information are segregated into two sen-sory channels.

Neuroscience students learn that, intouch, vision and hearing, signals evoked byexternal events are quickly and reliably channelled to their respective sensory areasin the cortex of the brain through ‘relay’ neu-rons in the thalamus (Fig. 1). These channelshave been termed ‘lemniscal’ pathways. Butneuroanatomists have long recognized thatthe lemniscal groups of cells (‘nuclei’) occu-py less than half the volume of the thalamusassociated with each type of sensation. Theremainder belongs to ‘paralemniscal’ sen-sory pathways. The physiological propertiesof paralemniscal neurons in the thalamus aredifficult to classify using typical sensorystimulation protocols. So it has been difficultto assign any specific function to these path-ways. But now, Ahissar et al.2 suggest that the paralemniscal pathway associated withtouch information picked up by a rat’swhiskers can relay to the cortex informationabout the position of touched objects.

Each full forwards–backwards sweep of a rat’s whiskers is called a whisking cycle. Forthe brain to obtain any useful informationabout the location of objects touched duringthis cycle, it needs to reconstruct the trajec-tory of the whiskers. Conceivably, the sensorycortex could receive data about the cycledirectly from the motor systems that gener-ate the whisker movements. But this does notappear to be the case. Motor cortex, althougha major source of input to sensory cortex,does not carry a strong signal about whiskerposition3. By default, then, informationabout the whisking cycle must arise througha signal transmitted from the whiskers.

To identify the signal that reveals whiskerposition, Ahissar et al.2 induced whiskermovement in anaesthetized rats by directingair-puffs along the nose. The whisker deflec-tions, in the range of 2–8 Hz, were taken asexperimentally controlled whisking cycles.

The flow of information from whiskers tocortex is shown in Fig. 1. Lemniscal signalsare transmitted from receptors at the base ofthe whiskers, via the brainstem trigeminalcomplex, to the medial portion of the ventralposterior nucleus of the thalamus (VPM),ending up in the tactile region of cortex.Paralemniscal signals are transmitted along a similar pathway, but through the medialportion of the posterior nucleus (POm) inthe thalamus. Ahissar et al. looked at the neuronal responses to whisker movement in each of these brain regions. For each fre-quency of whisker deflection, the authorsaligned the neuronal responses across cyclesto yield an average response per cycle. Theythen measured the response latency (thetime between the start of whisker movementand the beginning of neuronal activity) andresponse magnitude (the number of ‘spikes’of neuronal activity per whisker movement).

They found that neurons in the trigemi-

nal complex — where the lemniscal andparalemniscal pathways are not yet distinct— fired with an identical response latencyand magnitude regardless of whisking fre-quency. In both VPM and POm, by contrast,the response magnitude decreased as the frequency increased. But in VPM, responselatency was constant regardless of frequency,whereas in POm response latency increasedas whisking frequency increased. Neurons atthe cortical targets of the two pathways (cor-tical layer IV for VPM, and layer Va for POm)behaved similarly to their thalamic inputs.

Ahissar et al. propose that a feedback circuit between sensory cortex and POm is responsible for the latency shifts withincreasing whisking frequency. Corticalneurons in layer V have intrinsic oscillatorymechanisms4 and a powerful influence onPOm5. Ahissar et al. suggest that POm neu-rons activate cortical inhibitory neurons;these then inhibit the cortical oscillators,which in turn drive cortex-to-thalamusfeedback. The properties of this loop couldaccount for the changing response latencyseen in POm.

There is no direct proof that such feed-back loops can decode whisker position. ButAhissar et al. have shown that this varyingresponse to whisking frequency endows theparalemniscal system with the ability toprocess temporal information — such asobject location — that varies on timescales as slow as single whisking cycles. By compar-ing the timing of feedback descending fromthe cortex with the timing of informationascending from the whiskers, POm neuronscould signal where along the whisking sweepan object is located. The lemniscal pathway,by contrast, responds with a fixed latency, soit is better suited to representing object fea-tures — such as surface texture — that do notvary along the whisking cycle.

Can these ideas be generalized to other

news and views

NATURE | VOL 406 | 20 JULY 2000 | www.nature.com 247

sensations? What must be represented by thebrain is unique to each kind of sensation, so one cannot expect the nature of sensory|coding to be identical. The tactile sensorypathways form neural representations ofobjects that contact receptors in the skin; thevisual and auditory systems form represen-tations of objects at a distance.

