Abstract
Anterogradely labeled connections at the single-axon level provide unparalleled spatial and quantitative data as well as a novel perspective on laminar, columnar, hierarchical and other aspects of cortical organization. Here, I briefly summarize single-axon results from representative examples of thalamocortical, corticocortical, callosal, and lateral intrinsic connections, with attention to implications for cortical organization. Particularly worth emphasizing is the intricate spatial configuration and striking morphometric heterogeneity of individual axons even within the same system of connections. A short section touches on patterns of axonal trajectories in the distal, preterminal few millimeters. Emphasis is on studies in nonhuman primates from about 1983 to present, with non-viral tracers and 2-D reconstruction (i.e., compressed z-axis) in the early visual cortical pathway. The last section recapitulates what this approach can tell us about inter-areal communication and cortical organization, and possible implications for dynamics and effective connectivity, and concludes with comments on open questions and future directions.
Similar content being viewed by others
References
Abdeldim L, Matho KS, Clavreul S, Matho P, Sintes JM, Solinas X, Arganda-Carreras I, Turney SG, Lichtman JW et al (2019) Multicolor brain imaging with chromatic multi-photon serial microscopy. Nat Commun 10:1–14. https://doi.org/10.1038/s41467-019-09552-9
Anderson JC, Martin KA (2002) Connection from cortical area V2 to MT in the macaque monkey. J Comp Neurol 443:56–70
Anderson JC, Martin KA (2006) Synaptic connection from cortical area V4 to V2 in macaque monkey. J Comp Neurol 495:709–721
Anderson JC, Martin KAC (2009) The synaptic connections between cortical areas V1 and V2 in macaque monkey. J Neurosci 29:11283–11293. https://doi.org/10.1523/neurosci.5757-08.2009
Anderson JC, Binzegger T, Martin KA, Rockland KS (1998) The connection from cortical area V1 to V5: a light and electron microscopic study. J Neurosci 18:10525–10540
Barone P, Batardiere A, Knoblauch K, Kennedy H (2000) Laminar distribution of neurons in extrastriate areas projecting to areas V1 and V4 correlates with the hierarchical rank and indicates the operation of a distance rule. J Neurosci 20:3263–3281
Blasdel GG, Lund JS (1983) Termination of afferent axons in macaque striate cortex. J Neurosci 3:1389–1413
Borra E, Rockland KS (2011) Projections to early visual areas v1 and v2 in the calcarine fissure from parietal association areas in the macaque. Front Neuroanat 5:article 35. https://doi.org/10.3389/fnana.2011.00035
Bressoud R, Innocenti GM (1999) Typology, early differentiation, and exuberant growth of a set of cortical axons. J Comp Neurol 406:87–108
Buckner RL, DiNicola LM (2019) The brain’s default network: updated anatomy, physiology, and evolving insights. Nat Rev Neurosci 20:593–608. https://doi.org/10.1038/s41583-019-0212-7
Burkhalter A, Charles V (1990) Organization of local axon collaterals of efferent projection neurons in rat visual cortex. J Comp Neurol 302:920–934
Callaway EM (1992) Cell type specificity of local cortical connections. J Neurocytol 31:231–237
Cembrowsi MS, Spruston N (2019) Heterogeneity within the classical cell types is the rule: lessons from hippocampal pyramidal neurons. Nat Rev Neurosci 20:193–204. https://doi.org/10.1038/s41583-019-0125-5
Chen R, Wang F, Liang H, Li W (2017) Synergistic processing of visual contours across cortical layers in V1 and V2. Neuron 96:1388–1402. https://doi.org/10.1016/j.neuron.2017.11.004
Cheng K, Saleem KS, Tanaka K (1997) Organization of corticostriatal and corticoamygdalar projections arising from the anterior inferotemporal area TE of the macaque monkey: a Phaseolus vulgaris leucoagglutinin study. J Neurosci 17:7902–7925
