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 TMS

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 Coil Maximum magnetic field at center of figure-8 Frameless Stereotaxy 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

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 Studies Passive 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