科学学近期发表的论文

编译 | 李言

Science , 12 NOV 2021, VOL 374, ISSUE 6569

《科学》 2021年11月12日，第374卷，6569期

地球科学 Geoscience

Discovery of davemaoite, CaSiO3-perovskite, as a mineral from the lower mantle

下地幔中发现CaSiO3-钙钛矿

作者：OLIVER TSCHAUNER, SHICHUN HUANG, SHUYING YANG, MUNIR HUMAYUN et al.

链接：

摘要

下地幔中的硅酸盐钙钛矿CaSiO3具有重要的地球化学意义，因为它富集了上地幔中不相容的元素，包括产生热量的钍和铀，它们的半衰期比地球的地质 历史 还要长。

我们报告的CaSiO3-钙钛矿是一种已验证认可的矿物（IMA2020-012a），名称为毛钙硅石（davemaoite）。毛钙硅石天然标本证实了下地幔内部存在成分不均一性。

我们的观察表明，毛钙硅石在其结构中除了含有铀和钍外，还含有钾。因此，区域和全球毛钙硅石的丰度影响深部地幔的热收支，该矿物是热力学稳定的。

Abstract

Calcium silicate perovskite, CaSiO3, is arguably the most geochemically important phase in the lower mantle, because it concentrates elements that are incompatible in the upper mantle, including the heat-generating elements thorium and uranium, which have half-lives longer than the geologic history of Earth. We report CaSiO3-perovskite as an approved mineral (IMA2020-012a) with the name davemaoite. The natural specimen of davemaoite proves the existence of compositional heterogeneity within the lower mantle. Our observations indicate that davemaoite also hosts potassium in addition to uranium and thorium in its structure. Hence, the regional and global abundances of davemaoite influence the heat budget of the deep mantle, where the mineral is thermodynamically stable.

Age and composition of young basalts on the Moon, measured from samples returned by Chang’e-5

由嫦娥五号返回的样品测量月球上玄武岩的年龄和组成

作者：XIAOCHAO CHE, ALEXANDER NEMCHIN, DUNYI LIU, TAO LONGCHEN WANG et al.

链接：

摘要

轨道数据显示，月球上最年轻的火山是位于大洋州普罗塞拉姆的玄武岩熔岩，该地区具有高水平的产热元素钾、钍和铀。嫦娥五号任务收集了这些月球玄武岩样本，并将它们带回地球进行实验室分析。

我们认为这些熔岩的年龄为1963 5700万年，并确定了它们的化学和矿物组成。这个时间范围限制了太阳系内部的月球撞击年代学和月球的热演化。没有证据表明，在月球的地幔深处有高浓度的产热元素产生了这些熔岩，因此，对于月球岩浆活动的持久，还需要其他的解释。

Abstract

Orbital data indicate that the youngest volcanic units on the Moon are basalt lavas in Oceanus Procellarum, a region with high levels of the heat-producing elements potassium, thorium, and uranium. The Chang’e-5 mission collected samples of these young lunar basalts and returned them to Earth for laboratory analysis. We measure an age of 1963 57 million years for these lavas and determine their chemical and mineralogical compositions. This age constrains the lunar impact chronology of the inner Solar System and the thermal evolution of the Moon. There is no evidence for high concentrations of heat-producing elements in the deep mantle of the Moon that generated these lavas, so alternate explanations are required for the longevity of lunar magmatism.

化学 Chemistry

Methoxyl stable isotopic constraints on the origins and limits of coal-bed methane

甲氧基稳定同位素对煤层气成因和限制的制约

作者：M. K. LLOYD, E. TREMBATH-REICHERT, K. S. DAWSON, S. J. FEAKINS et al.

链接：

摘要

微生物煤层气是一种重要的经济资源，也是一种强效温室气体的来源，但人们对其形成的控制机制知之甚少。为了测试煤的微生物降解性是否限制微生物甲烷的生成，我们监测了从木材到烟煤成熟度范围内的全球样品组中的甲氧基去甲基反应——一种促进甲烷生成的反应。

残留甲氧基的碳同位素组成与热反应不一致。这表明，在地质时间尺度上，深层生物圈群落参与了植物到煤的转化，甲氧基丰度影响煤层气产量。微生物甲基化导致的碳同位素富集也解释了来自煤和常规烃矿床的微生物甲烷中碳-13含量的神秘抵消。

