Skip to main content

Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

  • Article
  • Published:

Vibrissa motor cortex activity suppresses contralateral whisking behavior

Nature Neuroscience volume 20, pages 82–89 (2017)Cite this article

Subjects

This article has been updated

Abstract

Anatomical, stimulation and lesion data implicate vibrissa motor cortex in whisker motor control. Work on motor cortex has focused on movement generation, but correlations between vibrissa motor cortex activity and whisking are weak. The exact role of vibrissa motor cortex remains unknown. We recorded vibrissa motor cortex neurons during various forms of vibrissal touch, which were invariably associated with whisker protraction and movement. Free whisking, object palpation and social touch all resulted in decreased cortical activity. To understand this activity decrease, we performed juxtacellular recordings, nanostimulation and in vivo whole-cell recordings. Social touch resulted in decreased spiking activity, decreased cell excitability and membrane hyperpolarization. Activation of vibrissa motor cortex by intracortical microstimulation elicited whisker retraction, as if to abort vibrissal touch. Various vibrissa motor cortex inactivation protocols resulted in contralateral protraction and increased whisker movements. These data collectively point to movement suppression as a prime function of vibrissa motor cortex activity.

This is a preview of subscription content, access via your institution

Access options

Buy this article

  • Purchase on Springer Link
  • Instant access to full article PDF
Buy now

Prices may be subject to local taxes which are calculated during checkout

Figure 1: Decrease of VMC activity during vibrissal touch.
Figure 2: Decreased activity, decreased excitability and hyperpolarization of VMC during social touch.
Figure 3: VMC activity is additively suppressed by both nose-to-nose touch and whisker protraction.
Figure 4: Unilateral microstimulation of VMC in awake rats leads to contralateral whisker retraction.
Figure 5: Unilateral blockade of VMC increases contralateral whisker movement and protraction.

Similar content being viewed by others

Change history

  • 17 November 2016

    In the version of this article initially published online, the x axes in Figure 1e,g,i were labeled "Time (Hz)"; the correct label is "Time (s)." Also, the P value in Figure 1j was given as 0.00018; the correct value is 0.0018. The errors have been corrected for the print, PDF and HTML versions of this article.

References

  1. Hall, R.D. & Lindholm, E.P. Organization of motor and somatosensory neocortex in the albino rat. Brain Res. 66, 23–38 (1974).

    Google Scholar 

  2. Gioanni, Y. & Lamarche, M. A reappraisal of rat motor cortex organization by intracortical microstimulation. Brain Res. 344, 49–61 (1985).

    CAS  PubMed  Google Scholar 

  3. Haiss, F. & Schwarz, C. Spatial segregation of different modes of movement control in the whisker representation of rat primary motor cortex. J. Neurosci. 25, 1579–1587 (2005).

    CAS  PubMed  PubMed Central  Google Scholar 

  4. Tandon, S., Kambi, N. & Jain, N. Overlapping representations of the neck and whiskers in the rat motor cortex revealed by mapping at different anaesthetic depths. Eur. J. Neurosci. 27, 228–237 (2008).

    PubMed  PubMed Central  Google Scholar 

  5. Brecht, M. et al. Organization of rat vibrissa motor cortex and adjacent areas according to cytoarchitectonics, microstimulation, and intracellular stimulation of identified cells. J. Comp. Neurol. 479, 360–373 (2004).

    PubMed  Google Scholar 

  6. Brecht, M., Schneider, M., Sakmann, B. & Margrie, T.W. Whisker movements evoked by stimulation of single pyramidal cells in rat motor cortex. Nature 427, 704–710 (2004).

    CAS  PubMed  Google Scholar 

  7. Matyas, F. et al. Motor control by sensory cortex. Science 330, 1240–1243 (2010).

    CAS  PubMed  Google Scholar 

  8. Neafsey, E.J. et al. The organization of the rat motor cortex: a microstimulation mapping study. Brain Res. 396, 77–96 (1986).

    CAS  PubMed  Google Scholar 

  9. Welker, W.I. Analysis of sniffing of the albino rat. Behaviour 22, 223–244 (1964).

    Google Scholar 

  10. Kleinfeld, D., Ahissar, E. & Diamond, M.E. Active sensation: insights from the rodent vibrissa sensorimotor system. Curr. Opin. Neurobiol. 16, 435–444 (2006).

