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Development of Cortical Circuits: Lessons from Ocular Dominance Columns

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Development of Cortical Circuits: Lessons from Ocular Dominance Columns REVIEWS DEVELOPMENT OF CORTICAL CIRCUITS: LESSONS FROM OCULAR DOMINANCE COLUMNS Lawrence C. Katz and Justin C. Crowley The development of ocular dominance columns has served as a Rosetta stone for understanding the mechanisms that guide the construction of cortical circuits. Traditionally, the emergence of ocular dominance columns was thought to be closely tied to the critical period, during which columnar architecture is highly susceptible to alterations in visual input. However, recent findings in cats, monkeys and ferrets indicate that columns develop far earlier, more rapidly and with considerably greater precision than was previously suspected. These observations indicate that the initial establishment of cortical functional architecture, and its subsequent plasticity during the critical period, are distinct developmental phases that might reflect distinct mechanisms. ORIENTATION COLUMNS Historically, neuronal development has been divided guided the interpretation of developmental events in Orientation tuning is a property into a sequence of events that leads from the initial spec- many other systems. Despite the powerful appeal of of visual cortical neurons that ification of neuronal cell fate to the eventual emergence this general model, and the experimental support that allows the detection of lines and of adult circuits1. In this formulation, many circuits accumulated over several decades, a number of recent edges within visual scenes by encoding their orientations. undergo a pivotal transition in which precise patterns of findings indicate that some of the assumptions under- Neurons that share the same synaptic connections emerge from an earlier stage of lying the conventional formulation might need to be orientation tuning are grouped more coarsely specified connections. Although the initial revised. Such revisions, in turn, indicate that alterna- into orientation columns. organization of neural circuits relies on a variety of mol- tive explanations for the patterning of connections ecular cues that guide axons to generally appropriate should be considered, and that the definition of and regions, the final specification of patterned connections evidence for ‘activity-dependent refinement’ requires is widely held to depend on patterns of neuronal activity, greater precision. generated either by circuits intrinsic to the developing brain or by early experience2–11. In particular, as postu- History of theories of column development lated by Hebb, correlations in presynaptic and post- Hubel and Wiesel initially described ocular dominance synaptic activity patterns strengthen and retain ‘correct’ columns in the early 1960s15. By making electrophysio- synapses, and eliminate ‘inappropriate’ connections. logical recordings in cat primary visual cortex, they In the central nervous system, and in the mam- noted that the two eyes differentially activated cortical Howard Hughes Medical malian neocortex in particular, much of this prolonged neurons (the physiological property of ocular domi- Institute and Department of Neurobiology, Box 3209, sculpting of neuronal connections is thought to occur nance). Cells with similar eye preference were grouped Duke University Medical during ‘critical periods’,when circuits are particularly together into columns, and eye dominance shifted Center, Durham, North susceptible to external sensory inputs. Such ideas orig- periodically across the cortex. On the basis of a few Carolina 27710, USA. inated from developmental studies of functional archi- recordings in very young, visually inexperienced cats, Correspondence to L.C.K. tecture in the mammalian visual cortex, especially the Hubel and Wiesel originally argued that ‘innate’ mech- e-mail: 12–14 [email protected] formation of ocular dominance columns (FIG. 1). anisms determined the organization of the cortex into DOI: 10.1038/nrn703 This highly influential body of work subsequently ocular dominance columns and ORIENTATION COLUMNS16 34 | JANUARY 2002 | VOLUME 3 www.nature.com/reviews/neuro REVIEWS Temporal Primary visual cortex layer 4 A substantial alteration in this formulation occurred retina when transneuronal transport of tritiated amino acids Ipsilateral (and later, sugars) — for example, 3H-proline — made it possible to directly visualize ocular dominance columns19–23. In adult monkeys19, injections of one eye Nasal retina Contralateral produced bands of transneuronally transported label that revealed the termination patterns of one eye’s rep- resentation in the cortex, alternating with dark bands, Nasal retina Ipsilateral nearly devoid of label, which corresponded to the LGN other eye’s thalamic input (FIG. 2a). Monocular