Development and Organization of the Retinal Orientation Selectivity Map.

Orientation or axial selectivity, the property of neurons in the visual system to respond preferentially to certain angles of a visual stimuli, plays a pivotal role in our understanding of visual perception and information processing. This computation is performed as early as the retina, and although much work has established the cellular mechanisms of retinal orientation selectivity, how this computation is organized across the retina is unknown. Using a large dataset collected across the mouse retina, we demonstrate functional organization rules of retinal orientation selectivity. First, we identify three major functional classes of retinal cells that are orientation selective and match previous descriptions. Second, we show that one orientation is predominantly represented in the retina and that this predominant orientation changes as a function of retinal location. Third, we demonstrate that neural activity plays little role on the organization of retinal orientation selectivity. Lastly, we use in silico modeling followed by validation experiments to demonstrate that the overrepresented orientation aligns along concentric axes. These results demonstrate that, similar to direction selectivity, orientation selectivity is organized in a functional map as early as the retina. One Sentence Summary: Development and organization of retinal orientation selectivity.

Circuit mechanisms underlying embryonic retinal waves.

Spontaneous activity is a hallmark of developing neural systems. In the retina, spontaneous activity comes in the form of retinal waves, comprised of three stages persisting from embryonic day 16 (E16) to eye opening at postnatal day 14 (P14). Though postnatal retinal waves have been well characterized, little is known about the spatiotemporal properties or the mechanisms mediating embryonic retinal waves, designated stage 1 waves. Using a custom-built macroscope to record spontaneous calcium transients from whole embryonic retinas, we show that stage 1 waves are initiated at several locations across the retina and propagate across a broad range of areas. Blocking gap junctions reduced the frequency and size of stage 1 waves, nearly abolishing them. Global blockade of nAChRs similarly nearly abolished stage 1 waves. Thus, stage 1 waves are mediated by a complex circuitry involving subtypes of nAChRs and gap junctions. Stage 1 waves in mice lacking the ß2 subunit of the nAChRs (ß2-nAChR-KO) persisted with altered propagation properties and were abolished by a gap junction blocker. To assay the impact of stage 1 waves on retinal development, we compared the spatial distribution of a subtype of retinal ganglion cells, intrinsically photosensitive retinal ganglion cells (ipRGCs), which undergo a significant amount of cell death, in WT and ß2-nAChR-KO mice. We found that the developmental decrease in ipRGC density is preserved between WT and ß2-nAChR-KO mice, indicating that processes regulating ipRGC numbers and distributions are not influenced by spontaneous activity.

Sleep, plasticity, and sensory neurodevelopment.

A defining feature of early infancy is the immense neural plasticity that enables animals to develop a brain that is functionally integrated with a growing body. Early infancy is also defined as a period dominated by sleep. Here, we describe three conceptual frameworks that vary in terms of whether and how they incorporate sleep as a factor in the activity-dependent development of sensory and sensorimotor systems. The most widely accepted framework is exemplified by the visual system where retinal waves seemingly occur independent of sleep-wake states. An alternative framework is exemplified by the sensorimotor system where sensory feedback from sleep-specific movements activates the brain. We prefer a third framework that encompasses the first two but also captures the diverse ways in which sleep modulates activity-dependent development throughout the nervous system. Appreciation of the third framework will spur progress toward a more comprehensive and cohesive understanding of both typical and atypical neurodevelopment.

Roles of visually evoked and spontaneous activity in the development of retinal direction selectivity maps.

Detecting the direction of motion underlies many visually guided behaviors, from reflexive eye movements to identifying and catching moving objects. A subset of motion sensitive cells are direction selective - responding strongly to motion in one direction and weakly to motion in other directions. In mammals, direction-selective cells are found throughout the visual system, including the retina, superior colliculus, and primary visual cortex. Direction selectivity maps are well characterized in the mouse retina, where the preferred directions of retinal direction-selective cells follow the projections of optic flow, generated by the movements animals make as they navigate their environment. Here, we synthesize recent findings implicating activity-dependent mechanisms in the development of retinal direction selectivity maps, with primary focus on studies in mice, and discuss the implications for the development of direction-selective responses in downstream visual areas.

The Retinal Basis of Light Aversion in Neonatal Mice.