    CAS  PubMed  Google Scholar 

  11. Georgopoulos, A.P., Schwartz, A.B. & Kettner, R.E. Neuronal population coding of movement direction. Science 233, 1416–1419 (1986).

    CAS  Google Scholar 

  12. Lemon, R. The output map of the primate motor cortex. Trends Neurosci. 11, 501–506 (1988).

    CAS  PubMed  Google Scholar 

  13. Carvell, G.E., Miller, S.A. & Simons, D.J. The relationship of vibrissal motor cortex unit activity to whisking in the awake rat. Somatosens. Mot. Res. 13, 115–127 (1996).

    CAS  PubMed  Google Scholar 

  14. Hill, D.N., Curtis, J.C., Moore, J.D. & Kleinfeld, D. Primary motor cortex reports efferent control of vibrissa motion on multiple timescales. Neuron 72, 344–356 (2011).

    CAS  PubMed  PubMed Central  Google Scholar 

  15. Friedman, W.A., Zeigler, H.P. & Keller, A. Vibrissae motor cortex unit activity during whisking. J. Neurophysiol. 107, 551–563 (2012).

    PubMed  Google Scholar 

  16. Gerdjikov, T.V., Haiss, F., Rodriguez-Sierra, O.E. & Schwarz, C. Rhythmic whisking area (RW) in rat primary motor cortex: an internal monitor of movement-related signals? J. Neurosci. 33, 14193–14204 (2013).

    CAS  PubMed  PubMed Central  Google Scholar 

  17. Asanuma, H. Recent developments in the study of the columnar arrangement of neurons within the motor cortex. Physiol. Rev. 55, 143–156 (1975).

    CAS  PubMed  Google Scholar 

  18. Penfield, W. & Rasmussen, T. The Cerebral Cortex of Man (Macmillan, 1952).

  19. Graziano, M.S., Taylor, C.S., Moore, T. & Cooke, D.F. The cortical control of movement revisited. Neuron 36, 349–362 (2002).

    CAS  PubMed  Google Scholar 

  20. Bobrov, E., Wolfe, J., Rao, R.P. & Brecht, M. The representation of social facial touch in rat barrel cortex. Curr. Biol. 24, 109–115 (2014).

    CAS  PubMed  Google Scholar 

  21. Wolfe, J., Mende, C. & Brecht, M. Social facial touch in rats. Behav. Neurosci. 125, 900–910 (2011).

    PubMed  Google Scholar 

  22. Lenschow, C. & Brecht, M. Barrel cortex membrane potential dynamics in social touch. Neuron 85, 718–725 (2015).

    CAS  PubMed  Google Scholar 

  23. Lee, E. et al. Enhanced neuronal activity in the medial prefrontal cortex during social approach behavior. J. Neurosci. 36, 6926–6936 (2016).

    CAS  PubMed  PubMed Central  Google Scholar 

  24. Schiemann, J. et al. Cellular mechanisms underlying behavioral state-dependent bidirectional modulation of motor cortex output. Cell Rep. 11, 1319–1330 (2015).

    CAS  PubMed  PubMed Central  Google Scholar 

  25. Moore, J.D. et al. Hierarchy of orofacial rhythms revealed through whisking and breathing. Nature 497, 205–210 (2013).

    CAS  PubMed  PubMed Central  Google Scholar 

  26. Deschênes, M. et al. Inhibition, not excitation, drives rhythmic whisking. Neuron 90, 374–387 (2016).

    PubMed  PubMed Central  Google Scholar 

  27. Dörfl, J. The musculature of the mystacial vibrissae of the white mouse. J. Anat. 135, 147–154 (1982).

    PubMed  PubMed Central  Google Scholar 

  28. Haidarliu, S., Simony, E., Golomb, D. & Ahissar, E. Muscle architecture in the mystacial pad of the rat. Anat. Rec. (Hoboken) 293, 1192–1206 (2010).

    Google Scholar 

  29. Herfst, L.J. & Brecht, M. Whisker movements evoked by stimulation of single motor neurons in the facial nucleus of the rat. J. Neurophysiol. 99, 2821–2832 (2008).

