For personal use. Only reproduce with permission from The Lancet Publishing Group.

THE LANCET Neurology Vol 1 November 2002 http://neurology.thelancet.com402

When we pick up an apple and bite, theaction is generated in the motor cortex,by the disjointed human figure that is counterpart to the sensory homuncu-lus. The existence of the motorhomunculus remains widely taught,and yet evidence is accruing that corticalrepresentation of muscle action lookseven stranger—findings that couldbenefit patients with motor lesions.

The motor cortex of human beingswas mapped initially by neurosurgeonWilder Penfield, during his 1930selectrical-stimulation studies in patientswith epilepsy. Subsequent research hasconfirmed “a very rough somatatopy”in the primary motor cortex—”essentially a body-part map”—says Michael Graziano (PrincetonUniversity, NY, USA). “But within aparticular body-part representation,there’s what looks like finer-grainedorganisation.”

The first question for Peter Strick(University of Pittsburgh School ofMedicine, PA, USA) is whether theorganisation represents muscles ormovements. Graziano reckons the datahave yet to support the notion that themotor cortex maps to individualmuscles, muscle firing, or joint angle.“Traditional views that don’t really fitthe data have been a frustratingstumbling block within motorphysiology. But there hasn’t been anoverarching view to replace them.”

Moreover, the motor cortex hasbeen previously mapped by short,intense stimulation bursts, whichproduce muscle twitches that relate inconfusing or conflicting ways to themotor cortex. But work on othercortical areas has suggested that longerstimulation may be more relevant tomap behaviour. So Graziano’s teamdecided to stimulate cortical motorareas for around 0·5 s in activemonkeys, “approximating the timescaleof normal reaching and graspingmovements”.

What resulted were coordinated,complex postures that involve manyjoints, such as those mimicking the actof bringing food to an opening mouth.What’s more, stimulation of a specificsite always induced action toward thesame final posture, regardless of the

movement required to attain it, evenduring anaesthesia. Stimulation of othersites, including the adjacent premotorcortex, evoked different postures,forming a cortical map of positionsaround the body. “We suggest that theseregions fit together into a single map ofthe workspace around the body”(Neuron 2002; 34: 841–51).

So, Graziano explains, theparameter that varies across the“somewhat warpy” motor map is “notthe location where the muscles areactivated, but rather the location nearthe body to which the movement isdirected”. Edward Tehovnik

(Massachisettes Institute of Technology,Cambridge, MA, USA) points out that“[similar] organisation resides in thedorsomedial frontal cortex, whichcontains a topographic maprepresenting the termination positionsof the eyes.” Moreover, John Schlag andhis team (University of California at LosAngeles, CA, USA) have found thateven short stimulation trains, applied inthis region when the eyes are moving,will alter the eyes’ trajectory “to reachthe point they would have reached if theeyes had been immobile at the time ofstimulation. This suggests that theoculomotor cortex can be organised interms of ‘goals’ of movement, ratherthan vectors.”

Yet, Marc Schieber (University ofRochester School of Medicine, NY,USA) is unconvinced that the Princetonreport “can be taken to mean that thereis a point-to-point representation ofevoked limb position in the cortex”.Instead, he favours an interpretationthat involves “some sort of gradient ofrepresentation of the limbmusculature”, in which representationsof smaller parts or muscles overlap

extensively, as indicated by his findingson finger movements (briefly reviewedin Adv Exp Med Biol 2002; 508: 411–16).Graziano finds his view hard toreconcile with the fact that “stimulationof one site in cortex can cause activity intotally different muscles depending onthe starting position of the limb.”

Both Schieber and Strick point outthat the intense and prolongedmicrostimulation used in the Princetonstudies would allow excitation to spreadwidely, stimulating networks ofdescending neurons, and adjacentcortex. Graziano counters that “thestimulation is supposed to spread—it’s neurophysiologically relevant”.However, Strick wonders whetherdifferent stimulation patterns mightactivate different descending systems(eg, low-threshold stimulation toactivate direct connections between thecortex and motor neurons in the spinalcord, and high-threshold stimulation toactivate cortical effects on motorneurons that are mediated byinterneuronal systems in the spinalcord; Nat Neurosci 2002; 5: 714–15).

Whether the effects of brainstimulation are natural or an artefact,the findings are clinically relevant.“Eventually”, Schieber points out,“long-train intracranial microstim-ulation might be helpful in facilitatingmuscle excitation in patients recoveringfrom partial lesions of the corticospinalpathway”. This approach depends onthe presence of some intact descendingpathways. But, Schlag notes, “there arenow promising attempts to designimplanted prostheses that will read theactivity in cortical areas and decode it”.

Already, Tehovnik, who isdeveloping electrical techniques to alterthe working of the visual cortex inmonkeys, anticipates the developmentof “an effective prosthetic device toimplant in the visual cortex of blindhumans”. Similarly, Schlag anticipatesthe use of motor cortical activity tooperate robots for paralysed patients.“Obviously”, he says, “if cortical areas—at least some of them—are organised torepresent ‘goals’ or ‘intents’ rather thanphysical movements, the prospect ofsuccess is much greater.”Kelly Morris

Remapping the motor cortex—death of a homunculus?

Newsdesk

Does he need to be redrawn?

Dav

id S

hand