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Luis Hueso and Neil Mathur are in the Department of Materials Science and Metallurgy, University of Cambridge, New Museums Site, Pembroke Street, Cambridge CB2 3QZ, UK. e-mails: [email protected] [email protected]

1. Levy, P., Leyva, A. G., Troiani, H. E. & Sánchez, R. D. Appl. Phys. Lett. 83, 5247–5249 (2003). 2. Ijima, S. Nature 354, 56–58 (1991). 3. Ishii, H. et al. Nature 426, 540–544 (2003). 4. Amaratunga, G. IEEE Spectrum 40, 28–32 (2003). 5. Hernandez, B. A. et al. Chem. Mater. 14, 480–482 (2002). 6. Luo, Y. et al. Appl. Phys. Lett. 83, 440–442 (2003). 7. Morrison, F. D., Ramsay, L. & Scott, J. F. J. Phys. Condens. Matter 15, L527–L532 (2003). 8. Volger, J. Physica 20, 49–54 (1954). 9. Jin, S. et al. Science 264, 413–415 (1994). 10. Mathur, N. & Littlewood, P. Physics Today 56, 25–30 (2003).

Psychology

Insight and the sleep committee Pierre Maquet and Perrine Ruby We all spend about a third of our lives asleep, an essential but seemingly unproductive state. Experimental evidence now emerges to support anecdotal evidence that sleep can stimulate creative thinking. oes this experience seem familiar? The solution to an unfathomable problem, left unresolved in the evening, effortlessly pops into your mind the following morning. Although many people believe that sleep plays a role in these flashes of insight, this is a hypothesis that has not been rigorously tested. On page 352 of this issue1, however, Wagner and collaborators provide evidence that sleep can have a beneficial effect on insight. The authors have applied a clever test that allows them to determine exactly when insight occurs in the time course of learning2.

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In this task, the participants have to transform a string of eight digits into a new string, the last digit of which is the final solution (see Fig. 1 on page 353). To do this, they are instructed to apply two simple rules sequentially, from one digit to the next. However, unknown to the subjects, another rule is hidden in the material: the last three responses mirror the three preceding ones.Discovering the hidden rule can speed up the execution of the task, as the final solution is known when the third digit is specified. All participants were trained in the task (three blocks of tasks), then retested (for ten

Figure 1 Instances of insight and creativity said to have followed sleep (or rather, except in the case of Massenet, occurring during or following a dream). Some may be apocryphal. But these examples foster the general feeling, tested in a controlled manner by Wagner et al.1, that sleep may aid insight.

blocks) after eight hours. During the eighthour period the subjects were either awake (during daytime or during the night) or sleeping. When they were retested, the proportion who gained insight in those allowed to sleep (60%) was more than twice that in those who remained awake (22%). If subjects were exposed to the task continuously for 13 blocks, without having been trained the day before, the proportion who gained insight was similar to that in the awake groups. In other words, the favourable effect of sleep on insight occurred only if a memory had been formed before the sleep period. The data further suggest that the conscious use of the hidden rule did not evolve from procedural learning — that is, from the unconscious acquisition of a skill through practice. Rather, it stemmed from separate mental representations that were rearranged during sleep after training had taken place. First, although in the sleep group the times taken to solve each sequence of the string of digits decreased overnight in both ‘solvers’ (the 60% who gained insight later on) and ‘nonsolvers’ (who did not), this overnight decrease in reaction time was much smaller in the solvers than in the nonsolvers. Second, compared with the nonsolvers,the responses to the first digits in a sequence were delayed in the solvers as early as the end of the training session. It seems that the solvers spent time analysing the task during the training and retest sessions. Nevertheless, the solvers were the fastest to find the final solution in a sequence because they were aware of and applied the hidden rule. The primitive elements of the task that the participants gleaned during training seem to have been reorganized during sleep, eventually leading them to become conscious of the hidden rule the following morning. Sleep has been implicated in learning and memory in the adult brain and is thought to favour the ‘off-line’ processing of new memories3. Wagner and colleagues’ data can be viewed as an extreme case of memory processing, in which the reorganization of primitive representations leads to a new conscious knowledge that entirely changes and improves the subject’s ability to crack the problem. This study, of course, raises plenty of questions. What are the neural correlates of the processing of the primitive representations during sleep that lead to the gain of insight the following day? During wakefulness, learning of the hidden rule is known to be related to activation of two parts of the brain in particular — the perirhinal cortex and superior parietal lobule2. Do these areas participate in the off-line processing during sleep? What are the neuronal interactions, reinforced during sleep, that underpin the emergence of the conscious knowledge? There is also the issue of whether all stages of sleep participate in these processes — that is,what does the ‘sleep committee’that

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composition, grain size, tube dimensions and tube distribution should reveal more exciting possibilities ahead. The future of nanotubes looks anything but hollow. ■