    PubMed  Google Scholar 

  30. Berg, R.W. & Kleinfeld, D. Vibrissa movement elicited by rhythmic electrical microstimulation to motor cortex in the aroused rat mimics exploratory whisking. J. Neurophysiol. 90, 2950–2963 (2003).

    PubMed  Google Scholar 

  31. Huber, D. et al. Multiple dynamic representations in the motor cortex during sensorimotor learning. Nature 484, 473–478 (2012).

    CAS  PubMed  PubMed Central  Google Scholar 

  32. Semba, K. & Komisaruk, B.R. Neural substrates of two different rhythmical vibrissal movements in the rat. Neuroscience 12, 761–774 (1984).

    CAS  PubMed  Google Scholar 

  33. Gao, P., Hattox, A.M., Jones, L.M., Keller, A. & Zeigler, H.P. Whisker motor cortex ablation and whisker movement patterns. Somatosens. Mot. Res. 20, 191–198 (2003).

    PubMed  Google Scholar 

  34. Kolb, B. Functions of the frontal cortex of the rat: a comparative review. Brain Res. 320, 65–98 (1984).

    CAS  PubMed  Google Scholar 

  35. Kawai, R. et al. Motor cortex is required for learning but not for executing a motor skill. Neuron 86, 800–812 (2015).

    CAS  PubMed  PubMed Central  Google Scholar 

  36. Stoltz, S., Humm, J.L. & Schallert, T. Cortical injury impairs contralateral forelimb immobility during swimming: a simple test for loss of inhibitory motor control. Behav. Brain Res. 106, 127–132 (1999).

    CAS  PubMed  Google Scholar 

  37. Zagha, E., Ge, X. & McCormick, D.A. Competing neural ensembles in motor cortex gate goal-directed motor output. Neuron 88, 565–577 (2015).

    CAS  PubMed  PubMed Central  Google Scholar 

  38. Mikuni, N. et al. Evidence for a wide distribution of negative motor areas in the perirolandic cortex. Clin. Neurophysiol. 117, 33–40 (2006).

    PubMed  Google Scholar 

  39. Filevich, E., Kühn, S. & Haggard, P. Negative motor phenomena in cortical stimulation: implications for inhibitory control of human action. Cortex 48, 1251–1261 (2012).

    PubMed  Google Scholar 

  40. Kuypers, H.G.J.M. A new look at the organization of the motor system. Prog. Brain Res. 57, 381–403 (1982).

    CAS  PubMed  Google Scholar 

  41. Grinevich, V., Brecht, M. & Osten, P. Monosynaptic pathway from rat vibrissa motor cortex to facial motor neurons revealed by lentivirus-based axonal tracing. J. Neurosci. 25, 8250–8258 (2005).

    CAS  PubMed  PubMed Central  Google Scholar 

  42. Sreenivasan, V., Karmakar, K., Rijli, F.M. & Petersen, C.C.H. Parallel pathways from motor and somatosensory cortex for controlling whisker movements in mice. Eur. J. Neurosci. 41, 354–367 (2015).

    PubMed  Google Scholar 

  43. Cheney, P.D. & Fetz, E.E. Functional classes of primate corticomotoneuronal cells and their relation to active force. J. Neurophysiol. 44, 773–791 (1980).

    CAS  PubMed  Google Scholar 

  44. Davidson, A.G., Chan, V., O'Dell, R. & Schieber, M.H. Rapid changes in throughput from single motor cortex neurons to muscle activity. Science 318, 1934–1937 (2007).

    CAS  PubMed  Google Scholar 

  45. Bonini, L., Maranesi, M., Livi, A., Fogassi, L. & Rizzolatti, G. Ventral premotor neurons encoding representations of action during self and others' inaction. Curr. Biol. 24, 1611–1614 (2014).

    CAS  PubMed  Google Scholar 

  46. Dushanova, J. & Donoghue, J. Neurons in primary motor cortex engaged during action observation. Eur. J. Neurosci. 31, 386–398 (2010).

    PubMed  PubMed Central  Google Scholar 

  47. Vigneswaran, G., Philipp, R., Lemon, R.N. & Kraskov, A. M1 corticospinal mirror neurons and their role in movement suppression during action observation. Curr. Biol. 23, 236–243 (2013).

