A segregated cortical stream for retinal direction selectivity

Visual features extracted by retinal circuits are streamed into higher visual areas (HVAs) after being processed along the visual hierarchy. However, how specialized neuronal representations of HVAs are built, based on retinal output channels, remained unclear. Here, we addressed this question by determining the effects of genetically disrupting retinal direction selectivity on motion-evoked responses in visual stages from the retina to HVAs in mice. Direction-selective (DS) cells in the rostrolateral (RL) area that prefer higher temporal frequencies, and that change direction tuning bias as the temporal frequency of a stimulus increases, are selectively reduced upon retinal manipulation. DS cells in the primary visual cortex projecting to area RL, but not to the posteromedial area, were similarly affected. Therefore, the specific connectivity of cortico-cortical projection neurons routes feedforward signaling originating from retinal DS cells preferentially to area RL. We thus identify a cortical processing stream for motion computed in the retina.

Visual features are streamed into higher visual areas (HVAs), but how representations in HVAs are built, based on retinal output channels, is unknown. Here, the authors show that specific connectivity of cortical neurons routes retina-originated direction-selective signaling into distinct HVAs.

Rapid Recovery From Cortical Blindness Caused by an Old Cerebral Infarction

When the primary visual cortex (V1) is damaged, cortical blindness results. However, visual information obtained from the superior colliculus (SC) or direct thalamic afferents to higher visual cortices produces unconscious visual functions called blindsight. Alarming visual stimuli suggesting the approach of a predator are known to trigger escape behaviors via visual information mediated by the SC and amygdala in small animals, and salient and dynamic visual stimuli also produce some conscious visual experience even in patients with blindsight. Fresh cortical blindness sometimes recovers spontaneously in patients with fresh cerebral damages, and recovery can be accelerated by early rehabilitation. However, the mechanisms underlying recovery are not well-known. We analyzed a patient with cortical blindness caused by an old cerebral infarction. After repeated presentation of alarming visual stimuli, the ability to detect visual stimuli in the impaired visual field showed behavioral short-term improvement (STI) within a few minutes. Repeated behavioral STI induction was followed by behavioral long-term improvement (LTI) lasting more than several days. After behavioral LTI, the patient partially recovered the ability to read letters presented in the impaired visual field. The behavioral STI experiment, which can be performed within 10 min, may serve as a clinical screening test for anticipating recovery from cortical blindness.

The Evolution of the Pulvinar Complex in Primates and Its Role in the Dorsal and Ventral Streams of Cortical Processing

Current evidence supports the view that the visual pulvinar of primates consists of at least five nuclei, with two large nuclei, lateral pulvinar ventrolateral (PLvl) and central lateral nucleus of the inferior pulvinar (PIcl), contributing mainly to the ventral stream of cortical processing for perception, and three smaller nuclei, posterior nucleus of the inferior pulvinar (PIp), medial nucleus of the inferior pulvinar (PIm), and central medial nucleus of the inferior pulvinar (PIcm), projecting to dorsal stream visual areas for visually directed actions. In primates, both cortical streams are highly dependent on visual information distributed from primary visual cortex (V1). This area is so vital to vision that patients with V1 lesions are considered "cortically blind". When the V1 inputs to dorsal stream area middle temporal visual area (MT) are absent, other dorsal stream areas receive visual information relayed from the superior colliculus via PIp and PIcm, thereby preserving some dorsal stream functions, a phenomenon called "blind sight". Non-primate mammals do not have a dorsal stream area MT with V1 inputs, but superior colliculus inputs to temporal cortex can be more significant and more visual functions are preserved when V1 input is disrupted. The current review will discuss how the different visual streams, especially the dorsal stream, have changed during primate evolution and we propose which features are retained from the common ancestor of primates and their close relatives.

