Our studies of speech motor control
focus on the role of the somatosensory system in both the
production and perception of speech and on the idea that
speech motor learning is associated with changes to
auditory perceptual function and sensory networks in the
brain.
Speech perception is known to be a
multimodal process, relying not only on auditory input
but also on the visual system and possibly on the motor
system as well. To date, there has been little work on
the potential involvement of the somatosensory system in
speech perception. In the present review, we identify
the somatosensory system as another contributor to
speech perception. First, we argue that evidence in
favor of a motor contribution to speech perception can
just as easily be interpreted as showing somatosensory
involvement. Second, physiological and neuroanatomical
evidence for auditory-somatosensory interactions across
the auditory hierarchy indicates the availability of a
neural infrastructure that supports somatosensory
involvement in auditory processing in general. Third,
there is accumulating evidence for somatosensory
involvement in the context of speech specifically. In
particular, tactile stimulation modifies speech
perception, and speech auditory input elicits activity
in somatosensory cortical areas. Moreover, speech sounds
can be decoded from activity in somatosensory cortex;
lesions to this region affect perception, and vowels can
be identified based on somatic input alone. We suggest
that the somatosensory involvement in speech perception
derives from the somatosensory-auditory pairing that
occurs during speech production and learning. By
bringing together findings from a set of studies that
have not been previously linked, the present article
identifies the somatosensory system as a presently
unrecognized contributor to speech perception.
Ohashi H, Ostry DJ (2021). Neural development of speech
sensorimotor learning. J Neurosci 41:4023-4035.
Abstract
PDF
The development of the human brain continues
through to early adulthood. It has been suggested that
cortical plasticity during this protracted period of
development shapes circuits in associative transmodal
regions of the brain. Here we considered how cortical
plasticity during development might contribute to the
coordinated brain activity required for speech motor
learning. Specifically, we examined patterns of brain
functional connectivity (FC), whose strength covaried
with the capacity for speech audio-motor adaptation in
children ages 5–12 and in young adults of both sexes.
Children and adults showed distinct patterns of the
encoding of learning in the brain. Adult performance
was associated with connectivity in transmodal regions
that integrate auditory and somatosensory information,
whereas children rely on basic somatosensory and motor
circuits. A progressive reliance on transmodal regions
is consistent with human cortical development and
suggests that human speech motor adaptation abilities
are built on cortical remodeling, which is observable
in late childhood and is stabilized in adults.
Ito T, Bai J, Ostry DJ (2020). Contribution of sensory
memory to speech motor learning. J Neurophysiol
124:1103-1109.
Abstract PDF
Speech learning requires precise motor
control, but it likewise requires transient storage of
information to enable the adjustment of upcoming
movements based on the success or failure of previous
attempts. The contribution of somatic sensory memory
for limb position has been documented in work on arm
movement; however, in speech, the sensory support for
speech production comes from both somatosensory and
auditory inputs, and accordingly sensory memory for
either or both of sounds and somatic inputs might
contribute to learning. In the present study,
adaptation to altered auditory feedback was used as an
experimental model of speech motor learning.
Participants also underwent tests of both auditory and
somatic sensory memory. We found that although
auditory memory for speech sounds is better than
somatic memory for speechlike facial skin
deformations, somatic sensory memory predicts
adaptation, whereas auditory sensory memory does not.
Thus even though speech relies substantially on
auditory inputs and in the present manipulation
adaptation requires the minimization of auditory
error, it is somatic inputs that provide the memory
support for learning.
Darainy M, Vahdat S, Ostry DJ (2019). Neural basis of
sensorimotor learning in speech motor adaptation. Cereb
Cortex 29:2876-2889.
Abstract PDF
When we speak, we get correlated sensory
feedback from speech sounds and from the muscles and
soft tissues of the vocal tract. Here we dissociate
the contributions of auditory and somatosensory
feedback to identify brain networks that underlie the
somatic contribution to speech motor learning. The
technique uses a robotic device that selectively
alters somatosensory inputs in combination with
resting-state fMRI scans that reveal learning-related
changes in functional connectivity. A partial
correlation analysis is used to identify connectivity
changes that are not explained by the time course of
activity in any other learning-related areas. This
analysis revealed changes related to behavioral
improvements in movement and separately, to changes in
auditory perception: Speech motor adaptation itself
was associated with connectivity changes that were
primarily in non-motor areas of brain, specifically,
to a strengthening of connectivity between auditory
and somatosensory cortex and between presupplementary
motor area and the inferior parietal lobule. In
contrast, connectivity changes associated with
alterations to auditory perception were restricted to
speech motor areas, specifically, primary motor cortex
and inferior frontal gyrus. Overall, our findings show
that during adaptation, somatosensory inputs result in
a broad range of changes in connectivity in areas
associated with speech motor control and learning.
Ito T, Coppola JH, Ostry DJ (2016). Speech motor
learning changes the neural response to both auditory
and somatosensory signals. Nature Sci Reports 6:25926.
Abstract PDF
In the present paper, we present evidence for
the idea that speech motor learning is accompanied by
changes to the neural coding of both auditory and
somatosensory stimuli. Participants in our experiments
undergo adaptation to altered auditory feedback, an
experimental model of speech motor learning which like
visuo-motor adaptation in limb movement, requires that
participants change their speech movements and
associated somatosensory inputs to correct for
systematic real-time changes to auditory feedback. We
measure the sensory effects of adaptation by examining
changes to auditory and somatosensory event-related
responses. We find that adaptation results in
progressive changes to speech acoustical outputs that
serve to correct for the perturbation. We also observe
changes in both auditory and somatosensory
event-related responses that are correlated with the
magnitude of adaptation. These results indicate that
sensory change occurs in conjunction with the
processes involved in speech motor adaptation.