    CAS  PubMed  PubMed Central  Google Scholar 

  48. Kraskov, A. et al. Corticospinal mirror neurons. Phil. Trans. R. Soc. Lond. B 369, 20130174 (2014).

    CAS  Google Scholar 

  49. Schieber, M.H. Mirror neurons: reflecting on the motor cortex and spinal cord. Curr. Biol. 23, R151–R152 (2013).

    CAS  PubMed  Google Scholar 

  50. Purves, D. et al. Neuroscience (Sinauer Associates, Sunderland, MA, 2004).

  51. Rao, R.P., Mielke, F., Bobrov, E. & Brecht, M. Vocalization-whisking coordination and multisensory integration of social signals in rat auditory cortex. eLife 3, 1–20 (2014).

    Google Scholar 

  52. Stockwell, R.G., Mansinha, L. & Lowe, R.P. Localization of the complex spectrum: the S transform. IEEE Trans. Signal Process. 44, 998–1001 (1996).

    Google Scholar 

  53. Schmitzer-Torbert, N., Jackson, J., Henze, D., Harris, K. & Redish, A.D. Quantitative measures of cluster quality for use in extracellular recordings. Neuroscience 131, 1–11 (2005).

    CAS  PubMed  Google Scholar 

  54. Barthó, P. et al. Characterization of neocortical principal cells and interneurons by network interactions and extracellular features. J. Neurophysiol. 92, 600–608 (2004).

    PubMed  Google Scholar 

  55. Vigneswaran, G., Kraskov, A. & Lemon, R.N. Large identified pyramidal cells in macaque motor and premotor cortex exhibit “thin spikes”: implications for cell type classification. J. Neurosci. 31, 14235–14242 (2011).

    CAS  PubMed  PubMed Central  Google Scholar 

  56. Houweling, A.R., Doron, G., Voigt, B.C., Herfst, L.J. & Brecht, M. Nanostimulation: manipulation of single neuron activity by juxtacellular current injection. J. Neurophysiol. 103, 1696–1704 (2010).

    PubMed  Google Scholar 

  57. Pinault, D. A novel single-cell staining procedure performed in vivo under electrophysiological control: morpho-functional features of juxtacellularly labeled thalamic cells and other central neurons with biocytin or Neurobiotin. J. Neurosci. Methods 65, 113–136 (1996).

    CAS  PubMed  Google Scholar 

  58. Tang, Q., Brecht, M. & Burgalossi, A. Juxtacellular recording and morphological identification of single neurons in freely moving rats. Nat. Protoc. 9, 2369–2381 (2014).

    CAS  PubMed  Google Scholar 

  59. Sürmeli, G. et al. Molecularly defined circuitry reveals input-output segregation in deep layers of the medial entorhinal cortex. Neuron 88, 1040–1053 (2015).

    PubMed  PubMed Central  Google Scholar 

  60. Clack, N.G. et al. Automated tracking of whiskers in videos of head fixed rodents. PLoS Comput. Biol. 8, e1002591 (2012).

    CAS  PubMed  PubMed Central  Google Scholar 

  61. MacDonald, C.J., Lepage, K.Q., Eden, U.T. & Eichenbaum, H. Hippocampal “time cells” bridge the gap in memory for discontiguous events. Neuron 71, 737–749 (2011).

    CAS  PubMed  PubMed Central  Google Scholar 

  62. Tehovnik, E.J., Tolias, A.S., Sultan, F., Slocum, W.M. & Logothetis, N.K. Direct and indirect activation of cortical neurons by electrical microstimulation. J. Neurophysiol. 96, 512–521 (2006).

    CAS  PubMed  Google Scholar 

  63. Aarts, E., Verhage, M., Veenvliet, J.V., Dolan, C.V. & van der Sluis, S. A solution to dependency: using multilevel analysis to accommodate nested data. Nat. Neurosci. 17, 491–496 (2014).

    CAS  PubMed  Google Scholar 

  64. Tehovnik, E.J. & Sommer, M.A. Effective spread and timecourse of neural inactivation caused by lidocaine injection in monkey cerebral cortex. J. Neurosci. Methods 74, 17–26 (1997).