Abstract

Microbial coal-bed methane is an important economic resource and source of a potent greenhouse gas, but controls on its formation are poorly understood. To test whether the microbial degradability of coal limits microbial methane, we monitored methoxyl group demethylation—a reaction that feeds methanogenesis—in a global sample suite ranging in maturity from wood to bituminous coal. Carbon isotopic compositions of residual methoxyl groups were inconsistent with a thermal reaction, instead implying a substrate-limited biologic process. This suggests that deep biosphere communities participated in transforming plant matter to coal on geologic time scales and that methoxyl abundance influences coal-bed methane yield. Carbon isotopic enrichments resulting from microbial methylotrophy also explain an enigmatic offset in the carbon-13 content of microbial methane from coals and conventional hydrocarbon deposits.

Real-space imaging of anisotropic charge of σ-hole by means of Kelvin probe force microscopy

利用开尔文显微镜对σ-空穴各向异性电荷进行实空间成像

作者：B. MALLADA, A. GALLARDO, M. LAMANEC, B. DE LA TORRE, V. ŠPIRKO et al.

链接：

摘要

各向异性电荷在单个原子上的分布，如σ-空穴，可能会强烈地影响系统的材料和结构特性。然而，原子上这种各向异性电荷分布的空间分辨率是一个长期存在的实验挑战。特别是卤素原子上σ-空穴的存在，只能通过含卤素有机分子的晶体结构测定或理论计算间接证明，因而需要直接的实验可视化。

我们证明了开尔文显微镜与适当的功能化探针可以成像的各向异性电荷的σ-空穴和一氧化碳分子的四极电荷。这为表征各向异性原子电荷起决定性作用的生物和化学系统提供了一种新的方法。

Abstract

An anisotropic charge distribution on inpidual atoms, such as σ-holes, may strongly affect the material and structural properties of systems. However, the spatial resolution of such anisotropic charge distributions on an atom represents a long-standing experimental challenge. In particular, the existence of the σ-hole on halogen atoms has been demonstrated only indirectly through the determination of the crystal structures of organic molecules containing halogens or with theoretical calculations, consequently calling for its direct experimental visualization. We show that Kelvin probe force microscopy with a properly functionalized probe can image the anisotropic charge of the σ-hole and the quadrupolar charge of a carbon monoxide molecule. This opens a new way to characterize biological and chemical systems in which anisotropic atomic charges play a decisive role.

动物学 Zoology

Origins and evolution of extreme life span in Pacific Ocean rockfishes

太平洋岩鱼长寿的起源和进化

作者：SREE ROHIT RAJ KOLORA, XGREGORY L. OWENS, JUAN MANUEL VAZQUEZ , ALEXANDER STUBBS et al.

链接：

摘要

太平洋岩鱼（石鲈属）在寿命上表现出极大的差异，其中一些种类是现存最长寿的脊椎动物。我们重组了88种岩鱼的基因组，并从这些已确认的长寿类群DNA修复通路和137个长寿相关基因中收集了这些基因。

这些基因通过胰岛素信号直接影响寿命，并通过体型和环境适应产生多效效应。对结构变异的全基因组筛选揭示了长寿物种免疫调节butyrophilin基因家族的拷贝数明显增加。

不同岩鱼生活史的进化与遗传多样性耦合，并重塑突变谱，在长寿物种中分离CpG TpG变异。这些分析强调了生命 历史 特征适应的基础上的遗传创新，以及它们如何塑造基因组多样性。

Abstract

Pacific Ocean rockfishes (genus Sebastes ) exhibit extreme variation in life span, with some species being among the most long-lived extant vertebrates. We de novo assembled the genomes of 88 rockfish species and from these identified repeated signatures of positive selection in DNA repair pathways in long-lived taxa and 137 longevity-associated genes with direct effects on life span through insulin signaling and with pleiotropic effects through size and environmental adaptations. A genome-wide screen of structural variation reveals copy number expansions in the immune modulatory butyrophilin gene family in long-lived species. The evolution of different rockfish life histories is coupled to genetic persity and reshapes the mutational spectrum driving segregating CpG TpG variants in long-lived species. These analyses highlight the genetic innovations that underlie life history trait adaptations and, in turn, how they shape genomic persity.