A Differential Circuit via Retino-Colliculo-Pulvinar Pathway Enhances Feature Selectivity in Visual Cortex through Surround Suppression

In the mammalian visual system, information from the retina streams into parallel bottom-up pathways. It remains unclear how these pathways interact to contribute to contextual modulation of visual cortical processing. By optogenetic inactivation and activation of mouse lateral posterior nucleus (LP) of thalamus, a homolog of pulvinar, or its projection to primary visual cortex (V1), we found that LP contributes to surround suppression of layer (L) 2/3 responses in V1 by driving L1 inhibitory neurons. This results in subtractive suppression of visual responses and an overall enhancement of orientation, direction, spatial, and size selectivity. Neurons in V1-projecting LP regions receive bottom-up input from the superior colliculus (SC) and respond preferably to non-patterned visual noise. The noise-dependent LP activity allows V1 to "cancel" noise effects and maintain its orientation selectivity under varying noise background. Thus, the retina-SC-LP-V1 pathway forms a differential circuit with the canonical retino-geniculate pathway to achieve context-dependent sharpening of visual representations.

Defined Cell Types in Superior Colliculus Make Distinct Contributions to Prey Capture Behavior in the Mouse

The superior colliculus (SC) plays a highly conserved role in visual processing and mediates visual orienting behaviors across species, including both overt motor orienting [1, 2] and orienting of attention [3, 4]. To determine the specific circuits within the superficial superior colliculus (sSC) that drive orienting and approach behavior toward appetitive stimuli, we explored the role of three genetically defined cell types in mediating prey capture in mice. Chemogenetic inactivation of two classically defined cell types, the wide-field (WF) and narrow-field (NF) vertical neurons, revealed that they are involved in distinct aspects of prey capture. WF neurons were required for rapid prey detection and distant approach initiation, whereas NF neurons were required for accurate orienting during pursuit as well as approach initiation and continuity. In contrast, prey capture did not require parvalbumin-expressing (PV) neurons that have previously been implicated in fear responses. The visual coding and projection targets of WF and NF cells were consistent with their roles in prey detection versus pursuit, respectively. Thus, our studies link specific neural circuit connectivity and function with stimulus detection and orienting behavior, providing insight into visuomotor and attentional mechanisms mediated by superior colliculus.

A projection specific logic to sampling visual inputs in mouse superior colliculus

Using sensory information to trigger different behaviors relies on circuits that pass through brain regions. The rules by which parallel inputs are routed to downstream targets are poorly understood. The superior colliculus mediates a set of innate behaviors, receiving input from >30 retinal ganglion cell types and projecting to behaviorally important targets including the pulvinar and parabigeminal nucleus. Combining transsynaptic circuit tracing with in vivo and ex vivo electrophysiological recordings, we observed a projection-specific logic where each collicular output pathway sampled a distinct set of retinal inputs. Neurons projecting to the pulvinar or the parabigeminal nucleus showed strongly biased sampling from four cell types each, while six others innervated both pathways. The visual response properties of retinal ganglion cells correlated well with those of their disynaptic targets. These findings open the possibility that projection-specific sampling of retinal inputs forms a basis for the selective triggering of behaviors by the superior colliculus.

