inhibitory processes during response preparation...
TRANSCRIPT
Inhibitory processes during response preparation are present in muscles involved in volitional actions, but not in postural stability.Labruna L., Paluy M., Vanderschelden B., Greenhouse I., Ivry, R.B.
Conclusions
Introduction
General Methods
Background:TMS studies have identi�ed two inhibitory mechanisms that in�uence action selection and response initiation:
Impulse Control (IC)Inhibition of selected action, presumably to prevent premature response initiation.
Competition Resolution (CR)Inhibition of non-selected actions, presumably arising from competition between candidate actions.
FIXATION CUE IMPERATIVE
TMS baseline TMS delay
+ ) )+ ) )
+ ( (+ ( (
900 ms100 ms 300 ms
(
EMG amp before
movement
EMG amp movement
TMS pulseand
MEP amp
MEP
s an
d EM
G a
mpl
itude
s (m
V)
0.6
0.4
0.2
0
-0.4
-0.2
-0.6
0.6
0.4
0.2
0
-0.4
-0.2
-0.60.6
0.4
0.2
0
-0.4
-0.2
-0.60.6
0.4
0.2
0
-0.4
-0.2
-0.6
time
lift -
BIC
EPS
lift -
LFD
Isq
ueez
e so
ft -
LFD
Isq
ueez
e ha
rd -
LFD
I
Will inhibitory processes be observed when a muscle is engaged for postural adjustment, similar to what is seen when a muscle is engaged for volitional response?
We assumed FDI inhibition in Exp. 1 was due to its involvement in postural adjustment. To test this, we compared a condition in which FDI was part of postural adjustmentto conditions in which it was task-irrelevant
Conditions:
- Lift Object: Objects are held in each hand at start of trial and cued hand is used to lift the object. Replication of Exp. 1 Postural condition (tonic recruitment of FDI + grip force change during response).
- Lift No Object: Arms extended (empty handed) at start of trial and cued hand is lifted. No FDI recruitment.
- Lift, Right Object only. Left hand response is lift only (no object). Right hand response is lift object. Only right FDI recruitment.
Pilot Results, 6 participants tested to date.
Postural Prediction: Inhibition should only be observed in the Lift Object condition.
Results:
Trend to replicate Exp. 1 �nding of inhibition in postural muscle.
Large inhibition of FDI even when it is not engaged in the task for both No Object and Right Object conditions. This e�ect is observed even when left hand is not selected.
Fails to support hypothesis of inhibition directed at task-relevant, postural muscle.
Experiment 1: FDI is Postural or Volitional Muscle? Experiment 2: Postural inhibition or spreading inhibition?
0.00
0.50
1.00
1.50
2.00
EMG
am
plitu
de (m
V)
Objects No Objects Right Object
before movement left FDIbefore movement right FDI
movement left FDImovement right FDI
Goal: Are these inhibitory processes evident in muscles associated with posturaladjustments in a task-relevant e�ector?
20
30
40
50
60
70
80
90
100
110
120
MEP
s pe
rcen
t of b
asel
ine
SqueezeSoft
SqueezeHard
Lift
** ** ** ** ** **
selected e�ector
non-selected e�ector
***
* -
20
30
40
50
60
70
80
90
100
110
120
Objects No Objects Right Object
MEP
s pe
rcen
t of b
asel
ine
* ** ** ** **
****
* *
selected e�ector
non-selected e�ector
Experiment 1 results suggest that inhibition can be observed in muscles required for task-relevant, postural response.
However, Experiment 2 results challenge this idea since FDI inhibition is pronounced when it is not activated during the task.
To address in future experimentsIs preparatory inhibition reduced when the targeted muscle is tonically active?
Is there spread of inhibition from proximal to distal muscles? Would biceps show inhibition if response required only index �nger response?
Characterize additional inhibitory processes that operate during response selection and preparation.
Generic inhibition, observed in tasks requiring reactive inhibition, may also be present in proactive, delayed response task ** See Greenhouse poster **
EMG
am
plitu
de (m
V)
0.00
1.00
2.00
3.00
4.00
5.00
before movement left FDIbefore movement right FDI
movement left FDImovement right FDI
SqueezeSoft
SqueezeHard
Lift
Conditions:
- Postural: Objects are held in each hand at start of trial. Response is to lift the object with the cued hand. FDI is tonically activated to hold object and shows increase in EMG during lift due to modulation of grip force.
- Volitional: Objects are held in each hand at start of trial. Response is to squeeze the object with the cued hand. Two levels of squeeze (separate blocks): Hard and Soft. FDI is tonically activated to hold object and shows increase in EMG when squeeze is applied.
16 participants, each completed three conditions.
Results:
EMG increase due to grip force change in Postural is similar to that of soft squeeze in Volitional.
Inhibition of left FDI is similar across conditions when left hand is selected. Inhibition related to Impulse Control is similar in Postural and Volitional conditions.
Inhibition of left FDI is attenuated when right hand is selected in the Postural condition. Inhibition related to Response Competition is attenuated when muscle is used for postural adjustment (non-volitional).
Inhibition of FDI when left hand is selected or non-selected is similar for the Soft and Hard Squeeze conditions.
Choice reaction time task between left and right e�ectors
Delayed response task involving a cue that speci�es forthcoming response, followed 900 ms later by an imperative signal.
We included Catch trials so paricipants couldn't anticipate the response.
Single pulse TMS over right motor cortex to measure motor evoked potentials (MEPs) in left �rst dorsal interosseous muscle (FDI) on the index �nger.TMS applied either at trial onset (baseline) or during delay period.
Selected: cued response is the left e�ector. Non-selected: cued response is the right e�ector.
EMG recorded from left and right FDI and from left and right Biceps (BB).
Distal MuscleFDI
Prossimal MuscleBB