公共卫生 Public Health

Monetary incentives increase COVID-19 vaccinations

货币激励增加了COVID-19疫苗接种率

作者：POL CAMPOS-MERCADE, ARMANDO N. MEIER, FLORIAN H. SCHNEIDER , STEPHAN MEIER et al.

链接：

摘要

低COVID-19疫苗接种率正在威胁公共卫生。为了提高疫苗接种率，世界各国政府都在考虑使用货币激励政策。在此，我们证实有保障的奖金对COVID-19疫苗接种的影响。我们在瑞典进行了一项大规模的预先注册随机对照试验（有8286名参与者），并将数据与全人群的行政接种记录联系起来。

我们发现，只需支付24美元（200瑞典克朗），疫苗接种率就从71.6%的基线接种率提高了4.2个百分点（P = 0.005）。相比之下，行为推动增加了接种疫苗的意愿，但对接种率的影响很小，在统计学上不显著。研究结果突出表明，适度的货币激励有可能提高疫苗接种率。

Abstract

The stalling of COVID-19 vaccination rates threatens public health. To increase vaccination rates, governments across the world are considering the use of monetary incentives. Here we present evidence about the effect of guaranteed payments on COVID-19 vaccination uptake. We ran a large preregistered randomized controlled trial (with 8286 participants) in Sweden and linked the data to population-wide administrative vaccination records. We found that modest monetary payments of 24 US dollars (200 Swedish kronor) increased vaccination rates by 4.2 percentage points (P = 0.005), from a baseline rate of 71.6%. By contrast, behavioral nudges increased stated intentions to become vaccinated but had only small and not statistically significant impacts on vaccination rates. The results highlight the potential of modest monetary incentives to raise vaccination rates.

编译 冯维维

Science , 23 July 2021, Vol 373, Issue 6553

《科学》 2021年7月23日，第373卷，6553期

天体物理学 Astrophysics

Peta–electron volt gamma-ray emission from the Crab Nebula

来自蟹状星云的高能光子

作者：The LHAASO Collaboration

链接：

摘要

蟹状星云含有一颗脉冲星，它能激发周围的气体释放出高能辐射。脉冲星的年轻及其附近的位置使得该星云成为天空中最亮的伽马射线源。

作者报道了这种源在1012到1015电子伏的能量下的观察结果，扩展了这种原型物体的光谱。

他们将这些数据与较低能量下的观测数据结合起来，以模拟辐射过程的物理过程。多波长数据可以用同步辐射和逆康普顿散射的结合来解释。

Abstract

The Crab Nebula contains a pulsar that excites the surrounding gas to emit high-energy radiation. The combination of the pulsar's youth and nearby location makes the nebula the brightest gamma-ray source in the sky. Cao et al. report observations of this source at energies of tera– to peta–electron volts, extending the spectrum of this prototypical object. They combine these data with observations at lower energies to model the physics of the emission process. The multiwave-length data can be explained by a combination of synchrotron radiation and inverse Compton scattering.

Upper mantle structure of Mars from InSight seismic data

“洞察号”地震数据揭示火星上地幔结构

作者：Amir Khan, Savas Ceylan, William B. Banerdt, etc.

链接：

摘要

两年来，“洞察号”着陆器一直在记录火星上的地震数据，这些数据对了解火星的结构和热化学状态至关重要。作者利用8个低频火星地震的直接（P和S）和地表反射（PP、PPP、SS和SSS）体波相的观测，将内部结构限制在800公里的深度。

他们发现了一种与低速带相容的结构，该低速带与热岩石圈比地球上厚得多，这可能与远震距离上的弱s波阴影带有关。通过结合地震约束和地球动力学模型，作者预测，相对于原始地幔，地壳的产热元素更丰富，是原始地幔的13 - 20倍。

这种富集比伽玛射线地表测绘所建议的要大，并且具有中等到高的地表热流。

Abstract

For 2 years, the InSight lander has been recording seismic data on Mars that are vital to constrain the structure and thermochemical state of the planet. We used observations of direct (P and S) and surface-reflected (PP, PPP, SS, and SSS) body-wave phases from eight low-frequency marsquakes to constrain the interior structure to a depth of 800 kilometers. We found a structure compatible with a low-velocity zone associated with a thermal lithosphere much thicker than on Earth that is possibly related to a weak S-wave shadow zone at teleseismic distances. By combining the seismic constraints with geodynamic models, we predict that, relative to the primitive mantle, the crust is more enriched in heat-producing elements by a factor of 13 to 20. This enrichment is greater than suggested by gamma-ray surface mapping and has a moderate-to-elevated surface heat flow.

