No TMS TMS

EMG

RT

(s)

0.15

0.2

0.25Choice RT Session 2

LeftRight

No TMS TMS

EMG

RT

(s)

0.15

0.2

0.25Choice RT Session 1

LeftRight

Greenhouse I, King M, and Ivry RBDepartment of Psychology, University of California, Berkeley

Transcranial magnetic stimulation measures of intrinsic motor system excitability and task-based inhibition exhibit intra-subject stability across weeks

This study was designed to examine individual differences in inhibitory brain mechanisms involved in the selection and initiation of volitional movement.

Measurement reliability is a prerequisite for studies of individual differences.

We tested 26 participants twice (13.7 ± 2.5 days apart) to assess whether the level of task-based inhibition is stable within an individual.

Transcranial magnetic stimulation (TMS) was used to measure motor evoked potentials (MEPs) from hand muscles during the preparation of responses in a choice delayed response task.

Introduction Experimental Design - Repeated Twice

Individual differences in resting motor excitability and inhibition during response preparation are reliable across weeks. The amount of inhibition does not appear to be determined by individual differences in resting excitability.

Individuals with larger MEP amplitudes measured at rest and outside the context of a task also had faster RTs. This effect is most pronounced when MEPs and RTs were measured from same hand.

The speed with which a planned response is executed may depend on the intrinsic excitability of the cortico-spinal pathway. MEP amplitudes measured at rest may index the excitability of this pathway.

Conclusions

1. Duque, J., Lew, D., Mazzocchio, R., Olivier, E., & Ivry, R. B. (2010). Evidence for two concurrent inhibitory mechanisms during response preparation. The Journal of neuroscience, 30(10), 3793–38022. Labruna, L., Lebon, F., Duque, J., Klein, P.-A., Cazares, C., & Ivry, R. B. (2014). Generic inhibition of the selected movement and constrained inhibition of nonselected movements during response preparation. Journal of Cognitive Neuroscience, 26(2), 269–278.

TMS Signatures of Motor Inhibition are Reliable

Choice RT & Resting MEP Amplitudes are Reliable

Resting MEP Amplitudes Predict RT

• RT (for the two hands combined) was highly reliable across sessions for trials both with and without TMS, although the impact of TMS on RT was highly variable across participants.

• Inhibition was observed whether the left hand was selected or non-selected.• Inhibition was greater in the selected hand. • Inhibition was reliable in the left hand when it was selected for the forthcoming response and

marginally reliable when the left hand was non-selected (i.e. right hand selected).

Reliable individual differences suggest the involvement of a stable underlying inhibitory brain mechanism.

In addition to task-based TMS measures, we also assessed:

1) Resting motor threshold (rMT) - the TMS intensity required to elicit MEPs on half of attempts.

2) MEP amplitudes at rest (115% rMT TMS intensity) - to assess individual differences in resting corticomotor excitability.

3) Response time (RT) in the presence and absence of TMS.

3. Delayed Choice Response Task (108 trials)

2. Pre-Task Baseline 20 MEPs at rest

During the task, MEPs were measured either at baseline or 800 ms into the preparatory delay period.

+

Fixation Cue Imperative

200 ms 900 ms 500 ms

+ ( o(

++ o ) )

Task Baseline MEPPre- and Post-Task MEP

vs. Task Baseline

Session 1 RT (s) 0.1 0.2 0.3

Sess

ion

2 R

T (s

)

0.1

0.2

0.3

R = 0.71 P = 0.00011

Choice RT (no TMS)

Session 1 RT (s) 0.1 0.2 0.3

Sess

ion

2 R

T (s

)

0.1

0.2

0.3

R = 0.72 P = 8e-05

Choice RT (TMS)

• RT was very similar across the two sessions. • TMS during the delay period hastened RT.

Reaction Time

Resting Motor Threshold

Session 1 RMT (% MSO) 30 40 50 60 70

Sess

ion

2 RM

T (%

MSO

)

30

40

50

60

70

R = 0.92 P = 1.5e-10

Session 1 Session 2

Left

FDI M

EP a

mpl

itude

(% b

asel

ine)

-75

-50

-25

0

25MEP Amplitudes Across Visits

selectednon-selectedBaseline

Session 1 MEP (% baseline) -120 -70 -20

Sess

ion

2 M

EP (%

bas

elin

e)

-120

-70

-20

R = 0.49 P = 0.016

Selected

Session 1 MEP (% baseline) -120 -70 -20

Sess

ion

2 M

EP (%

bas

elin

e)

-120

-70

-20

R = 0.36 P = 0.082

Non-selected

Session 1 task baseline MEP (mV) -1 0 1 2 3 4

Sess

ion

2 ta

sk b

asel

ine

MEP

(mV)

-1

0

1

2

3

4

R = 0.64 P = 0.0011

• rMT and task baseline MEP amplitudes were very reliable across visits. • Task baseline MEP amplitudes were highly correlated with resting measures of excitability.

Pre & post MEP (mV) -1 0 1 2 3 4

Task

Bas

elin

e M

EP (m

V)

-1

0

1

2

3

4

R = 0.9 P = 3.4e-09

Non-selected MEP (mV) -1 0 1 2 3 4

Task

Bas

elin

e M

EP (m

V)

-1

0

1

2

3

4

R = 0.61 P = 0.002

Selected MEP (mV) -1 0 1 2 3 4

Task

Bas

elin

e M

EP (m

V)

-1

0

1

2

3

4

R = 0.64 P = 0.0011

• Individual differences in inhibition are not determined by resting baseline MEP amplitudes.

• Combined pre- and post-task MEP amplitudes predict individual differences in Choice RT. This effect was especially pronounced for the left hand, from which MEPs were measured.

Resting MEP (mV) -0.5 0.5 1.5 2.5

RT

(s)

0.1

0.2

0.3

R = -0.41 P = 0.05

Both Hands

Resting MEP (mV) -0.5 0.5 1.5 2.5

RT

(s)

0.1

0.2

0.3

R = -0.47 P = 0.023

Left Hand

1. rMT determined prior to testing

4. Post-Task Baseline 20 MEPs at rest

MEP

artifact

)

0 200 400 600 800 1000 1200

×10-3

-3

-2

-1

0

1

2

3

EMG Burts