Lametti DR, Nasir S, Ostry DJ (2012). Sensory preference
in speech production revealed by simultaneous alteration
of auditory and somatosensory feedback. J Neurosci
32:9351-9359. Abstract
PDF
The idea that humans learn and maintain
accurate speech by carefully monitoring auditory
feedback is widely held. But this view neglects the
fact that auditory feedback is highly correlated with
somatosensory feedback during speech production.
Somatosensory feedback from speech movements could be
a primary means by which cortical speech areas monitor
the accuracy of produced speech. We tested this idea
by placing the somatosensory and auditory systems in
competition during speech motor learning. To do this,
we combined two speech-learning paradigms to
simultaneously alter somatosensory and auditory
feedback in real time as subjects spoke. Somatosensory
feedback was manipulated by using a robotic device
that altered the motion path of the jaw. Auditory
feedback was manipulated by changing the frequency of
the first formant of the vowel sound and playing back
the modified utterance to the subject through
headphones. The amount of compensation for each
perturbation was used as a measure of sensory
reliance. All subjects were observed to correct for at
least one of the perturbations, but auditory feedback
was not dominant. Indeed, some subjects showed a
stable preference for either somatosensory or auditory
feedback during speech.
Nasir SM, Ostry DJ (2009). Auditory plasticity and
speech motor learning. Proc Natl Acad Sci U S A
106:20470–20475.
Abstract PDF
Is plasticity in sensory and motor systems
linked? Here, in the context of speech motor learning
and perception, we test the idea sensory function is
modified by motor learning and, in particular, that
speech motor learning affects a speaker’s auditory
map. We assessed speech motor learning by using a
robotic device that displaced the jaw and selectively
altered somatosensory feedback during speech. We found
that with practice speakers progressively corrected
for the mechanical perturbation and after motor
learning they also showed systematic changes in their
perceptual classification of speech sounds. The
perceptual shift was tied to motor learning.
Individuals that displayed greater amounts of learning
also showed greater perceptual change. Perceptual
change was not observed in control subjects that
produced the same movements, but in the absence of a
force field, nor in subjects that experienced the
force field but failed to adapt to the mechanical
load. The perceptual effects observed here indicate
the involvement of the somatosensory system in the
neural processing of speech sounds and suggest that
speech motor learning results in changes to auditory
perceptual function.
Ito T, Tiede M, Ostry DJ (2009). Somatosensory function
in speech perception. Proc Natl Acad Sci U S A
106:1245–1248.
Abstract PDF
Somatosensory signals from the facial skin and
muscles of the vocal tract provide a rich source of
sensory input in speech production. We show here that
the somatosensory system is also involved in the
perception of speech. We use a robotic device to
create patterns of facial skin deformation that would
normally accompany speech production. We find that
when we stretch the facial skin while people listen to
words, it alters the sounds they hear. The systematic
perceptual variation we observe in conjunction with
speech-like patterns of skin stretch indicates that
somatosensory inputs affect the neural processing of
speech sounds and shows the involvement of the
somatosensory system in the perceptual processing in
speech.
Nasir SM, Ostry DJ (2008). Speech motor learning in
profoundly deaf adults. Nat Neurosci 11:1217–1222.
Abstract PDF
Speech production, like other sensorimotor
behaviors, relies on multiple sensory inputs—audition,
proprioceptive inputs from muscle spindles, and
cutaneous inputs from mechanoreceptors in the skin and
soft tissues of the vocal tract. However, the capacity
for intelligible speech by deaf speakers suggests that
somatosensory input alone may contribute to speech
motor control and perhaps even to speech learning. We
assessed speech motor learning in cochlear implant
recipients who were tested with their implants turned
off. A robotic device was used to alter somatosensory
feedback by displacing the jaw during speech. We found
that implant subjects progressively adapted to the
mechanical perturbation with training. Moreover, the
corrections that we observed were for movement
deviations that were exceedingly small, on the order
of millimeters, indicating that speakers have precise
somatosensory expectations. Speech motor learning is
substantially dependent on somatosensory input.
Tremblay S, Shiller DM, Ostry DJ (2003). Somatosensory
basis of speech production. Nature 423:866-869.
Abstract PDF Commentary
The hypothesis that speech goals are defined
acoustically and maintained by auditory feedback is a
central idea in speech production research. An
alternative proposal is that speech production is
organized in terms of control signals that subserve
movements and associated vocal-tract configurations.
Indeed, the capacity for intelligible speech by deaf
speakers suggests that somatosensory inputs related to
movement play a role in speech production—but studies
that might have documented a somatosensory component
have been equivocal. For example, mechanical
perturbations that have altered somatosensory feedback
have simultaneously altered acoustics. Hence, any
adaptation observed under these conditions may have
been a consequence of acoustic change. Here we show
that somatosensory information on its own is
fundamental to the achievement of speech movements.
This demonstration involves a dissociation of
somatosensory and auditory feedback during speech
production. Over time, subjects correct for the
effects of a complex mechanical load that alters jaw
movements (and hence somatosensory feedback), but
which has no measurable or perceptible effect on
acoustic output. The findings indicate that the
positions of speech articulators and associated
somatosensory inputs constitute a goal of speech
movements that is wholly separate from the sounds
produced.