depriva- tion during the postnatal critical period led to a Contralateral remarkable and satisfying correspondence between the Temporal electrophysiological loss and gain of responsiveness, retina and the shrinkage and expansion of eye-specific bands in layer 4 (REFS 22–24). Figure 1 | Segregation of eye-specific information at the early stages of visual processing. In mammals with binocular vision, the nasal portion of one retina encodes the same part of the The same approach was then applied to the develop- visual world as the temporal portion of the other retina. The axons of retinal ganglion cells from the ing visual system to determine how ocular dominance nasal portion of each retina cross the optic chiasm and project to the same lateral geniculate columns first formed22,23. Unlike in the adult, injections nucleus (LGN) as the axons from the temporal portion of the other eye. These projections form of amino acids into cats’ eyes before the onset of the criti- discrete, eye-specific LGN layers. The projection from the LGN to layer 4 of the primary visual cal period yielded a homogeneous band of label in layer 4, cortex maintains this eye-specific segregation by terminating in eye-specific patches that are the regardless of which eye was injected. Beginning at about anatomical basis for ocular dominance columns. Ocular dominance columns can therefore be considered to correspond to an eye (left or right) or a retinal location (nasal or temporal). 3 weeks after birth, and continuing over the next month, the adult pattern of segregated ocular dominance stripes gradually appeared. The timing of the emergence of the (‘innate’ was used interchangeably with ‘genetic’ in their adult-like pattern overlapped beautifully with the period early writings17). Monocular eye closure during the first when columns were susceptible to alterations of visual few months of life — the critical period — decreased experience13,14,25. Physiological evidence, based on sin- the numbers of cells activated by the closed eye and gle-unit recordings, also indicated a higher proportion markedly increased the number of neurons activated of binocular neurons at early ages, perhaps reflecting the exclusively by the open eye12,13,18. The initial descrip- greater degree of overlap of afferents representing the tions of the effects of eye closure during the critical two eyes23. In contrast to the original Hubel/Wiesel for- period indicated that pre-existing connections had sub- mulation, these observations indicated that the precise sequently been altered, through a competitive process, organization of columns in layer 4 was not innately to cause the loss of cells driven by the closed eye. In specified, but was gradually moulded by the same mech- their interpretations, Hubel and Wiesel clearly distin- anisms that guided columnar rearrangement during guished between the innate mechanisms that guide the the critical period — correlation-based synaptic com- initial formation of cortical functional architecture, and petition. Indeed, computer models based on initially the experience-dependent, competition-based mecha- overlapping inputs and differential activity patterns can nisms responsible for their later modification during produce columns with patterns strikingly similar to the critical period. those observed in vivo 3–5,8,9,26–28. Figure 2 | Early developmental organization of ocular dominance columns. Ocular dominance segregation in primates and carnivores precedes the onset of the critical period for ocular dominance column plasticity. a | Adult-like ocular dominance segregation occurs in the macaque monkey before birth. A surface view of radioactive proline labelling in layer 4 after injection of one eye shows clear alternating columns (alternating bright and dark bands) in an animal that had received no visual stimulation. b | Ocular dominance column segregation in the ferret occurs by postnatal day 16 (P16). In this coronal section from a P21 ferret (equivalent to a P0 cat), labelled axons form segregated patches in layer 4 of the visual cortex (pia is at the top). Scale bar, 500 µm. c,d | Ocular dominance columns form before the critical period in cat. Intrinsic signal optical imaging (c) and improved transneuronal transport methods (d) show ocular dominance segregation at P14; a reconstruction of the pattern in a P14 cat shows segregated columns at this early time. Part a reproduced with permission from REF.31 © 1996 Society for Neuroscience; part b reproduced with permission from REF.58 © 2000 American Association for the Advancement of Science; parts c and d reproduced with permission from REF.48 © 2001 John Wiley & Sons, Inc. NATURE REVIEWS | NEUROSCIENCE VOLUME 3 | JANUARY 2002 | 35 REVIEWS However, the carefully executed work of LeVay et al.23 waves seem to be crucial for the emergence of segregated revealed a complication of
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