How does TMS work?

Uses inductance to get electrical energy across the scalp

Coil of wire gets changing currents run through it Rapid magnetic field changes >> electric current About 2T

Magnetic field created at scalp with figure-8 coil Strength of magnetic field depends on the # of turns of

the wire and the magnitude of the current First TMS study Barker, Jalinous, & Freeston,

1985

What does TMS do?

Electric current induced in neurons in cortex Adds noise, disrupts coordinated activity Temporary “lesion”

Without the kind of compensation that develops w/ long-term lesions

Apply to different areas of scalp to disrupt function Disruption does NOT mean brain regions directly under coil

responsible for function Only that it’s involved somehow in the function OR connected to regions involved in the function

Get distal effects through connections (“diaschisis”)

Principles of TMShttp://www.biomag.hus.fi/tms/Thesis/dt.html

Repetitive TMS (rTMS)

Rapidly repeated trains of magnetic pulses Because single pulses weren’t found to have much

effect on gross measures of behavior early on Longer lasting effects compared to single pulse rTMS is thought to effect long-term potentiation

between neurons Two repetition rates

Slow= below 1 kHz Fast= above 1 kHz

TMS CoilMaximum magnetic field at center of figure-8

http://www.bu.edu/naeser/aphasia/

Frameless Stereotaxy

http://www.icn.ucl.ac.uk/Experimental-Techniques/Transcranial-magnetic-stimulation/TMS.htm

Therapeutic Uses

OCD Seizures Tinnitus ALS Chronic pain Depression Stroke Phantom limb pain Migraine

Drawbacks of TMS

Possible risk of side effects Seizure, particularly with rTMS

Headache and/or muscle aches caused by activation of neck and shoulder muscles

The equipment is loud, about 100 dB Loud enough to cause hearing loss

http://www.biomag.hus.fi/tms/Thesis/dt.html

A Mediating Role of the Premotor Cortex in

Phoneme Segmentation

Marc Sato, Pascale Tremblay, & Vincent Gracco (2009)

Auditory theory vs. Motor theory of speech perception

Speech perception driven by auditory mechanisms This is based on invariant

properties of the acoustic signal

Not mediated by the motor system

Speech sounds perceived by same mechanism for audition and perceptual learning

The perception of speech is a sensorimotor process Perception of articulatory

gestures Speech gestures are

represented as motor control structures Marianna’s question

Support for the Motor Theory from Imaging StudiesPassive auditory, visual and AV speech perception

Posterior part of left inferior frontal gyrus (Ojanen et al., 2005)

Broca’s area Ventral premotor cortex

Single pulse TMS stimulating left primary premotor cortex (Fadiga et al., 2002)

Lip or tongue MEP’s enhanced during passive speech listening and viewing Increased activity in Broca’s area and ventral premotor cortex

Motor facilitation stronger when the muscle activity and auditory stimuli are for the same articulator (Fadiga et al., 2002; Roy et al.,

2008)

Similar patterns of motor activity in ventral premotor cortex while listening to or producing lip/tongue phonemes

Do speech motor centers contribute to speech perception?

The use of rTMS and electrocortical stimulation can help to answer questions about causality which cannot be answered through passive speech perception experiments Creation of a transient ‘virtual lesion’ (Boatman, 2004)

Possible functional role of Broca’s area and the superior ventral premotor cortex (svPMC) for auditory speech processing has not bee determined

Evidence from rTMS studies

Temporary disruption of the left inferior frontal gyrus doesn’t impair ability on auditory speech discrimination tasks (Boatman, 2004; Boatman & Miglioretti, 2005)

Judgments require WM and subvocal rehearsal Lucy’s Question

rTMS stimulation of left svPMC (active in syllable production and perception) resulted impaired ability to identify auditory syllables (Meister, Wilson, Deblieck, Wu & Iacoboni, 2007)

Interpretation: premotor cortex contributes to top-down modulation of the auditory cortex

Note that this study was done with masking noise in the background

Goal of the Present Study

Extend/refine results of Meister, et al., (2007), presentation of auditory stimuli without background noise 1 kHz rTMS, frameless stereotaxy to disrupt the svPMC

Phoneme identification Solely auditory, no motor system needed

Syllable identification Similar to phoneme identification

Phoneme discrimination Segment initial phonemes to make same/different judgment This task would see the strongest effect of rTMS on accuracy

and reaction time

Participants

10 healthy adults (7 females) Mean age 27 ± 5 years 9 native speakers of French-Canadian, 1

native speaker of French All right handed No history of hearing loss Corrected-to-normal vision