    CAS  PubMed  Google Scholar 

Download references

Acknowledgements

We thank B. Geue, U. Schneeweiß and J. Diederichs for technical assistance and V. Bahr and F. Mielke for assistance with programming. We thank M. Rüsseler for assistance with video tracking and R.P. Rao and E. Bobrov for sharing tracked whisker traces of behaving rats. We thank S. Helgheim Tawfiq for behavior drawings. We thank A. Neukirchner, E. Chorev, S. Ray, P. Bennett and A. Clemens for comments on the manuscript. This work was supported by Humboldt-Universität zu Berlin, the Bernstein Center for Computational Neuroscience Berlin, the German Federal Ministry of Education and Research (BMBF, Förderkennzeichen 01GQ1001A, M.B.) and NeuroCure. M.B. was a recipient of a European Research Council grant and the Gottfried Wilhelm Leibniz Prize.

Author information

Author notes

  1. Guy Doron

    Present address: Present address: NeuroCure Cluster of Excellence, Humboldt-Universität zu Berlin, Berlin, Germany.,

Authors and Affiliations

  1. Bernstein Center for Computational Neuroscience Berlin, Humboldt-Universität zu Berlin, Berlin, Germany

    Christian Laut Ebbesen, Guy Doron, Constanze Lenschow & Michael Brecht

  2. Berlin School of Mind and Brain, Humboldt-Universität zu Berlin, Berlin, Germany

    Christian Laut Ebbesen

Authors
  1. Christian Laut Ebbesen
    View author publications

    You can also search for this author in PubMed Google Scholar

  2. Guy Doron
    View author publications

    You can also search for this author in PubMed Google Scholar

  3. Constanze Lenschow
    View author publications

    You can also search for this author in PubMed Google Scholar

  4. Michael Brecht
    View author publications

    You can also search for this author in PubMed Google Scholar

Contributions

C.L.E., G.D. and M.B. designed the study. C.L.E. performed tetrode experiments. C.L.E. and G.D. performed juxtacellular experiments. G.D. and C.L. performed whole-cell recordings. C.L.E. and G.D. performed microstimulation and blockade experiments. C.L.E. analyzed the data and performed statistical modeling. C.L.E. and M.B. wrote the first version of the manuscript. All authors assisted with analyzing data and contributed to writing the manuscript.

Corresponding author

Correspondence to Michael Brecht.

Ethics declarations

Competing interests

The authors declare no competing financial interests.

Integrated supplementary information

Supplementary Figure 1 Whisker tracking procedure and additional modeling

(a)Example high-speed (250 frames/s) video frame showing whiskers of a head-fixed rat during a juxtacellular recording experiment. The pivot point (red dot) and the whisker tracking ROI (green dots) are manually clicked for tracking each video.

(b)Example traces demonstrating the tracking procedure. We rotated adjacent frames around the pivot point shown in (a) to maximize the correlation between the frames within the whisking ROI (‘Pearson’s ρ‘, top trace) and estimated the mean change in angles between adjacent frames (‘ΔAngle’, middle traces). Datapoints with sudden spikes in the correlation between frames due to video artifacts were removed from the traces (example marked by black arrow). To estimate the whisking angle, we linearly interpolated, numerically integrated and band-pass filtered the change in angle between frames (‘Angle’, bottom trace). Grey bar indicates a nose-to-nose touch.

(c)Top: Distribution of βAmpl for all cells is not different from zero (P = 0.204, Wilcoxon signed-rank test, also shown in Fig 3c). Bottom: When we plot only significant cells (assessed by a likelihood ratio test), the pattern is mixed: 10 cells are suppressed (red bars) and 6 cells are activated (blue bars). As a population, they are not different from zero (P = 0.098, Wilcoxon signed-rank test), but we note that the suppressed cells tend to be more strongly modulated that the activated cells: (median |βAmpl|= 0.221/0.128 for suppressed/activated cells, P = 0.00025, Mann-Whitney U-test).

(d)Soma of example juxtacellularly labeled Ctip2-negative cell.