Anatomy and Physiology of Neurons in Layer 9 of the Chicken Optic Tectum

Visual information in birds is to great extent processed in the optic tectum (TeO), a prominent laminated midbrain structure. Retinal input enters the TeO in its superficial layers, while output is limited to intermediate and deeper layers. In addition to visual information, the TeO receives multimodal input from the auditory and somatosensory pathway. The TeO gives rise to a major ascending tectofugal projection where neurons of tectal layer 13 project to the thalamic nucleus rotundus, which then projects to the entopallium. A second tectofugal projection system, called the accessory pathway, has however not been studied as thoroughly. Again, cells of tectal layer 13 form an ascending projection that targets a nucleus known as either the caudal part of the nucleus dorsolateralis posterior of the thalamus (DLPc) or nucleus uveaformis (Uva). This nucleus is known for multimodal integration and receives additional input from the lateral pontine nucleus (PL), which in turn receives projections from layer 8–15 of the TeO. Here, we studied a particular cell type afferent to the PL that consists of radially oriented neurons in layer 9. We characterized these neurons with respect to their anatomy, their retinal input, and the modulation of retinal input by local circuits. We found that comparable to other radial neurons in the tectum, cells of layer 9 have columnar dendritic fields and reach up to layer 2. Sholl analysis demonstrated that dendritic arborization concentrates on retinorecipient layers 2 and 4, with additional arborization in layers 9 and 10. All neurons recorded in layer 9 received retinal input via glutamatergic synapses. We analyzed the influence of modulatory circuits of the TeO by application of antagonists to γ-aminobutyric acid (GABA) and acetylcholine (ACh). Our data show that the neurons of layer 9 are integrated in a network under strong GABAergic inhibition, which is controlled by local cholinergic activation. Output to the PL and to the accessory tectofugal pathway thus appears to be under strict control of local tectal networks, the relevance of which for multimodal integration is discussed.

Prefrontal Corticotectal Neurons Enhance Visual Processing through the Superior Colliculus and Pulvinar Thalamus

Top-down modulation of visual processing is mediated in part by direct prefrontal to visual cortical projections. Here, we show that the mouse cingulate cortex (Cg) regulates visual processing not only through corticocortical neurons projecting to the visual cortex but also through corticotectal neurons projecting subcortically. Bidirectional optogenetic manipulation demonstrated a prominent contribution of Cg corticotectal neurons to visually guided behavior, which is mediated by their collateral projections to both the motor-related layers of the superior colliculus (SC) and the lateral posterior nucleus of the thalamus (LP, analogous to the primate pulvinar). Whereas the Cg innervates the anterior LP (LPa), the SC innervates the posterior LP (LPp). Activating each stage of the Cg→SC→LPp or the Cg→LPa pathway strongly enhanced visual performance of the mouse and the sensory responses of visual cortical neurons. These results delineate two subcortical pathways by which a subtype of prefrontal pyramidal neurons exerts a powerful top-down influence on visual processing. VIDEO ABSTRACT.

Spatial Clustering of Inhibition in Mouse Primary Visual Cortex

Whether mouse visual cortex contains orderly feature maps is debated. The overlapping pattern of geniculocortical inputs with M2 muscarinic acetylcholine receptor-rich patches in layer 1 (L1) suggests a non-random architecture. Here, we found that L1 inputs from the lateral posterior thalamus (LP) avoid patches and target interpatches. Channelrhodopsin-2-assisted mapping of excitatory postsynaptic currents (EPSCs) in L2/3 shows that the relative excitation of parvalbumin-expressing interneurons (PVs) and pyramidal neurons (PNs) by dLGN, LP, and cortical feedback is distinct and depends on whether the neurons reside in clusters aligned with patches or interpatches. Paired recordings from PVs and PNs show that unitary inhibitory postsynaptic currents (uIPSCs) are larger in interpatches than in patches. The spatial clustering of inhibition is matched by dense clustering of PV terminals in interpatches. The results show that the excitation/inhibition balance across V1 is organized into patch and interpatch subnetworks, which receive distinct long-range inputs and are specialized for the processing of distinct spatiotemporal features.

V1 microcircuits underlying mouse visual behavior

Visual behavior is based on the concerted activity of neurons in visual areas, where sensory signals are integrated with top-down information. In the past decade, the advent of new tools, such as functional imaging of populations of identified single neurons, high-density electrophysiology, virus-assisted circuit mapping, and precisely timed, cell-type specific manipulations, has advanced our understanding of the neuronal microcircuits underlying visual behavior. Studies in head-fixed mice, where such tools can routinely be applied, begin to provide new insights into the neural code of primary visual cortex (V1) underlying visual perception, and the micro-circuits of attention, predictive processing, and learning.