Clasca F, Rubio-Garrido P, Jabaudon D (2012) Unveiling the diversity of thalamocortiocal neuron subtypes. Eur J Neursci 35:1524–1532. https://doi.org/10.1111/j.1460-9568.2012.08033.x
Clasca F, Porrero C, Galazo M, Rubio-Garrido P, Evangelio M (2016) Anatomy and development of multi-specific thalamocortical axons: implications for cortical dynamics and evolution. In: Rockland KS (ed) Axons and brain architecture. Elsevier, Amsterdam, pp 69–92. https://doi.org/10.1016/B978-0-12-801393-9.00004-9
DeFelipe J, Conley M, Jones EG (1986) Long-range focal collateralization of axons arising from cortiococortical cells in monkey sensory-motor cortex. J Neurosci 6:3749–3766
Ding SL, Van Hoesen G, Rockland KS (2000) Inferior parietal lobule projections to the presubiculum and neighboring ventromedial temporal cortical areas. J Comp Neurol 425:510–530
Drawitsch F, Karimi A, Boergens KM, Helmstaedter M (2018) FluoEM, virtual labeling of axons in three-dimensional electron microscopy data for long-range connectomics. Elife 14:7. https://doi.org/10.7554/eLife.38976
Economo MN, Winnubst J, Bas E, Ferreira TA, Chandrashekar J (2019) Singe-neuron axonal reconstruction: the search for a wiring diagram of the brain. J Comp Neurol 527:2190–2199. https://doi.org/10.1002/cne.24674
Erickson SL, Lewis DA (2004) Cortical connections of the lateral mediodorsal thalamus in cynomolgus monkeys. J Comp Neurol 473:107–127. https://doi.org/10.1002/cne.20084
Freund TF, Martin KA, Soltesz I, Somogyi P, Whitteridge D (1989) Arborisation pattern and postsynaptic targets of physiologically identified thalamocortical afferents in striate cortex of the macaque monkey. J Comp Neurol 289:315–336. https://doi.org/10.1002/cne.902890211
Gabbott PLA, Martin KAC, Whitteridge D (1987) Connections between pyramidal neurons in layer 5 of cat visual cortex (area 17). J Comp Neurol 259:364–381
Garraghty PE, Sur M (1990) Morphology of single intracellularly stained axons terminating in area 3b of macaque monkeys. J Comp Neurol 294:583–593
Gerfen CR, Sawchenko PE (1984) An anterograde neuroanatomical tracing method that shows the detailed morphology of neurons, their axons and terminals: Immunohistochemical localization of an axonally transported plant lectin, Phaseolus vulgaris-leucoagglutinin (PHA-L). Brain Res 290:219–238
Gilbert CD, Wiesel TN (1983) Clustered intrinsic connections in cat visual cortex. J Neurosci 3:1116–1133
Gilson M, Kouaris NE, Deco G, Mangin JF, Poupon C, Lefranc S, Riviere D, Zamora-lopez G (2019) Network analysis of whole-brain fMRI dynamics: a new framework based on dynamic communicability. Neuroimage. https://doi.org/10.1016/j.neuroimage.2019.116007
Goldman-Rakic PS, Schwartz ML (1982) Interdigitation of contralateral and ipsilateral columnar projections to frontal association cortex in primates. Science 216:755–757
Gong H, Zeng S, Yan C, Lv X, Yang Z, Xu T, Feng Z, Ding W, Qi X et al (2013) Continuously tracing brain-wide long-distance axonal projections in mice at a one-micron resolution. Neuroimage 74:87–98. https://doi.org/10.1016/j.neuroimage.2013.02.005
Han Y, Kebschull JM, Campbell RAA, Cowan D, Imhof F, Zador AM, Mrsic-Flogel TD (2018) The logic of single-cell projections from visual cortex. Nature 556:51–56. https://doi.org/10.1038/nature26159
Harris JA, Mihalas S, Hirokawa KE, Whitesell JD, Choi H, Bernard A, Bohn P et al (2019) Hierarchical organization of cortical and thalamic connectivity. Nature 575:195–202. https://doi.org/10.1038/s41586-019-1716-z
Hashikawa T, Molinari M, Rausell E, Jones EG (1995) Patchy and laminar terminations of medial geniculate axons in monkey auditory cortex. J Comp Neurol 362:195–208. https://doi.org/10.1002/cne.903620204