(e)Example recorded data and fitted model from the neuron shown in (d). The top traces show the occurrence of nose-no-nose touches (grey bars), the juxtacellular recording trace with spikes (high-pass filtered at 300 Hz, top trace) and the whisker angle and whisking amplitude (tracked by high-speed videography). Below we show the estimate of the instantaneous firing rate of the best fitted model (green line, smoothed with a Gaussian with σ = 75 ms) plotted on top of an estimate of the observed firing rate (grey area, calculated by convolving the spike train with a Gaussian with σ = 75 ms, clipped at 10 Hz for plotting). This cell was suppressed by nose touch, whisker protraction and by increased whisking amplitude (maximum likelihood estimates: β0 = 0.03, βNose = −0.70, βAngle = −0.30 (°)-1, βAmpl = −0.38 (°)-1)

(f)Fitted betas, when we run the model shown in Fig 3 on stepwise orthogonalized data. In this model, β’Nose measures how the spike rate depends on nose touch, β’Angle measures how the spike rate depends on ‘the variation in whisker angle, which is orthogonal to variations in nose touch’, and β’Ampl measures how the spike rate depends on ‘the variation in whisking amplitude, which is orthogonal to variations in nose touch and variations in whisker angle’. Bars indicate median β, error bars indicate 95% confidence intervals of the median.

Supplementary Figure 2 Unilateral microstimulation of vibrissa motor cortex shortens social facial touch episodes

(a)Cumulative histograms of the duration of social facial interactions (from first to last whisker-to-whisker touch) on days with VMC microstimulation during interactions (red lines) and days with sham stimulation during interactions (black lines) for one example rat.

(b)Interactions are shorter with VMC microstimulation than during sham stimulation (N = 4 rats, dots indicate median interaction duration, lines indicate slope (with rat-specific intercept) from LME model, colors indicate rats).

Supplementary Figure 3 Unilateral blockade of vibrissa motor cortex (by AMPA & NMDA antagonists) increases contralateral whisker movement and protraction

(a)Example image of anaesthetized rat after unilateral VMC blockade (right hemisphere) by superfusion of APV (an NMDA antagonist) and NBQX (an AMPA antagonist).

(b)Example ipsilateral (blue) and contralateral (red) whisking traces of whisker micromovements, which escape light anaesthesia (Whisker arc 1). The contralateral whiskers are more protracted (~27° vs. ~32°) and the contralateral micromovements have a larger amplitude.

(c)After VMC blockade, the whisker set point is higher contralaterally (red markers) than ipsilaterally (blue markers) to the blocked hemisphere (N = 3 rats).

(d)After VMC blockade, the whisking power is much higher (~5 fold) in the contralateral whiskers than in the ipsilateral whiskers (Markers indicate ratio of contralateral to ipsilateral whisker power).

Supplementary Figure 4 Unilateral blockade of vibrissa motor cortex (by muscimol injection) increases contralateral whisker movement and protraction.

(a)Top: Example image of lightly anaesthetized mouse after unilateral VMC blockade (left hemisphere) by muscimol injection, showing protraction of contralateral whiskers. Bottom: Example whisking pattern from the same mouse showing large whisker movements contralaterally, and smaller whisker movements ipsilaterally.

(b)After VMC blockade, the whisker set point is higher contralaterally (red markers) than ipsilaterally (blue markers) to the blocked hemisphere (N = 4 mice). Round markers indicate that only deep VMC was blocked, square markers indicate that both deep and superficial VMC was blocked.

(c)After VMC blockade, the whisking power is much higher (~8 fold) in the contralateral whiskers than in the ipsilateral whiskers (Markers indicate ratio of contralateral to ipsilateral whisker power). Round markers indicate that only deep VMC was blocked, square markers indicate that both deep and superficial VMC was blocked.

(d)Example ipsilateral (blue) and contralateral (red) whisking traces of whisker micromovements which escape light anaesthesia (Whisker arc 1) in another mouse, showing the whisking patterns at a longer time scale.

Supplementary information

Rights and permissions

Reprints and permissions

About this article

Check for updates. Verify currency and authenticity via CrossMark

Cite this article

Ebbesen, C., Doron, G., Lenschow, C. et al. Vibrissa motor cortex activity suppresses contralateral whisking behavior. Nat Neurosci 20, 82–89 (2017). https://doi.org/10.1038/nn.4437

Download citation

  • Received:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI: https://doi.org/10.1038/nn.4437

This article is cited by

Search

Advanced search

Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing