Involvement of motor cortex
in speech perception
Riikka Möttönen
Department of Experimental Psychology
University of Oxford
Is speech perception embodied?
• Yes
– Motor theory of speech perception (Liberman et al., 1967)
– Speech perception and production mechanisms are tightly
linked
• No
– Auditory theories
– Speech sounds are processed as any other sounds
– Humans and other animal process speech sounds using similar
auditory mechanisms
Transcranial Magnetic Stimulation (TMS)
A tool to modulate normal
functioning of brain areas in
a controlled manner
Based on electromagnetic
induction (Faraday’s law)
Stimulating the hand and lip representations in
the primary motor (M1) cortex
Motor Evoked Potentials
= MEPs
TMSpulse
MEP
50 ms
MEP
First dorsal interrossius
1 mV
HAND
LIP
Orbicularis oris
Hearing and seeing speech enhances
excitability of the lip area in the left M1 cortex
Lip area in Left M1
Lip area in Right M1
Hand area in Left M1
Watkins et al. (2003)
Does the motor cortex contribute to speech
perception?
Speech sounds are highly variable
Kuhl, 2004
Categorical perception of speech sounds
– Identification
– Discrimination
Experimental Design
Tasks:
– Identification (Id)
– Discrimination (Di)
Low-frequency repetitive TMS (rTMS):
– 0.6 Hz for 15 min
– Monophasic pulses
– Over lip or hand area of left M1
– Sub-threshold intensity (active
motor threshold)
Möttönen and Watkins, 2009, J Neurosci
TMS-induced disruptions in the motor cortex
before
7 min
after
15 min
after
• Low-frequency rTMS
– Suppressed excitability of the
targeted area in M1
Möttönen and Watkins, 2009
Speech sounds
b/p
– Experiment 1:
• 8-step continuum from ‘ba’ to ‘da’
• 8-step continuum from ‘ka’ to ‘ga’
d/t
g/k
Categorical perception of speech sounds
•
Identification
Category boundary
•
Discrimination
Across-category pairs
Möttönen and Watkins, 2009, J Neurosci
Influences of TMS-induced motor disruptions on
categorical perception
•Identification:
• Disruption of the lip representation
reduced the slope of the ‘ba’-’da’
boundary (p < 0.05)
• no effects on positions of
category boundaries
•Discrimination:
• Disruption of the lip representation
reduced the proportion of “different”
responses to ‘ba’-’da’ acrosscategory pairs (p < 0.001)
• no effects on discrimination of
within-category pairs
Möttönen and Watkins, 2009, J Neurosci
Speech sounds
– Experiment 1:
• 8-step continuum from ‘ba’ to ‘da’
• 8-step continuum from ‘ka’ to ‘ga’
– Experiment 2:
• 8-step continuum from ‘pa’ to ‘ta’
• 8-step continuum from ‘da’ to ‘ga’
b/p
d/t
g/k
Influences of TMS-induced motor disruptions on
categorical perception
•Identification:
• Disruption of the lip representation
reduced the slope of the ‘ba’-’da’
boundary (p < 0.05)
• no effects on positions of
category boundaries
•Discrimination:
• Disruption of the lip representation
reduced the proportion of “different”
responses to ‘pa’- ‘ta’ acrosscategory pairs (p < 0.001)
• no effects on discrimination of
within-category pairs
Möttönen and Watkins, 2009, J Neurosci
Summary and Conclusions
• Representations of articulators in the left M1 cortex contribute to
categorical perception of speech sounds
• The motor system generates internal motor models of speaker’s
articulatory movements during speech perception
Möttönen and Watkins, 2009, J Neurosci
Open questions
• Does motor cortex contribute to
– early stages of speech processing in the auditory cortex?
– later “task-dependent” stages of speech processing?
• Are these motor influences
– automatic?
– dependent on attention?
Does the motor cortex contribute to early
auditory processing of speech sounds ?