Thickness and structure of the martian crust from InSight seismic data

“洞察号”地震数据揭示火星地壳厚度和结构

作者：Brigitte Knapmeyer-Endrun, Mark P. Panning, Felix Bissig, W. Bruce Banerdt, etc.

链接：

摘要

行星的地壳见证了行星的形成和演化 历史 ，但对于火星来说，还没有对地壳厚度的绝对测量。作者利用火星地震记录和周围波场确定了洞察号登陆点下方地壳的结构。

通过分析在地下界面反射和转换的地震相，他们发现观测结果与模型相符，至少有两个界面，也可能有三个界面。如果第2个界面为地壳边界，厚度为20 5公里；如果第3个界面为地壳边界，厚度为39 8公里。

全球重力和地形图允许对整个星球进行这种点测量，显示火星地壳的平均厚度在24到72公里之间。独立的体积组成和地球动力学约束表明，较厚的模型与观测到的浅层地壳产热元素的丰度一致，而较薄的模型则需要在深度上更大的浓度。

Abstract

A planet’s crust bears witness to the history of planetary formation and evolution, but for Mars, no absolute measurement of crustal thickness has been available. Here, we determine the structure of the crust beneath the InSight landing site on Mars using both marsquake recordings and the ambient wavefield. By analyzing seismic phases that are reflected and converted at subsurface interfaces, we find that the observations are consistent with models with at least two and possibly three interfaces. If the second interface is the boundary of the crust, the thickness is 20 5 kilometers, whereas if the third interface is the boundary, the thickness is 39 8 kilometers. Global maps of gravity and topography allow extrapolation of this point measurement to the whole planet, showing that the average thickness of the martian crust lies between 24 and 72 kilometers. Independent bulk composition and geodynamic constraints show that the thicker model is consistent with the abundances of crustal heat-producing elements observed for the shallow surface, whereas the thinner model requires greater concentration at depth.

Seismic detection of the martian core

火核的地震探测

作者：Simon C. Stähler, Amir Khan, W. Bruce Banerdt, Suzanne E. Smrekar

链接：

摘要

一颗行星的地质 历史 线索包含在它的内部结构中，尤其是它的核心。作者利用“洞察号”地震数据探测到来自火星核-地幔边界的地震波反射，并将其与大地测量数据进行反演，将液态金属核的半径限制在1830 40公里。

巨大的地核表明火星地幔在矿物学上类似于地球上的上地幔和过渡带，但与地球不同的是没有以桥曼石为主的下地幔。我们推断出核的平均密度为每立方厘米5.7至6.3克，这就需要大量的轻元素溶解在铁镍核中。

从洞察号的位置上看到的地震核心阴影覆盖了火星表面的一半，包括大多数潜在的活跃区域。这可能限制了可探测到的火星地震的数量。

Abstract

Clues to a planet’s geologic history are contained in its interior structure, particularly its core. We detected reflections of seismic waves from the core-mantle boundary of Mars using InSight seismic data and inverted these together with geodetic data to constrain the radius of the liquid metal core to 1830 40 kilometers. The large core implies a martian mantle mineralogically similar to the terrestrial upper mantle and transition zone but differing from Earth by not having a bridgmanite-dominated lower mantle. We inferred a mean core density of 5.7 to 6.3 grams per cubic centimeter, which requires a substantial complement of light elements dissolved in the iron-nickel core. The seismic core shadow as seen from InSight’s location covers half the surface of Mars, including the majority of potentially active regions—e.g., Tharsis—possibly limiting the number of detectable marsquakes.