Houzel J-C, Milleret C, Innocenti G (1994) Morphology of callosal axons interconnecting areas 17 and 18 of the cat. Eur J Neurosci 6:898–917
Humphrey AL, Sur M, Uhlrich DJ, Sherman SM (1985a) Projection patterns of individual X- and Y-cell axons from the lateral geniculate nucleus to cortical area 17 in the cat. J Comp Neurol 233:159–189
Humphrey AL, Sur M, Uhlrich DJ, Sherman SM (1985b) Termination patterns of individual X- and Y-cell axons in the visual cortex of the cat: projections to area 18, to the 17/18 border region, and to both areas 17 and 18. J Comp Neurol 233:190–212
Ichinohe N, Matsushita A, Ohta K, Rockland KS (2010) Pathway-specific utilization of synaptic zinc in the macaque ventral visual cortical areas. Cereb Cortex 20:2818–2831. https://doi.org/10.1093/cercor/bhq028
Imura K, Rockland KS (2007) Giant neurons in the macaque pulvinar: a distinct relay subpopulation. Front Neuroanat 1:1–8. https://doi.org/10.3389/neuro.05.002.2007(article 2)
Innocenti GM, Caminiti R (2017) Axon diameter relates to synaptic bouton size: structural properties define computationally different types of cortical connections in primates. Brain Struct Funct 222:1169–1177. https://doi.org/10.1007/s00429-016-1266-1
Innocenti GM, Lehmann P, Houzel J-C (1994) Computational structure of visual callosal axons. Eur J Neurosci 6:918–935
Innocenti GM, Vercelli A, Caminiti R (2014) The diameter of cortical axons depends both on the area of origin and target. Cereb Cortex 24:2178–2188. https://doi.org/10.1093/cercor/bht070
Jones EG (2001) The thalamic matrix and thalamocortical synchrony. Trends Neurosci 24:595–601
Jones EG (2007) The thalamus. Cambridge University Press, Cambridge
Katz LC (1987) Local circuitry of identified projection neurons in cat visual cortex brain slices. J Neurosci 7:1223–1249
Kennedy H, Bullier J (1985) A double-labeling investigation of the afferent connectivity to cortical areas V1 and V2 of the macaque monkey. J Neurosci 5:2815–2830
Kita T, Kita H (2012) The subthalamic nucleus is one of multiple innervation sites for long-range corticofugal axons: a single axon tracing study in the rat. J Neurosci 32:5990–5999
Kobbert C, Apps R, Bechmann I, Lanciego JL, Mey J, Thanos S (2000) Current concepts in neuroanatomical tracing. Prog Neurobiol 62:327–351
Kuramoto E, Pan S, Tanaka YR, Iwai H, Yamanaka A, Ohno S, Kaneko T, Goto T, Hioki H (2017) Individual mediodorsal thalamic neurons project to multiple areas of the rat prefrontal cortex: a single neuron-tracing study using virus vectors. J Comp Neurol 525:166–185. https://doi.org/10.1002/cne.24054
Kuypers HG, Szwarcbart MK, Mishkin M, Rosvold HE (1965) Occipitotemporal corticocortical connections in the rhesus monkey. Exp Neurol 11:245–262
Levitt JB, Lewis DA, Yoshioka T, Lund JS (1993) Topography of pyramidal neuron intrinsic connections in macaque monkey prefrontal cortex (areas 9 and 46). J Comp Neurol 338:360–376
Levitt JB, Yoshioka T, Lund JS (1995) Connections between the pulvinar complex and cytochrome oxidase-defined compartments in visual area V2 of macaque monkey. Exp Brain Res 104:419–430
Liang H, Gong X, Chen M, Yan Y, Li W, Gilbert CD (2017) Interactions between feedback and lateral connection sin the primary visual cortex. Proc Natl Acad Sci USA 114:8637–8642. https://doi.org/10.1073/pnas.1706183114
Lin H-M, Kuang J-X, Sun P, Li N, Lv X, Zhang Y-H (2018) Reconstruction of intratelencephalic neurons in the mouse secondary motor cortex reveals the diverse projection patterns of single neurons. Front Neuroanat 12:86. https://doi.org/10.3389/fnana.2018.00086
Lu Y, Yin J, Chen Z, Gong H, Liu Y, Qian L, Li X, Liu R, Andolina IM, Wang W (2018) Revealing detail along the visual hierarchy: neural clustering preserves acuity from V1 to V4. Neuron 98:417–428. https://doi.org/10.1016/j.neuron.2018.03.009