- combined TMS and EEG
Mismatch Negativity (MMN)
–
–
–
–
–
can be recorded using EEG and MEG
is elicited by infrequent changes in the sound sequence
peaks 100-200 ms after the change
generated in the auditory cortex
automatic
Do TMS-induced disruptions in the left M1 cortex
suppress MMN responses to changes in speech
sounds?
Sound sequence
• Oddball:
Da Da Da Ga Da Da Da Da Da Ba Da Da Da...
80%
10%
10%
Möttönen, Dutton and Watkins (in prep)
Experimental Design
Möttönen, Dutton and Watkins (in prep)
Experiment 1:
TMS-induced disruption of the lip representation
suppressed MMN responses to ‘ba’ and ‘ga’
POST < PRE: 166-188 ms (p< 0.05)
MMN =
Response to deviant sounds
minus
Response to standard sounds
POST < PRE: 170-210 ms (p< 0.05)
Responses were bandpass filtered 1-30 Hz and
referenced to the left and right mastoids.
Möttönen, Dutton and Watkins (in prep)
Are these effects specific to the stimulation
site?
Does TMS over the hand area suppress MMN
responses?
Experiment 2:
TMS-induced disruption of the hand representation did
not suppress MMN responses to ‘ba’ and ‘ga’
Responses were bandpass filtered 1-30 Hz and
referenced to the left and right mastoids.
Möttönen, Dutton and Watkins (in prep)
Summary
• “Speech motor cortex” modulates early automatic
discrimination of speech sounds
– Disruption of the lip, but not hand, representation
suppressed MMNs
– Not specific to articulatory features of speech sounds
• MMNs to both lip- and tongue-articulated sounds were
suppressed
Does TMS over the lip representation suppress processing of
any kind of acoustic changes in speech sounds?
Möttönen, Dutton and Watkins (in prep)
Are these effects specific to phonetic changes
in speech sounds?
Does disruption of lip area suppress processing
of any kind of acoustic changes in speech
sounds?
Experiment 3
• Oddball:
Da Da Da Da Da Da Da Da Da Da Da Da Da...
80%
10%
10%
Duration change
+ 70 ms
Intensity change
- 6dB
Möttönen, Dutton and Watkins (in prep)
Experiment 3:
Effects of TMS-induced disruption of the lip representation on
MMN responses to acoustic changes in speech sounds
POST < PRE:
148-170 ms & 186-202 ms (p< 0.05)
Responses were bandpass filtered 1-30 Hz and
referenced to the left and right mastoids.
Möttönen, Dutton and Watkins (in prep)
Is this effect specific to speech?
Does disruption of the lip area suppress MMN
responses to changes in tones?
Experiment 4
• Oddball:
♪ ♪ ♪
80%
♪
♪
♪
♪ ♪ ♪ ♪
10%
10%
Duration change
+ 70 ms
Intensity change
- 6dB
Möttönen, Dutton and Watkins (in prep)
Experiment 4:
TMS-induced disruption of the lip representation did not suppress
MMN responses to changes in tones
Responses were bandpass filtered 1-30 Hz and
referenced to the left and right mastoids.
Möttönen, Dutton and Watkins (in prep)
P < 0.05
P < 0.01
P < 0.05
The MMN mean amplitudes were calculated as the mean voltage across a 40-ms window
centred at the peak latency in the grand-average response at FCZ.
Conclusions
• Motor representations of articulators
– contribute to discrimination of speech sounds
• The auditory system interacts with the motor system at early
stages of speech processing
– Interaction supports discrimination of speech sounds even when
they are outside the focus of attention
– Attention may however modulate this interaction
• Combination of TMS and EEG is a powerful tool to investigate
auditory-motor speech processing with high temporal resolution
– Combination of TMS and MEG
Thank you
–
–
–
–
–
–
–
Kate Watkins
Rebekah Dutton
Julia Erb
Rowan Boyles
Jennifer Chesters
Mervyn Hardiman
Dorothy Bishop