45; right). The regions outlined in black show premotor and primary motor
cortical activity while producing the same syllables. Other areas were also
activated in the motor conditions, but are not shown. Arrowheads show thelocation of the central sulcus. Motor areas activated by both speech
perception and production were observed in every subject. Extensive
activations in the superior temporal gyrus and surrounding areas were also
observed in all subjects. All participants gave informed consent, and thestudy was approved by the UCLA and UCSD institutional review boards.

located primarily in the superior part of ventral premotor cortex(PMv), extending toward primary motor cortex (see below).
Activations were bilateral in four subjects, left-lateralized in two andright-lateralized in four. At lower thresholds or with reduced minimum
cluster sizes, however, responses were bilateral for all subjects. Other
activated areas are reported in Supplementary Table 1 online.
Motor areas activated by production of the same syllables are shownin Figure 1 (outlined in black; see also Supplementary Table 1). Motor
responses were bilateral in all subjects. Comparison of the regions activated by listening to and producing the syllables showed substantial
overlap for all subjects. Across subjects, 73 ± 7% of voxels in precentral
gyrus and central sulcus regions activated by listening to speech werealso activated by speech production. We defined regions of interest foreach subject as clusters of voxels in the precentral gyrus or central sulcus that were responsive to either listening to speech or listening to

1Ahmanson-Lovelace Brain Mapping Center, 2Neuroscience Interdepartmental Program, 3Department of Psychiatry and Biobehavioral Sciences, University ofCalifornia, Los Angeles, California 90095, USA. 4Department of Cognitive Science, University of California, San Diego, La Jolla, California 92093, USA.
Correspondence should be addressed to S.M.W. (stephenw@ucla.edu).

Published online 6 June 2004; doi:10.1038/nn1263

NATURE NEUROSCIENCE VOLUME 7 | NUMBER 7 | JULY 2004701© 2004 Nature Publishing Group http://www.nature.com/natureneuroscience

B R I E F C O M M U N I C AT I O N S
Figure 2 Characterization of the relationships between listening and motor
areas. (a) Hemodynamic responses in motor regions that were responsive tolistening to either speech or nonspeech (P < 10–4). The bars show the time
periods during which auditory stimuli were presented (16 s) and during which
subjects were producing speech (3 s). Error bars here and in other panels
represent s.e.m. (b) Motor areas for speech production in areas 4p and 4a/6 ina representative subject. In the coronal view, two distinct peaks can be seen in
the right hemisphere, but in the left hemisphere, area 4p cannot be seen as it
is posterior to the plane. Across subjects, the mean MNI coordinates of the 4ppeaks were (–45, –13, 34; left) and (48, –10, 35; right), and of the 4a/6peaks (–51, –11, 46; left) and (56, –8, 44; right). CS, central sulcus; SMA,supplementary motor area. (c) The locations of listening and production peaks
overlaid on probabilistic cytoarchitectonic maps of areas 4a, 4p and 6 in theleft hemisphere. The peaks lay close to the plane shown (x = –49). The map
for the right hemisphere was similar. sPMv, superior PMv; iPMv, inferior PMv;
PrCS, precentral sulcus; SylF, Sylvian fissure. (d) Maximum percent signal
change by condition for three of the peaks in panel c.

nonspeech sounds, and then plotted the mean time course (Fig. 2a).
A robust response to speech production confirmed the motor properties of these listening-responsive voxels, and activity was much greaterfor speech than for nonspeech sounds.
Closer examination of speech production–related activity revealed
several distinct peaks within each cluster. In 16 of 20 hemispheres, weobserved a ventral peak with 30 ≤ z ≤ 39 and a dorsal peak with 40 ≤ z≤ 50 (except in two hemispheres where this peak was a few millimeters
more dorsal). The ventral peaks were located deep in the central sulcus, whereas the dorsal peaks lay more laterally on the anterior lip of
the sulcus (Fig. 2b); activations always spanned the central sulcus, but
distinct sensory peaks were never observed. Based on probabilistic
cytoarchitectonic maps10,11, the ventral peaks were located in
Brodmann area 4p, whereas the dorsal peaks lay on the border of areas
4a and 6 (Fig. 2c). Previous imaging studies of speech production havenot distinguished two areas and generally report group-averaged peak
coordinates that lie between the two peaks we observed12.
In relation to these peaks for speech production, the mean center ofmass for listening to speech was located 4.5 ± 0.7 mm anterior to thearea 4a/6 production peak (P = 0.0005) but not significantly medial, lateral, superior or inferior to it (all Ps > 0.05). This slightly anterior location means that it falls most likely in area 6, in the superior portion ofPMv2 (Fig. 2c). Note that an inferior region of PMv is also involved inspeech production12 (see location in Fig. 2c), but this region did notrespond to listening to speech. We next examined listening responses inpeak production voxels (Fig. 2d). Peak area 4a/6 voxels responded to listening to speech but not nonspeech sounds, suggesting that the area
activated by listening to speech may extend into the most anterior partof primary motor cortex. In contrast, peak area 4p voxels did not
respond in either listening condition. This functional distinction may beanalogous to the recently reported greater involvement of area 4a than702

4p in motor imagery13. The bimanual task allowed us to confirm thatthese motor areas are specific to the mouth. Finger/hand motor areas
responsive to the bimanual task were located medial, posterior andsuperior to the regions activated for listening to and producing speech(Supplementary Table 1).
These findings are consistent with the view that speech perception
involves the motor system in a process of auditory-to-articulatory
mapping to access a phonetic code with motor properties1. Whether or
not the superior PMv region we identified is necessary for normal
speech perception is not known. Frontal lesions can severely compromise speech perception14, but which lesions in particular lead to perception deficits is not clear. In addition to responding robustly tospeech, the superior PMv region also showed diminished responses tononspeech sounds. Premotor cortex is involved in coding attended
environmental features, including auditory stimuli, in a body-based
but highly abstract form15, and the level of activity in this region may
reflect the extent to which an articulatory representation can beobtained for any acoustic input.
Note: Supplementary information is available on the Nature Neuroscience website.
ACKNOWLEDGMENTS
We thank K. Amunts for providing cytoarchitectonic maps, the UCSD Center for
Functional MRI, and the Ahmanson-Lovelace Brain Mapping Center. Supportedby NSF grants REC 0107077 and BCS 0224321.
COMPETING INTERESTS STATEMENT
The authors declare that they have no competing financial interests.
Received 29 March; accepted 5 May 2004
Published online at http://www.nature.com/natureneuroscience/
1. Liberman, A.M., Cooper, F.S. Shankweiler, D.P. & Studdert-Kennedy, M. Psychol.
Rev. 74, 431–461 (1967).
2. Rizzolatti, G., Fogassi, L. & Gallese, V. Curr. Opin. Neurobiol. 12, 149–154(2002).
3. Binder, J.R. et al. Cereb. Cortex 10, 512–528 (2000).
4. Scott, S.K. & Wise, R.J.S. Cognition 92, 13–45 (2004).
5. Benson, R.R. et al. Brain Lang. 78, 364–396 (2001).
6. Callan, D.E. et al. Neuroreport 14, 2213–2218 (2003).
7. Bookheimer, S. Annu. Rev. Neurosci. 25, 151–188 (2002).
8. Fadiga, L., Craighero, L., Buccino, G. & Rizzolatti, G. Eur. J. Neurosci. 15,399–402 (2002).
9. Watkins, K.E., Strafella, A.P. & Paus, T. Neuropsychologia 41, 989–994 (2003).
10. Geyer, S. et al. Nature 382, 805–807 (1996).
11. Geyer, S. The Microstructural Border Between the Motor and the Cognitive Domain
in the Human Cerebral Cortex (Springer, Berlin, 2004).
12. Fox, P.T. et al. NeuroImage 13, 196–209 (2001).
13. Ehrsson, H.H., Geyer, S. & Naito, E. J. Neurophysiol. 90, 3304–3316 (2003).
14. Blumstein, S.E., Baker, E. & Goodglass, H. Neuropsychologia 15, 19–30(1977).
15. Schubotz, R.I. & von Cramon, D.Y. NeuroImage 20, S120–S131 (2003).

VOLUME 7 | NUMBER 7 | JULY 2004 NATURE NEUROSCIENCE