粒子物理学 Particle Physics

Coherent manipulation of an Andreev spin qubit

安德烈夫自旋量子位的相干操纵

作者：M. Hays, V. Fatemi, D. Bouman, J. Cerrillo, S. Diamond, K. Serniak, T. Connolly, P. Krogstrup, J. Nygård, A. Levy Yeyati, A. Geresdi, M. H. Devoret

链接：

摘要

到目前为止，发展量子信息处理系统最有希望的固态方法是基于超导电路的循环超电流和操纵半导体量子点中电子的自旋特性。

作者结合了这两种方法的可取之处，超导电路的可扩展性和量子点的紧凑空间，来设计和制造超导自旋量子比特。

这个所谓的安德烈夫自旋量子位提供了开发一个新的量子信息处理平台的机会。

Abstract

To date, the most promising solid-state approaches for developing quantum information-processing systems have been based on the circulating supercurrents of superconducting circuits and manipulating the spin properties of electrons in semiconductor quantum dots. Hays et al. combined the desirable aspects of both approaches, the scalability of the superconducting circuits and the compact footprint of the quantum dots, to design and fabricate a superconducting spin qubit (see the Perspective by Wendin and Shumeiko). This so-called Andreev spin qubit provides the opportunity to develop a new quantum information processing platform.

Inhibited nonradiative decay at all exciton densities in monolayer semiconductors

在单层半导体的所有激子密度下抑制无辐射衰减

作者：Hyungjin Kim, Shiekh Zia Uddin, Naoki Higashitarumizu, Eran Rabani, Ali Javey

链接：

摘要

由于激子发生非辐射湮灭，单层过渡金属二卤族的光致发光量子产额在高发射强度下普遍下降。作者指出，这一过程在这些材料中通过它们的联合态密度的van Hove奇点共振放大。

然而，小的机械应变（约0.5%）的应用改变了van Hove奇点，抑制了非辐射过程。在剥离的硫化钼、硫化钨和硒化钨单分子层以及化学气相沉积的厘米级硫化钨单分子层中，都可以看到高激子密度下的近统一光致发光量子产额。

Abstract

The photoluminescence quantum yield in monolayer transition metal dichalcogenides generally drops at high emission intensities because the excitons undergo nonradiative annihilation. Kim et al. show that this process is resonantly amplified in these materials by van Hove singularities in their joint density of states. However, application of small mechanical strains ( 0.5%) shifted the van Hove singularities and suppressed the nonradiative processes. Near-unity photoluminescence quantum yield at high exciton densities was seen in exfoliated monolayers of molybdenum sulfide, tungsten sulfide, and tungsten selenide, as well as centimeter-scale tungsten sulfide monolayers grown by chemical vapor deposition.

Atomically resolved single-molecule triplet quenching

原子分辨单分子三态猝灭

作者：Jinbo Peng, Sophia Sokolov, Daniel Hernangómez-Pérez, Ferdinand Evers, Leo Gross4, John M. Lupton, Jascha Repp

链接：

摘要

分子的非平衡三重态在光催化、有机光伏和光动力治疗等方面发挥着重要作用。作者通过引入原子力显微镜中的电子泵浦探针方法，报告了直接测量单个并五苯分子在绝缘表面上的三重态寿命的原子分辨率。

如果氧分子在近距离内共吸附，可以观察到三重态寿命的强烈猝灭。通过单分子操作技术，与氧分子的不同排列被创造出来，并以原子精度表征，允许分子排列与淬灭三重态的寿命直接相关。

这种单分子长寿命三联体的电寻址，结合原子尺度的操作，为控制和研究局部自旋-自旋相互作用提供了以前未 探索 的途径。

Abstract

The nonequilibrium triplet state of molecules plays an important role in photocatalysis, organic photovoltaics, and photodynamic therapy. We report the direct measurement of the triplet lifetime of an inpidual pentacene molecule on an insulating surface with atomic resolution by introducing an electronic pump-probe method in atomic force microscopy. Strong quenching of the triplet lifetime is observed if oxygen molecules are coadsorbed in close proximity. By means of single-molecule manipulation techniques, different arrangements with oxygen molecules were created and characterized with atomic precision, allowing for the direct correlation of molecular arrangements with the lifetime of the quenched triplet. Such electrical addressing of long-lived triplets of single molecules, combined with atomic-scale manipulation, offers previously unexplored routes to control and study local spin-spin interactions.