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news and views The role that sleep plays in human creativity will be a mystery for some time yet. But at the very least, Wagner et al. give us good reason to fully respect our periods of sleep — especially given the current trend to recklessly curtail them. ■ Pierre Maquet and Perrine Ruby are at the Cyclotron Research Centre, University of Liège, Allée du 6 Août, 8 Sart Tilman, B-4000 Liège, Belgium. e-mail: [email protected] 1. Wagner, U., Gais, S., Haider, H., Verleger, R. & Born, J. Nature 427, 352–355 (2004). 2. Rose, M., Haider, H., Weiller, C. & Buchel, C. Neuron 36, 1221–1231 (2002). 3. Maquet, P., Smith, C. & Stickgold, R. (eds) Sleep and Brain Plasticity (Oxford Univ. Press, New York, 2003). 4. Plihal, W. & Born, J. Psychophysiology 36, 571–582 (1999). 5. Stickgold, R., Whidbee, D., Schirmer, B., Patel, V. & Hobson, J. A. J. Cogn. Neurosci. 12, 246–254 (2000). 6. Gais, S., Plihal, W., Wagner, U. & Born, J. Nature Neurosci. 3, 1335–1339 (2000).

Biogeochemistry

Carbon budget in the black Michael W. I. Schmidt A significant fraction of a common organic component of marine sediments has an unexpected source, providing a fresh context for studies of the global carbon cycle in oceanic and terrestrial settings. he global carbon cycle is both a consequence and a determinant of the state of the planet. Not surprisingly, then, a small army of scientists is at work studying the exchange of various guises of the element between land, atmosphere and ocean. One of those guises is ‘black carbon’, a relatively inert form that is produced by incomplete combustion during wildfires or the industrial burning of fossil fuels. It initially accumulates in soils, and is then distributed to lake and river sediments, and eventually to marine sediments. On page 336 of this issue, Dickens et al.1 describe the results of their investigations into how much black carbon is stored in marine sediments. They have analysed sediment laid down in pre-industrial times, and conclude that the fire-derived carbon content has been overestimated. As much as half of the organic carbon previously identified as black carbon instead appears to be fossil ‘graphitic’ black carbon — that is, carbon derived from the weathering of rocks. This conclusion is based on two pillars of evidence. One is that although it was previously not possible to distinguish chemically between fire- and rock-derived carbon, Dickens et al. have done so using a novel chemical approach. They have therefore been able to isolate graphitic black carbon. The other arises from application of the isotope 14C as an analytical tool. Organic carbon that originates in terrestrial wildfires has a 14C signature. Carbon derived from fossil-fuel burning in the industrial era, or

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from geologically much older sources (such as metamorphic rocks), has no such signature — that is, it is ‘radiocarbon dead’. The literature on this topic is rather scant. But by combining their own data from

pre-industrial samples with previous results, Dickens et al. make the case that the proportion of fire-derived black carbon in global marine sediments has been overestimated. Here, however, a comment by Alexandre Dumas in the nineteenth century comes to mind: “All generalizations are dangerous, even this one.” The results of Dickens et al. stem from a limited number of samples, originating mostly from the continental shelf off the American Pacific northwest. They may therefore be representative of such regions — tectonically active continental margins, with high erosion rates on land and high sedimentation rates in the adjacent marine environment. But there are not enough data from other places to be sure that the results apply more widely to the same extent. For example, sediments from open ocean areas are represented by only one sample from the central Pacific. As always, more data would be helpful, and would increase confidence in Dickens and colleagues’ conclusions. But to be fair, Dumas’ point applies to much research in this field, and the authors do present a plausible case. If their results are indeed typical for many regions of the world, they will have considerable influence on both oceanographic and terrestrial studies of the carbon cycle. The authors themselves describe some of the implications. For instance, they consider that radiocarbon dating of sedimentary organic carbon may need a rethink.And they calculate that 71011 g of graphitic black Figure 1 Carbonized conundrum. Black carbon is formed during wildfires in ecosystems such as savannah grasslands and high-latitude coniferous forests. a, Immediately after a fire, the local area is covered by large pieces and dust-sized particles of black carbon. b, After 60 years, a few charred tree stumps remain among a new generation of trees, and only small quantities of black carbon can be detected in the soil. c, Its fate could be export by rivers into the oceans (the Yenessei River, which runs into the Arctic Ocean, is shown here). But given the finding1 that marine sediments contain less fire-derived black carbon than previously thought, some of the carbon must be degraded at an earlier stage or stored elsewhere.

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stimulates insight consist of? Explicit memory tasks are generally thought to be more sensitive to deprivation of slow-wave sleep4.On the other hand,most of the famous cases of scientific insight and artistic creativity (Fig. 1) are reported as emerging from a dream, which is a mental activity that occurs more frequently during the sleep pattern known as rapid-eyemovement (REM) sleep. Another possibility is that both non-REM and REM sleep are sequentially needed to optimize the memory5,6.Wagner et al. give us the tools to explore these questions experimentally. Their paper constitutes steps forward in investigating the unpredictable and elusive phenomenon of insight, and in broadening the scope of the research on sleep, cognition and brain plasticity (the brain’s ability to persistently change its structure and/or function).