How does premotor cortex prepare the hand for different grips?
A pulse to ventral premotor cortex just before a pulse to motor cortex selectively boosted the hand muscle that the person was about to use, depending on whether they were preparing a pinch or a whole-hand grasp.
Source
Ventral premotor to primary motor cortical interactions during object-driven grasp in humans
Study at a glance
- Design
- Human experiment — Within-subject dual-coil conditioning-test TMS (ventral premotor to M1, M1 to M1 control, and M1 paired-pulse) delivered 800 ms after a pen or disc appeared, while people prepared to grasp it.
- N
- N=11 · 11 right-handed volunteers aged 20 to 30, all tested in every condition.
- Population
- Healthy right-handed young adults in London
- Outcome
- Motor evoked potential amplitude in the first dorsal interosseous and abductor digiti minimi hand muscles, relative to test pulse alone
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What they did
Eleven healthy right-handers saw either a pen (to be picked up with a thumb-index precision grip) or a disc (to be grasped with the whole hand) and, 800 ms later, a TMS pulse over primary motor cortex (M1) cued them to reach. On many trials a weaker conditioning pulse was first given over ventral premotor cortex (PMv) at intervals from 1 to 15 ms, and muscle responses (motor evoked potentials) were recorded from an index-finger muscle and a little-finger muscle. Two control conditions stimulated M1 twice, to check for current spread and to probe other inputs to M1.
What they found
Conditioning PMv 6 or 8 ms before the M1 pulse strongly facilitated the index muscle only when a precision grip was being prepared and the little-finger muscle only when a whole-hand grasp was being prepared. Across people, the size of this grasp-specific boost tracked how differently the muscles were later used in the two grasps (r of about .78 to .79). Conditioning over M1 itself produced the usual inhibition and facilitation regardless of grasp, arguing against current spread, and paired pulses over M1 gave a smaller grasp-specific facilitation than the PMv route.
The limits
What it doesn't show
The sample was only 11 young adults and the paper has no dedicated limitations section. Only two objects, two muscles and a single preparation time (800 ms) were tested, so the timing and generality of the effect are unknown. TMS reveals a net influence of PMv on M1 but cannot tell whether it is direct or relayed, and interpretations about grasp-specific neuron populations rest on monkey data rather than measurements in these participants.
Key terms
- Ventral premotor cortex (PMv)
- A frontal region in front of motor cortex that contains neurons tuned to particular grasp types.
- Primary motor cortex (M1)
- The strip of cortex whose output neurons send commands down the spinal cord to muscles.
- Motor evoked potential (MEP)
- A muscle twitch recorded by EMG after a TMS pulse to motor cortex; its size indexes corticospinal excitability.
- Conditioning-test TMS
- A dual-coil method in which a first pulse to one area shortly precedes a second pulse to M1, revealing how the first area influences M1 output.
- Precision grip versus whole-hand grasp
- Holding a small object between thumb and index finger versus wrapping all fingers around a larger object.
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Quiz yourself
When a precision grip was prepared, which muscle did PMv conditioning facilitate?
Common questions
Why stimulate M1 twice as a control?
If the premotor pulse simply spread to M1, conditioning M1 directly should look the same; it did not change with grasp, so the effect likely travels through premotor-to-motor connections.
Did TMS affect how fast people moved?
No; reaction time and movement duration were unaffected by any TMS condition or interval.
Why does it matter that the interaction is facilitatory here?
The same group previously found PMv inhibits M1 at rest, so the switch to muscle-specific facilitation shows the pathway is actively reconfigured when a grasp is being planned.
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