Aversive responses to bright light (photoaversion) require signaling from the eye to the brain. Melanopsin-expressing intrinsically photosensitive retinal ganglion cells (ipRGCs) encode absolute light intensity and are thought to provide the light signals for photoaversion. Consistent with this, neonatal mice exhibit photoaversion before the developmental onset of image vision, and melanopsin deletion abolishes photoaversion in neonates. It is not well understood how the population of ipRGCs, which constitutes multiple physiologically distinct types (denoted M1-M6 in mouse), encodes light stimuli to produce an aversive response. Here, we provide several lines of evidence that M1 ipRGCs that lack the Brn3b transcription factor drive photoaversion in neonatal mice. First, neonatal mice lacking TRPC6 and TRPC7 ion channels failed to turn away from bright light, while two photon Ca2+ imaging of their acutely isolated retinas revealed reduced photosensitivity in M1 ipRGCs, but not other ipRGC types. Second, mice in which all ipRGC types except for Brn3b-negative M1 ipRGCs are ablated exhibited normal photoaversion. Third, pharmacological blockade or genetic knockout of gap junction channels expressed by ipRGCs, which reduces the light sensitivity of M2-M6 ipRGCs in the neonatal retina, had small effects on photoaversion only at the brightest light intensities. Finally, M1s were not strongly depolarized by spontaneous retinal waves, a robust source of activity in the developing retina that depolarizes all other ipRGC types. M1s therefore constitute a separate information channel between the neonatal retina and brain that could ensure behavioral responses to light but not spontaneous retinal waves.SIGNIFICANCE STATEMENT At an early stage of development, before the maturation of photoreceptor input to the retina, neonatal mice exhibit photoaversion. On exposure to bright light, they turn away and emit ultrasonic vocalizations, a cue to their parents to return them to the nest. Neonatal photoaversion is mediated by intrinsically photosensitive retinal ganglion cells (ipRGCs), a small percentage of the retinal ganglion cell population that express the photopigment melanopsin and depolarize directly in response to light. This study shows that photoaversion is mediated by a subset of ipRGCs, called M1-ipRGCs. Moreover, M1-ipRGCs have reduced responses to retinal waves, providing a mechanism by which the mouse distinguishes light stimulation from developmental patterns of spontaneous activity.

The influence of spontaneous and visual activity on the development of direction selectivity maps in mouse retina.

In mice, retinal direction selectivity is organized in a map that aligns to the body and gravitational axes of optic flow, and little is known about how this map develops. We find direction selectivity maps are largely present at eye opening and develop normally in the absence of visual experience. Remarkably, in mice lacking the beta2 subunit of neuronal nicotinic acetylcholine receptors (ß2-nAChR-KO), which exhibit drastically reduced cholinergic retinal waves in the first postnatal week, selectivity to horizontal motion is absent while selectivity to vertical motion remains. We tested several possible mechanisms that could explain the loss of horizontal direction selectivity in ß2-nAChR-KO mice (wave propagation bias, FRMD7 expression, starburst amacrine cell morphology), but all were found to be intact when compared with WT mice. This work establishes a role for retinal waves in the development of asymmetric circuitry that mediates retinal direction selectivity via an unknown mechanism.

Light Prior to Eye Opening Promotes Retinal Waves and Eye-Specific Segregation.

Retinal waves are bursts of correlated activity that occur prior to eye opening and provide a critical source of activity that drives the refinement of retinofugal projections. Retinal waves are thought to be initiated spontaneously with their spatiotemporal features dictated by immature neural circuits. Here we demonstrate that, during the second postnatal week in mice, changes in light intensity dictate where and when a subset of retinal waves are triggered via activation of conventional photoreceptors. Propagation properties of triggered waves are indistinguishable from spontaneous waves, indicating that they are activating the same retinal circuits. Using whole-brain imaging techniques, we demonstrate that light deprivation prior to eye opening diminishes eye-specific segregation of the retinal projections to the dorsolateral geniculate nucleus of the thalamus, but not other retinal targets. These data indicate that light that passes through the closed eyelids plays a critical role in the development of the image-forming visual system.

Gating of reafference in the external cuneate nucleus during self-generated movements in wake but not sleep.

Nervous systems distinguish between self- and other-generated movements by monitoring discrepancies between planned and performed actions. To do so, corollary discharges are conveyed to sensory areas and gate expected reafference. Such gating is observed in neonatal rats during wake-related movements. In contrast, twitches, which are self-generated movements produced during active (or REM) sleep, differ from wake movements in that they reliably trigger robust neural activity. Accordingly, we hypothesized that the gating actions of corollary discharge are absent during twitching. Here, we identify the external cuneate nucleus (ECN), which processes sensory input from the forelimbs, as a site of movement-dependent sensory gating during wake. Whereas pharmacological disinhibition of the ECN unmasked wake-related reafference, twitch-related reafference was unaffected. This is the first demonstration of a neural comparator that is differentially engaged depending on the kind of movement produced. This mechanism explains how twitches, although self-generated, trigger abundant reafferent activation of sensorimotor circuits in the developing brain.

The Case of the Disappearing Spindle Burst.

Sleep spindles are brief cortical oscillations at 10-15 Hz that occur predominantly during non-REM (quiet) sleep in adult mammals and are thought to contribute to learning and memory. Spindle bursts are phenomenologically similar to sleep spindles, but they occur predominantly in early infancy and are triggered by peripheral sensory activity (e.g., by retinal waves); accordingly, spindle bursts are thought to organize neural networks in the developing brain and establish functional links with the sensory periphery. Whereas the spontaneous retinal waves that trigger spindle bursts in visual cortex are a transient feature of early development, the myoclonic twitches that drive spindle bursts in sensorimotor cortex persist into adulthood. Moreover, twitches-and their associated spindle bursts-occur exclusively during REM (active) sleep. Curiously, despite the persistence of twitching into adulthood, twitch-related spindle bursts have not been reported in adult sensorimotor cortex. This raises the question of whether such spindle burst activity does not occur in adulthood or, alternatively, occurs but has yet to be discovered. If twitch-related spindle bursts do occur in adults, they could contribute to the calibration, maintenance, and repair of sensorimotor systems.