Marion R, Li K, Purushothaman G, Jiang Y, Casagrande VA (2013) Morphological and neurochemical comparisons between pulvinar and V1 projections to V2. J Comp Neurol 521:813–832. https://doi.org/10.1002/cne.23203
Markov NT, Vezoli J, Chameau P, Falchier A, Quilodran R, Huissoud C, Lamy C, Misery P, Giroud P, Ullman S, Barone P, Dehay C, Knoblauch K, Kennedy H (2014) Anatomy of hierarchy: feedforward and feedback pathways in macaque visual cortex. J Comp Neurol 522:225–259. https://doi.org/10.1002/cne.23458
Martin KAC, Roth S, Rusch ES (2014) Superficial layer pyramidal cells communicate heterogeneously between multiple functional domains of cat primary visual cortex. Nat Commun 5:5252. https://doi.org/10.1038/ncomms6252
Martin KAC, Roth S, Rusch ES (2017) A biological blueprint for the axons of superficial layer pyramidal cell sin cat primary visual cortex. Brain Struct Funct 222:3407–34309. https://doi.org/10.1007/s00429-017-1410-6
Menzel M, Axer M, Amunts K, De Raedt H, Michielsen K (2019) Diattenuation imaging reveals different brain tissue properties. Sci Rep 9:1039. https://doi.org/10.1038/s41598-019-38506-w
Moore B, Li K, Kaas JH, Liao C-C, Boal AM, Mavity-Hudson J, Casagrande V (2018) Cortical projections to the two retinotopic maps of primate pulvinar are distinct. J Comp Neurol 527:577–588. https://doi.org/10.1002/cne.24515
Morecraft RJ, Ugolini G, Lanciego JL, Wouterlood FG, Pandya DN (2014) Classic and contemporary neural tract-tracing techniques. In: Johansen-Berg H, Behrens TEJ (eds) Diffusion MRI, 2nd edn. Elsevier, Amsterdam, pp 359–399. https://doi.org/10.1016/B978-0-12-396460-1.00017-2
Nassi JJ, Cepko CL, Born RT, Beier KT (2015) Neuroanatomy goes viral! Front Neuroanat 9:80. https://doi.org/10.3389/fnana.2015.00080
Negyessy L, Palfi E, Ashaber M, Palmer C, Jakli B, Friedman RM, Chen LM, Roe AW (2013) Intrinsic horizontal connections process global tactile features in the primary somatosensory cortex: neuroanatomical evidence. J Comp Neurol 521:2798–2817. https://doi.org/10.1002/cne.23317
Ojima H, Honda CN, Jones EG (1991) Patterns of axon collateralization of identified supragranular pyramidal neurons in the cat auditory cortex. Cereb Cortex 1:80–94
Ojima H, Honda CN, Jones EG (1992) Characteristics of intracellularly injected infragranular pyramidal neurons in cat primary auditory cortex. Cereb Cortex 2:197–216
Parent M, Parent A (2006) Single-axon tracing study of corticostriatal projections arising from primary motor cortex in primates. J Comp Neurol 496:202–213. https://doi.org/10.1002/cne.20925
Phillips JW, Schulmann A, Hara E, Winnubst J, Liu C, Valakh V, Wang L et al (2019) A repeated molecular architecture across thalamic pathways. Nat Neurosci 22:1925–1935. https://doi.org/10.1038/s41593-019-0483-3
Pucak ML, Levitt JB, Lund JS, Lewis DA (1996) Patterns of intrinsic and associational circuitry in monkey prefrontal cortex. J Comp Neurol 376:614–630
Reiner A, Veenman CL, Medina L, Jiao Y, Del Mr N, Honig MG (2000) Pathway tracing using biotinylated dextran amines. J Neurosci Methods 103:23–37
Rockland KS (1989) Bistratified distribution of terminal arbors of individual axons projecting from area V1 to middle temporal area (MT) in the macaque monkey. Vis Neurosci 3:155–170
Rockland KS (1992) Configuration, in serial reconstruction, of individual axons projecting from area V2 to V4 in the macaque monkey. Cereb Cortex 2:353–374
Rockland KS (1994) The organization of feedback connections from area V2 (18) to V1 (17). In: Peters A, Rockland KS (eds) Cerebral cortex primary visual cortex in primates, vol 10. Plenum Press, New York, pp 261–299
Rockland KS (1995) The morphology of individual axons projecting from area V2 to MT in the macaque. J Comp Neurol 355:15–26
