Viral Tracers

18 SCIENTIFIC AMERICAN March 1996

tion under way in the rem-nant of supernova 1006,this process probably oc-curs in other young super-nova remnants,Ó notes Rob-ert Petre of the GoddardSpace Flight Center, one ofKoyamaÕs collaborators. Butsome rays are so potentthat even supernova eventsprobably cannot accountfor their existence. (Themost extreme of these rayscontain as much energy asa Nolan Ryan fastballÑcrammed into a single sub-atomic particle!)

Many researchers haveassumed that the high-energy cosmic rays must originate in even greatershocksÑthose surroundingactive, or Òexploding,Ó gal-axies. In a recent paper inScience, however, G�nter Sigl of theUniversity of Chicago and his colleaguessuggest a more exotic possibility. SiglÕsgroup analyzed data from the FlyÕs Eyedetector in Utah and other, similar ex-periments that study the ßash of lightand spray of particles unleashed whencosmic rays collide with atoms in the

earthÕs upper atmosphere. The research-ers Þnd an odd ÒgapÓ in the data: atprogressively higher energies, the num-ber of cosmic rays seems to trail oÝ butthen abruptly increases again.

No known process could producesuch a gap, so why is it there? One pos-sibility is that the highest-energy cos-

mic rays are the product ofan entirely new, still hypo-thetical physical mecha-nismÑthe evaporation ofcosmic strings, for instance,or the decay of proposed su-permassive particles. On theother hand, the total num-ber of high-energy cosmicrays detected is quite small,so the perceived gap Òcouldbe a statistical ßuctuation,ÓSigl admits. ÒWe donÕt havethe dataÓ to tell for sure, helaments.

Help may soon be on theway. Last November physi-cists from 19 countriescommitted themselves tobuilding the Pierre AugerCosmic Ray Laboratory, a$100-million detector thatwould far exceed the sensi-tivity of any existing de-

vice. Tentatively scheduled to begin op-erating at the beginning of the nextcentury, the Auger Laboratory couldquickly settle many current questionsabout cosmic rays. ÒIt could conÞrmnew physics, or it could rule it out,ÓSigl reßects. ÒEither way, it will be veryinteresting.Ó ÑCorey S. Powell

EXPANDING REMNANTS of supernova explosions (such as theCrab Nebula, above) may be the birthplace of many cosmic rays.

Imagine trying to make sense of arailway map if none of the lineswere labeled. It would be nearly im-

possible to know which trains ran be-tween which towns. Neuroscientists longfaced a similar problem: the chemicalsthey used to trace lines of communica-tion between brain regions vanished af-ter a single stop. ÒThey only went fromone station to the next,Ó says Peter L.Strick of the Veterans AdministrationMedical Center in Syracuse, N.Y. Know-ing but short stretches of certain tracks,he adds, made it exceedingly diÛcultto determine where any one trainÑornerve signalÑultimately went.

Recently, though, Strick has turnedto a new, more powerful technique, onethat enlists itinerant viruses to chartbrain circuits in monkeys. ÒThe virusesmove from one neuron to another,right on down the line,Ó he notes. ÒHap-pily, there are strains of virus that dothis by crossing over synaptic connec-tions.Ó These viruses cross in only onedirection. A strain of the herpes simplextype I virus, for example, follows theßow of nerve impulses through neigh-boring cells: the virus particles passdown a neuronÕs axon, across a synapse,

into another neuron, down its axon, overanother synapse and so on. A diÝerentstrain moves in the opposite direction.

Unlike conventional tracers, a little vi-rus goes a long way. Because the strainsare living, they replicate in every cell,thus increasing in number before eachleg of the journey. ÒYou get an on-lineampliÞcation of sorts of the tracer sig-nal,Ó Strick points out. ÒSo we can seethe signal more clearly than we evercould before.Ó Already the method hasrevealed new facts about the cerebellumin primates. Traditionally, scientists be-lieved that this structure integrated in-formation from the cerebral cortexwith sensory input from the muscles. Itthen presumably sent nerve signalsback to other motor regions in thebrain, enabling the body to performskilled movements.

Strick, among others, has found thatthe cerebellum may also coordinate themovement of thoughts. Using viral trac-ers, he demonstrated that the cerebel-lum sent signals, via the thalamus, to re-gions in the cerebral cortex used solelyfor cognition, among them areas in theprefrontal cortex involved in short-termmemory and decision making. ÒPeople

proposed that the cerebellum had cog-nitive functions back in the 1980s,ÓStrick says, Òbut I thought they werenuts. Now IÕm a believer.Ó

Most recently, he has discovered somefar-reaching contacts that the basal gan-glia make. These structures were alsothought to preside primarily over mo-tor functions. But viral tracers exposedoutput from them to sections of thetemporal cortex responsible for visualtasks, such as recognizing objects. TheÞnding, Strick suggests, could help ex-plain why ParkinsonÕs disease patientswho take dopamine can experience vi-sual hallucinations as a side eÝect. Thedopamine given to humans may act onthose same cells in the basal gangliathat in monkeys talk to visual areas inthe temporal cortex.

Among other projects, Strick plansto determine whether the cerebellumplays a role in focusing attention. Dam-age to it may well provide the physicalbasis for the attentional deÞcits in au-tistic children. ÒI have spent some 30years studying motor areas, but thistechnique is allowing me to look moreglobally at the circuits in the brain,ÓStrick comments. In time, he adds, theviral tracing technique could elucidatesome of the circuits that malfunctionin a number of mental and neurologi-cal illnesses. ÑKristin Leutwyler

Viral TracersNeuroscientists use viruses to map out pathways in the brain

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Copyright 1996 Scientific American, Inc.