生态学 Ecology

Innovation and geographic spread of a complex foraging culture in an urban parrot

城市鹦鹉复杂觅食文化的创新和地理传播

作者：Barbara C. Klump, John M. Martin, Sonja Wild, Jana K. Hörsch, Richard E. Major, Lucy M. Aplin

链接：

摘要

文化创新的出现、传播和建立可以促进对人类变化的适应性反应。作者描述了一个发展适应城市环境的文化的假定案例：野生硫冠凤头鹦鹉打开家庭垃圾箱。

社区科学报告的空间网络分析揭示了通过 社会 学习，鹦鹉打开从澳大利亚郊区的3个垃圾箱到悉尼市区44个垃圾箱的地理分布。对160个直接观察结果的分析揭示了个体风格和特定地点的差异。

作者描述了从创新的传播到地理变异的出现的完整路径，证明了鹦鹉的觅食文化，并表明了鹦鹉文化复杂性的存在。垃圾箱的打开与人类提供的机会直接相关，突出了文化促进对人类变化的行为反应的潜力。

Abstract

The emergence, spread, and establishment of innovations within cultures can promote adaptive responses to anthropogenic change. We describe a putative case of the development of a cultural adaptation to urban environments: opening of household waste bins by wild sulphur-crested cockatoos. A spatial network analysis of community science reports revealed the geographic spread of bin opening from three suburbs to 44 in Sydney, Australia, by means of social learning. Analysis of 160 direct observations revealed inpidual styles and site-specific differences. We describe a full pathway from the spread of innovation to emergence of geographic variation, evidencing foraging cultures in parrots and indicating the existence of cultural complexity in parrots. Bin opening is directly linked to human-provided opportunities, highlighting the potential for culture to facilitate behavioral responses to anthropogenic change.

生命科学 Life Science

Enterically derived high-density lipoprotein restrains liver injury through the portal vein

肠内高密度脂蛋白通过门静脉抑制肝损伤

作者：Yong-Hyun Han, Emily J. Onufer, Li-Hao Huang, Robert W. Sprung, W. Sean Davidson, Rafael S. Czepielewski, Mary Wohltmann, Mary G. Sorci-Thomas, Brad W. Warner, Gwendalyn J. Randolph

链接：

摘要

高密度脂蛋白（HDL）对胆固醇代谢很重要，可能具有抗炎和抗菌特性。虽然HDL主要由肝脏产生，但肠道也是一个来源。

作者在小鼠实验中发现，肠道高密度脂蛋白不会进入体循环。而是以HDL3的形式，通过肝门静脉直接输送到肝脏。在那里，它将细菌的脂多糖从肠道中隔离出来，这些脂多糖会引发炎症和肝脏损伤。

在各种肝损伤模型中，肠道高密度脂蛋白的丢失加重了病理。相比之下，提高肠道高密度脂蛋白的药物可改善疾病结果。HDL - 3在门静脉血液中富集，提示肠内HDL可能是治疗肝病的靶点。

Abstract

High-density lipoprotein (HDL) is important for cholesterol metabolism and may have anti-inflammatory and antimicrobial properties. Although HDL is mainly produced by the liver, the intestine is also a source. Han et al. show in mice that intestinal HDL is not routed to the systemic circulation. Rather, in the form of HDL3, it is directly transported to the liver through the hepatic portal vein. There, it sequesters bacterial lipopolysaccharide from the gut that can trigger inflammation and liver damage. In various models of liver injury, loss of enteric HDL exacerbated pathology. By contrast, drugs elevating intestinal HDL improved disease outcomes. HDL3 is enriched in human portal venous blood, suggesting that enteric HDL may be targetable for the treatment of liver disease.

Retinal waves prime visual motion detection by simulating future optic flow

视网膜波通过模拟未来光流实现视觉运动检测

作者：Xinxin Ge, Kathy Zhang, Alexandra Gribizis, Ali S. Hamodi, Aude Martinez Sabino, Michael C. Crair

链接：

摘要

当一只老鼠在森林的地面上向前奔跑时，它所经过的风景是向后流动的。作者的研究表明，发育中的小鼠视网膜提前实践了眼睛在小鼠移动时必须处理的东西。

视网膜活动的自发波与几天后在环境中实际运动时产生的波形相同。这种自发的模式活动改善了大脑上丘细胞的反应能力，上丘接收来自视网膜的神经信号以处理方向性信息。

Abstract

As a mouse runs forward across the forest floor, the scenery that it passes flows backwards. Ge et al. show that the developing mouse retina practices in advance for what the eyes must later process as the mouse moves. Spontaneous waves of retinal activity flow in the same pattern as would be produced days later by actual movement through the environment. This patterned, spontaneous activity refines the responsiveness of cells in the brain's superior colliculus, which receives neural signals from the retina to process directional information.