Rockland KS (1997) Elements of cortical architecture: hierarchy revisited. In: Rockland KS, Kaas JH, Peters A (eds) Cerebral cortex extrastriate cortex in primates, vol 12. Plenum Press, New York, pp 243–293
Rockland KS (2002a) Non-uniformity of extrinsic connections and columnar organization. J Neurocytol 31:247–253
Rockland KS (2002b) Visual cortical organization at the single axon level: a beginning. Neurosci Res 42:155–166
Rockland KS (2018) White matter tracts visualized by parvalbumin in nonhuman primates, Chapter 10. In: Burke M, Ptito M (eds) Primates. IntechOpen, Rijeka, pp. 163–178. https://doi.org/10.5772/intechopen.70510
Rockland KS, Drash GW (1996) Collateralized divergent feedback connections that target multiple cortical areas. J Comp Neurol 373:529–548
Rockland KS, Knutson T (2000) Feedback connections from area MT of the squirrel monkey to areas V1 and V2. J Comp Neurol 425:345–368
Rockland KS, Knutson T (2001) Axon collaterals of Meynert cells diverge over large portions of area V1 in the macaque monkey. J Comp Neurol 441:134–147
Rockland KS, Lund JS (1983) Intrinsic laminar lattice connections in primate visual cortex. J Comp Neurol 216:303–318. https://doi.org/10.1002/cne.902160307
Rockland KS, Pandya DN (1979) Laminar origins and terminations of cortical connections of the occipital lobe in the rhesus monkey. Brain Res 179:3–20
Rockland KS, Virga A (1989) Terminal arbors of individual “feedback” axons projecting from area V2 to V1 in the macaque monkey: a study using immunohistochemistry of anterogradely transported Phaseolus vulgaris leucoagglutinin. J Comp Neurol 285:54–72. https://doi.org/10.1002/cne.902850106
Rockland KS, Virga A (1990) Organization of individual cortical axons projecting from area V1 (area 17) to area V2 (area 18) in the macaque monkey. Vis Neurosci 4:11–28
Rockland KS, Saleem KS, Tanaka K (1994) Divergent feedback connections from areas V4 and TEO in the macaque. Vis Neurosci 11:579–600. https://doi.org/10.1017/S0952523800002480
Rockland KS, Andresen J, Cowie RJ, Robinson DL (1999) Single axon analysis of pulvinocortical connections to several visual areas in the macaque. J Comp Neurol 406:221–250
Romanski LM, Giguere M, Bates JF, Goldman-Rakic PS (1997) Topographic organization of medial pulvinar connections with the prefrontal cortex in the rhesus monkey. J Comp Neurol 379:313–332
Ropireddy D, Scorcioni R, Lasher B, Buzsáki G, Ascoli GA (2011) Axonal morphometry of hippocampal pyramidal neurons semi-automatically reconstructed after in vivo labeling in different CA3 locations. Brain Struct Funct 216:1–15. https://doi.org/10.1007/s00429-010-0291-8
Sakaguchi R, Leiwe MN, Imai T (2018) Bright multicolor labeling of neuronal circuits with fluorescent proteins and chemical tags. eLife. https://doi.org/10.7554/eLife.40350
Saleem KS, Tanaka K, Rockland KS (1993) Specific and columnar projections from area TEO to TE in the macaque inferotemporal cortex. Cereb Cortex 3:454–464. https://doi.org/10.1093/cercor/3.5.454
Shipp S (2016) Neural elements for predictive coding. Front Psychol. https://doi.org/10.3389/fpsyg.2016.01792(article 1792)
Stettler DD, Das A, Bennett J, Gilbert CD (2002) Lateral connectivity and contextual interactions in macaque primary visual cortex. Neuron 36:739–750
Suzuki W, Saleem KS, Tanaka K (2000) Divergent backward projections from the anterior part of the inferotemporal cortex (area TE) in the macaque. J Comp Neurol 422:206–228
Tanigawa H, Wang Q, Fujita I (2005) Organization of horizontal axons in the inferior temporal cortex and primary visual cortex of the macaque monkey. Cereb Cortex 15:1887–1899
Thomson AM (2010) Neocortical layer 6, a review. Front Neuroanat. https://doi.org/10.3389/fnana.2010.00013(article 13)