A valuable and promising method for recording brain activity in behaving newborn rodents.

Neurophysiological recording of brain activity has been critically important to the field of neuroscience, but has contributed little to the field of developmental psychobiology. The reasons for this can be traced largely to methodological difficulties associated with recording neural activity in behaving newborn rats and mice. Over the last decade, however, the evolution of methods for recording from head-fixed newborns has heralded a new era in developmental neurophysiology. Here, we review these recent developments and provide a step-by-step primer for those interested in applying the head-fix method to their own research questions. Until now, this method has been used primarily to investigate spontaneous brain activity across sleep and wakefulness, the contributions of the sensory periphery to brain activity, or intrinsic network activity. Now, with some ingenuity, the uses of the head-fix method can be expanded to other domains to benefit our understanding of brain-behavior relations under normal and pathophysiological conditions across early development.

Myoclonic Twitching and Sleep-Dependent Plasticity in the Developing Sensorimotor System.

As bodies grow and change throughout early development and across the lifespan, animals must develop, refine, and maintain accurate sensorimotor maps. Here we review evidence that myoclonic twitches-brief and discrete contractions of the muscles, occurring exclusively during REM (or active) sleep, that result in jerks of the limbs-help animals map their ever-changing bodies by activating skeletal muscles to produce corresponding sensory feedback, or reafference. First, we highlight the spatiotemporal characteristics of twitches. Second, we review findings in infant rats regarding the multitude of brain areas that are activated by twitches during sleep. Third, we discuss evidence demonstrating that the sensorimotor processing of twitches is different from that of wake movements; this state-related difference in sensorimotor processing provides perhaps the strongest evidence yet that twitches are uniquely suited to drive certain aspects of sensorimotor development. Finally, we suggest that twitching may help inform our understanding of neurodevelopmental disorders, perhaps even providing opportunities for their early detection and treatment.

Self-generated movements with "unexpected" sensory consequences.

The nervous systems of diverse species, including worms and humans, possess mechanisms for distinguishing between sensations arising from self-generated (i.e., expected) movements from those arising from other-generated (i.e., unexpected) movements [1-3]. To make this critical distinction, animals generate copies, or corollary discharges, of motor commands [4, 5]. Corollary discharge facilitates the selective gating of reafferent signals arising from self-generated movements, thereby enhancing detection of novel stimuli [6-10]. However, for a developing nervous system, such sensory gating would be counterproductive if it impedes transmission of the very activity upon which activity-dependent mechanisms depend [11]. In infant rats during active (or REM) sleep--a behavioral state that predominates in early infancy [12-16]--neural circuits within the brainstem [17, 18] trigger hundreds of thousands of myoclonic twitches each day [19]. The putative contribution of these self-generated movements to the activity-dependent development of the sensorimotor system is supported by the observation that reafference from twitching limbs reliably and substantially triggers brain activity [20-23]. In contrast, under identical testing conditions, even the most vigorous wake movements reliably fail to trigger reafferent brain activity [21-23]. One hypothesis that accounts for this paradox is that twitches, uniquely among self-generated movements, lack corollary discharge [23]. Here, we test this hypothesis in newborn rats by manipulating the degree to which self-generated movements are expected and, therefore, their presumed recruitment of corollary discharge. We show that twitches, although self-generated, are processed as if they are unexpected.

Rapid whisker movements in sleeping newborn rats.

Spontaneous activity in the sensory periphery drives infant brain activity and is thought to contribute to the formation of retinotopic and somatotopic maps. In infant rats during active (or REM) sleep, brainstem-generated spontaneous activity triggers hundreds of thousands of skeletal muscle twitches each day; sensory feedback from the resulting limb movements is a primary activator of forebrain activity. The rodent whisker system, with its precise isomorphic mapping of individual whiskers to discrete brain areas, has been a key contributor to our understanding of somatotopic maps and developmental plasticity. But although whisker movements are controlled by dedicated skeletal muscles, spontaneous whisker activity has not been entertained as a contributing factor to the development of this system. Here we report in 3- to 6-day-old rats that whiskers twitch rapidly and asynchronously during active sleep; furthermore, neurons in whisker thalamus exhibit bursts of activity that are tightly associated with twitches but occur infrequently during waking. Finally, we observed barrel-specific cortical activity during periods of twitching. This is the first report of self-generated, sleep-related twitches in the developing whisker system, a sensorimotor system that is unique for the precision with which it can be experimentally manipulated. The discovery of whisker twitching will allow us to attain a better understanding of the contributions of peripheral sensory activity to somatosensory integration and plasticity in the developing nervous system.