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The Journal of Neuroscience, November 14, 2007, 27(46):12651-12654; doi:10.1523/JNEUROSCI.4308-07.2007

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Brief Communications
On-Line Control of Grasping Actions: Object-Specific Motor Facilitation Requires Sustained Visual Input

Gita Prabhu,¹ Roger Lemon,¹ and Patrick Haggard²

¹Sobell Department of Motor Neuroscience and Movement Disorders, Institute of Neurology, and ²Institute of Cognitive Neuroscience, UCL, London WC1N 3AR, United Kingdom

Correspondence should be addressed to Professor Patrick Haggard, Institute of Cognitive Neuroscience, UCL, 17 Queen Square, London WC1N 3AR. Email: p.haggard{at}ucl.ac.uk

Dorsal stream visual processing is generally considered to underlie visually driven action, but when subjects grasp an object from memory, as visual information is not available, ventral stream characteristics emerge. In this study we use paired-pulse transcranial magnetic stimulation (TMS) to investigate the importance of the current visual input during visuomotor grasp. Previously, the amplitude of the paired-pulse motor evoked potentials (MEPs) in hand muscles before movement onset have been shown to predict the subsequent pattern of muscle activity during grasp. Specific facilitation of paired-pulse MEPs may reflect premotor–motor (PMC–M1) cortex connectivity. Here we investigate the paired-pulse MEPs evoked under memory-cued and visually driven conditions before grasping one of two possible target objects (a handle or a disc). All trials began with a delay period of 1200 ms. Then, a TMS pulse served as the cue to reach, grasp and hold the target object for 0.5 s. Total trial length was 5 s. Both objects were continually visible in both conditions, but the way in which the target object was designated differed between conditions. In the memory-cued condition, the target object was illuminated for the first 200 ms of the trial only. In the visually driven condition, the target object was illuminated throughout the 5 s trial. Thus, the conditions differed in whether or not the object to be grasped was designated at the time of movement initiation. We found that the pattern of paired-pulse MEP facilitation matched the pattern of object-specific muscle activity only for the visually driven condition. The results suggest that PMC–M1 connectivity contributes to action selection only when immediate sensory information specifies which action to make.

Key words: transcranial magnetic stimulation; corticospinal; motor cortex; dorsal visual stream; grasping; affordance

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Received May 29, 2007; accepted Oct. 6, 2007.

Correspondence should be addressed to Professor Patrick Haggard, Institute of Cognitive Neuroscience, UCL, 17 Queen Square, London WC1N 3AR. Email: p.haggard{at}ucl.ac.uk

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