Tomasi S, Caminiti R, Innocenti GM (2012) Areal differences in diameter and length of corticofugal projections. Cereb Cortex 22:1463–1472. https://doi.org/10.1093/cercor/bhs011
Van Kerkoerle T, Marik SA, zum Alten Borgloh SM, Gilbert CD (2018) Axonal plasticity associated with perceptual learning in adult macaque primary visual cortex. Proc Natl Acad Sci USA 115:10464–10469. https://doi.org/10.1073/pnas.1812932115
Vogt Weisenhorn DM, Illing R-B, Spatz WB (1995) Morphology and connections of neurons in area 17 projecting to extrastriate areas MT and 19DM and to the superior colliculus in the monkey Callitrix jacchus. J Comp Neurol 362:233–255. https://doi.org/10.1002/cne.903620207
Winnubst J, Bas E, Ferreira TA, Wu Z, Economo MN, Edson P, Arthur BJ, Bruns C et al (2019) Reconstruction of 1,000 projection neurons reveals new cell types and organization of long-range connectivity in the mouse brain. Cell 179:268–281. https://doi.org/10.1101/537233
Wouterlood FG, Bloem B, Mansvelder HD, Luchicchi A, Deisseroth K (2014) A fourth generation of neuroanatomical tracing techniques: exploiting the offspring of genetic engineering. J Neurosci Methods 235:331–348. https://doi.org/10.1016/j.jneumeth.2014.07.021
Yamashita T, Vavladeli A, Pala A, Galan K, Cirochet S, Petersen SSA, Petersen CCH (2018) Diverse long-range axonal projections of excitatory layer 2/3 neurons in mouse barrel cortex. Front Neuroanat. https://doi.org/10.3389/fnana.2018.00033(article 33)
Yarch J, Larsen H, Chen M, Angelucci A (2019) Morphological cell types projecting from V1 layer 4B to V2 thick and thin stripes. J Neurosci 39:7501–7512. https://doi.org/10.1523/JNEUROSCI.1096-19.2019
Zeineh MM, Palomero-Gallagher N, Axer M, Grassel D, Goubran M, Wree A, Woods R, Amunts K, Zilles K (2017) Direct visualization and mapping of the spatial course of fiber tracts at microscopic resolution in the human hippocampus. Cereb Cortex 27:1779–1794. https://doi.org/10.1093/cercor/bhw010
Zhang QF, Li H, Chen M, Guo A, Wen Y, Poo MM (2018) Functional organization of intrinsic and feedback presynaptic inputs in the primary visual cortex. Proc Natl Acad Sci USA 115:E5174–E5182. https://doi.org/10.1073/pnas.1719711115
Zhang Q, Lee WA, Paul DL, Ginty DD (2019) Multiplexed peroxidase-based electron microscopy labeling enables simultaneous visualization of multiple cell types. Nat Neurosci 22:828–839. https://doi.org/10.1038/s41593-019-0358-7
Zhong YM, Rockland KS (2003) Inferior parietal lobule projections to anterior inferotemporal cortex (area TE) in macaque monkey. Cereb Cortex 13:527–540. https://doi.org/10.1093/cercor/13.5.527
Acknowledgements
I thank Drs. Jennifer Luebke, Farzad Mortazavi, Mihovil Pletikos, and R. Jarrett Rushmore for reading the manuscript and for helpful discussion.
Funding
No funding has been received for the present work by the author(ksr). As this is a review paper, issues of informed consent and of treatment of human (not applicable) or animal subjects do not apply. Animal work had been approved at several relevant institutions (Boston University, U. of Iowa, or RIKEN Brain Science Institute) as stated in the original research papers. Credit has been explicitly given to any original research mentioned in the paper.
Author information
Authors and Affiliations
Corresponding author
Ethics declarations
Conflict of interest
The author declares no direct or indirect conflicts of interest with this work.
Additional information
Publisher's Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Rights and permissions
About this article
Cite this article
Rockland, K.S. What we can learn from the complex architecture of single axons. Brain Struct Funct 225, 1327–1347 (2020). https://doi.org/10.1007/s00429-019-02023-3
Received:
Accepted:
Published:
Issue Date:
DOI: https://doi.org/10.1007/s00429-019-02023-3