But the observations made by Ahissar etal. raise some interesting ideas about hownon-tactile stimuli may be represented. Forexample, neuroscientists debate whether thetransmission of information is based onneuronal firing counts over relatively longtime windows (‘spike-count’ coding) or onvariations in neuronal firing within brieftime windows (‘spike-time’ coding). Ahissaret al. have shown that it may be efficient forthese codes to coexist. POm neurons trans-mit information about the frequency ofwhisker movements by shifts in responselatency — a clear spike-time code. But theresponse latency determines the time win-dow available for firing, so these timing shiftslead to spike-count differences, too. So fre-quency information is present in both thetotal spike count and the spike timing.

Time will tell whether or not these results can be applied to other sensory systems. But it seems that, at least for touch,the parallel pathways long recognized byneuroanatomists are beginning to acquirefunctional significance. ■

Mathew E. Diamond is in the CognitiveNeuroscience Sector, International School forAdvanced Studies, Via Beirut 2-4, 34014 Trieste,Italy.e-mail: diamond@sissa.it1. Carvell, G. E. & Simons, D. J. J. Neurosci. 10, 2638–2648 (1990).2. Ahissar, E., Sosnik, R. & Haidarliu, S. Nature 406, 302–306 (2000).3. Fee, M. S., Mitra, P. P. & Kleinfeld, D. J. Neurophysiol. 78,

1144–1149 (1997).4. Silva, L. R., Amitai, Y. & Connors, B. W. Science 251, 432–435

(1991).5. Diamond, M. E., Armstrong-James, M., Budway, M. J. & Ebner,

F. F. J. Comp. Neurol. 319, 66–84 (1992).

Atmospheric physics

Enlightening water vapourBrian J. Soden

The extent to which the Earth’s climatewill warm as a result of increasing car-bon dioxide in the atmosphere depends

largely on the response of water vapour in the upper levels of the troposphere (roughly5–10 km above the surface). But our abilityto monitor changes in water vapour there islimited by the scarcity of observing systemswith sufficient accuracy and longevity todocument its global variation and to detecttrends.

On page 290 of this issue1, Price dem-onstrates a remarkably robust relationshipbetween upper tropospheric water vapour

and global lightning activity. This observationnot only supports the idea that atmosphericconvection moistens the upper troposphere,a characteristic of climate models that hasbeen the subject of spirited debate2, but mayalso provide a unique tool for monitoringpurposes.

The importance of water vapour in regu-lating climate is without question. It is thedominant greenhouse gas, trapping more ofthe Earth’s heat than any other gaseous con-stituent of the atmosphere. If water vapourconcentrations increase in a warmer world,as is widely believed, the added absorption

© 2000 Macmillan Magazines Ltd

Imagine that you are with a group of peoplefrom your community, and have beengiven a sum of money on condition that

you share it with an anonymous member ofthe group. You make them an offer of a pro-portion of the cash, but neither you nor theother player will get a penny unless theyaccept your offer. There is only one round ofthe game, and confidentiality is maintained.

What do you think is a fair offer? Theanswers to this question, for a range of cul-tures from around the world, were the subjectof a symposium* held last month: economicgame theory met human behavioural ecol-ogy with fascinating results.

The rational way to play the game, knownin economics as the ultimatum game, is for

the first player to offer as little as possible andfor the second player to accept it — some-thing is always better than nothing. But whengroups of university students play, the mostcommon offer is 50% of the money, with amean of about 40%. Offers below 20% arealmost always rejected. Rejection is purespite, as no one gains, but it is the only way ofpunishing the proposer for a mean offer. Arange of other experiments confirm a generaltendency in humans both to be generous todistantly related individuals and to punishcheaters, sometimes at great personal cost(H. Gintis, Univ. Massachusetts, Amherst).

Western university students are the socialscientist’s equivalent of the laboratory rat, yet evolutionary psychologists and econo-mists are often tempted to assume that resultsobtained with them represent universalhuman preferences. But Westerners, not

will act to further amplify the initial warm-ing. Mathematical models of the Earth’s climate suggest that this would provide con-siderable positive feedback, more than dou-bling the sensitivity of surface temperatureto anthropogenic greenhouse gases. Currentmodels predict that water vapour concen-trations in the upper troposphere couldincrease by as much as 40% over the nexthalf-century3. This amplified moistening inresponse to the predicted warming not onlyhighlights the significance of water vapour asa feedback mechanism, but also underscoresthe need for long-term monitoring of upper-tropospheric water vapour in helping todetect climate change and identify its causes.

Some climate researchers have challengedthis view. They argue that the treatment ofwater vapour in climate models is overly sim-plified and that concentrations in the uppertroposphere might actually decrease in awarmer climate. The mechanisms and effectsof atmospheric convection and related cloudprocesses have been central to this debate.

Lindzen2 postulated that the increasedconvective overturning of the atmosphere inresponse to warmer surface temperatureswould bring more dry air down from higheraltitudes and decrease the moisture in theupper troposphere. That atmospheric con-vection locally moistens the environment inwhich it occurs is indisputable. Observationsfrom weather satellites (Fig. 1) clearly showincreased levels of humidity (yellow to red)in regions surrounding the areas of activeconvection, as can be seen from the upper-level cloud cover (grey). However, on a globalscale, the net effect of an increase in convec-tive overturning is less obvious. Rising air inconvective towers must be balanced in sur-rounding regions by sinking air, which, inthe absence of other sources of moisture, canlead to extremely dry conditions in the uppertroposphere (blue in Fig. 1).

Price1 uses global lightning activity as aproxy for atmospheric convection. This is rea-sonable because convection not only deter-mines the vertical transport of moisture, butalso influences the electrification processesresponsible for generating lightning activity.He shows that variations in lightning are wellcorrelated with the observed fluctuations inglobal upper-tropospheric moisture on bothweekly and seasonal timescales. Some mayrightly question the reliability of current sys-tems for observing moisture levels, such asthe satellite output that Price has used. But heshows that similar variations in water vapourare obtained using independent measure-ments from different observing systems,which lends further credibility to the results.

By itself, the strong correlation betweenlightning activity and upper troposphericwater vapour offers a valuable opportunityfor testing model simulations of the responseof moisture to changes in atmospheric con-vection. But Price also points out that global

inexpensive method for monitoring the long-term trends in upper-tropospheric moisture.

Water vapour is notoriously difficult tomeasure and such a capability would be ofgreat value to the climate research commu-nity. Water vapour is highly variable in bothspace and time (Fig. 1), and its concentrationin the atmosphere varies by over three ordersof magnitude. Although an international network of weather balloons has carriedwater vapour sensors for nearly half a cen-tury, changes in instrumentation and poorcalibration make such sensors unsuitable fordetecting trends in upper-tropospheric watervapour. Likewise, although weather satelliteshave provided global measurements of watervapour for over two decades, little effort hasgone into making the data appropriate forlong-term climate monitoring.

Price has shown that seasonal changes inglobal lightning activity are well correlatedwith changes in water vapour in the uppertroposphere, but it remains to be seenwhether the long-term trends in these quan-tities exhibit similar consistency. If Price’soptimism proves correct, and future trendsin lightning activity can indeed be linked totrends in upper-tropospheric water vapour,observations of Schumann resonancescould provide a great deal of enlightenmentin tracking climate change. ■

Brian J. Soden is at the NOAA Geophysical FluidDynamics Laboratory, Princeton University, POBox 308, Princeton, New Jersey 08542, USA.e-mail: bjs@gfdl.gov 1. Price, C. Nature 406, 290–293 (2000).2. Lindzen, R. S. Bull. Am. Meteorol. Soc. 71, 288–299

(1990).3. Rind, D. Science 281, 1152–1153 (1998).

news and views

248 NATURE | VOL 406 | 20 JULY 2000 | www.nature.com

Figure 1 Water vapour in the upper troposphere.This satellite image shows that higher levels ofhumidity (yellow to red) are associated with theareas of active atmospheric convection evidentfrom clouds (grey). Price1 takes global lightningas a proxy for convection, and shows that itmight be used to monitor water vapourconcentrations in the upper troposphere.

Human behaviour

Fair gameRuth Mace

*Reciprocity and Human Sociality, Human Behaviour and Evolution

Society, Amherst College, Massachusetts, USA, 7–11 June 2000.

lightning activity can be readily monitoredfrom a single location through so-calledSchumann resonances, the low-frequencyelectromagnetic waves discharged by light-ning. Because these waves become trapped inthe wave guide formed by the Earth’s iono-sphere, they can travel around the globe sever-al times before dissipating. This means thatglobal monitoring can be done from a singlelocation. Given the close relationship betweenlightning and upper-tropospheric water vapour, Price suggests that measurements of Schumann resonances could provide an

© 2000 Macmillan Magazines Ltd