Stimuli

CVC syllables naturally recorded Marianna’s question

Spoken by native French-Canadian Six utterances

/put/ /but/ /pyd/ /byd/ /pon/ /bon/

Procedure

Participants seated 50 cm in front of a computer monitor Acoustic stimuli presented through loudspeakers Two experimental sessions

rTMS session Sham session

Experimental tasks Phoneme identification

Initial syllable /p/ or /b/ Syllable discrimination

Initial phoneme same /put/ /put/, or not /put/ /but/ Phoneme discrimination

Initial phoneme of syllable pairs same /put/ /put/-/but/ /byt/, or not /pon/ /bon/-/pon/ /byd/

Non-verbal matching control Letter shown after fixation cross

Experimental Session All tasks, fixation cross in center of

screen for 250 ms, blank screen for 2500 ms at end

Structural MRI, frameless stereotaxy TMS stimulation applied with a 70

mm air cooled figure 8 coil Resting motor threshold (RMT): minimum

stimulus intensity capable of evoking a motor response

600 pulses applied at 1 kHz with an intensity of 110% of RMT, inhibition lasts up to 10 minutes

Sessions separated by 1 hour

Sham Session

Recorded TMS machine noise was presented through loudspeakers Ear plugs were worn for both sessions

Same tasks as the experimental session rTMS coil positioned over svPMC, however

no TMS stimulation was presented Participants not told which session was the

sham and which one was experimental

Data Analysis

Button press reaction times were examined RT’s slower than 2000 ms considered errors, omitted from

the analysis RT’s calculated

Onset of the second fixation cue in control task Onset of the presented syllable in phoneme identification

task Onset of the second presented syllable in the phoneme

and syllable discrimination task Repeated measures ANOVA performed on the

percentage of correct responses and median RT’s

Results

Main effect of task Lower percent correct for the phoneme

discrimination task Albert’s Question

Faster reaction times in control task compared to phoneme discrimination and other tasks

Main effect of stimulation Slower RT’s after rTMS compared to sham Interaction: slower RT’s after rTMS compared to

sham for the phoneme discrimination task

A= percent correct B= RT

Limitations of rTMS

Inter-participant anatomical differences Length of inhibitory effects of rTMS

About 10 minutes, task was 6 minutes Israel’s question Effect of rTMS on phoneme discrimination

task was not attention or sensory related No effect observed in the other auditory tasks, or

the visual matching task

Results Compared to Previous Investigations

No effect in phoneme identification and syllable discrimination tasks similar to previous work (Demonet, Thierry, & Cardebat,

2005)

Activation in the left, posterior part of the inferior frontal gyrus and vPMC along with auditory regions

For phoneme monitoring and discrimination tasks These areas are active for phoneme recoding and

segmentation, recruited for planning and executing speech gestures (Bohland & Guenther, 2006)

Present study supports this and provides evidence for the participation on the svPMC in the segmentation of the speech stream

Pawel’s Question

Phoneme Discrimination Results

Previous work showed rTMS disrupts left posterior inferior frontal gyrus (Romero, et al., 2006)

Phoneme discrimination ability effected The present study and previous work indicate

the inferior frontal gyrus and the svPMC are important for speech processing when WM demands are high and articulatory rehearsal is needed Also top-down influence on the temporal lobe for

phoneme segmentation needs

Effects of rTMS

rTMS stimulation of the left inferior frontal lobe or PMC does not impair ability to discriminate syllable pairs Phoneme identification and discrimination require

auditory analysis, not influenced by the inhibition of the rTMS stimulated areas

The phoneme discrimination task was effected by the stimulation Suggests that the svPMC plays role in speech

segmentation, especially when WM demands are high

Which theory is supported? Dual-stream model (Hickok & Poeppel, 2001, 2004, 2007)

Dorsal auditory-motor circuit maps sounds on articulatory based representations

Auditory fields in the superior temporal gyrus are involved in early stages of speech perception

Later in life the ventral stream projects to the PMC and inferior frontal gyrus for speech/vocabulary development

Recruitment of motor representation when WM demands are high Results of the phoneme identification and syllable discrimination

tasks do not fit the motor theory

Results support an integrated view of speech perception