The Visual Cortex in Context

In this article, we review the anatomical inputs and outputs to the mouse primary visual cortex, area V1. Our survey of data from the Allen Institute Mouse Connectivity project indicates that mouse V1 is highly interconnected with both cortical and subcortical brain areas. This pattern of innervation allows for computations that depend on the state of the animal and on behavioral goals, which contrasts with simple feedforward, hierarchical models of visual processing. Thus, to have an accurate description of the function of V1 during mouse behavior, its involvement with the rest of the brain circuitry has to be considered. Finally, it remains an open question whether the primary visual cortex of higher mammals displays the same degree of sensorimotor integration in the early visual system.

Neurons and networks in the entorhinal cortex: A reappraisal of the lateral and medial entorhinal subdivisions mediating parallel cortical pathways

In this review, we aim to reappraise the organization of intrinsic and extrinsic networks of the entorhinal cortex with a focus on the concept of parallel cortical connectivity streams. The concept of two entorhinal areas, the lateral and medial entorhinal cortex, belonging to two parallel input-output streams mediating the encoding and storage of respectively what and where information hinges on the claim that a major component of their cortical connections is with the perirhinal cortex and postrhinal or parahippocampal cortex in, respectively, rodents or primates. In this scenario, the lateral entorhinal cortex and the perirhinal cortex are connectionally associated and likewise the postrhinal/parahippocampal cortex and the medial entorhinal cortex are partners. In contrast, here we argue that the connectivity matrix emphasizes the potential of substantial integration of cortical information through interactions between the two entorhinal subdivisions and between the perirhinal and postrhinal/parahippocampal cortices, but most importantly through a new observation that the postrhinal/parahippocampal cortex projects to both lateral and medial entorhinal cortex. We suggest that entorhinal inputs provide the hippocampus with high-order complex representations of the external environment, its stability, as well as apparent changes either as an inherent feature of a biological environment or as the result of navigating the environment. This thus indicates that the current connectional model of the parahippocampal region as part of the medial temporal lobe memory system needs to be revised.

The Foveal Visual Representation of the Primate Superior Colliculus

A defining feature of the primate visual system is its foveated nature. Processing of foveal retinal input is important not only for high-quality visual scene analysis but also for ensuring precise, albeit tiny, gaze shifts during high-acuity visual tasks. The representations of foveal retinal input in the primate lateral geniculate nucleus and early visual cortices have been characterized. However, how such representations translate into precise eye movements remains unclear. Here, we document functional and structural properties of the foveal visual representation of the midbrain superior colliculus. We show that the superior colliculus, classically associated with extra-foveal spatial representations needed for gaze shifts, is highly sensitive to visual input impinging on the fovea. The superior colliculus also represents such input in an orderly and very specific manner, and it magnifies the representation of foveal images in neural tissue as much as the primary visual cortex does. The primate superior colliculus contains a high-fidelity visual representation, with large foveal magnification, perfectly suited for active visuomotor control and perception.

TrkB Activation during a Critical Period Mimics the Protective Effects of Early Visual Experience on Perception and the Stability of Receptive Fields in Adult Superior Colliculus

During a critical period in development, spontaneous and evoked retinal activity shape visual pathways in an adaptive fashion. Interestingly, spontaneous activity is sufficient for spatial refinement of visual receptive fields (RFs) in superior colliculus (SC) and visual cortex (V1), but early visual experience is necessary to maintain inhibitory synapses and stabilize RFs in adulthood (Carrasco et al., 2005, 2011; Carrasco and Pallas, 2006; Balmer and Pallas, 2015a). In V1, BDNF and its high-affinity receptor TrkB are important for development of visual acuity, inhibition, and regulation of the critical period for ocular dominance plasticity (Hanover et al., 1999; Huang et al., 1999; Gianfranceschi et al., 2003). To examine the generality of this signaling pathway for visual system plasticity, the present study examined the role of TrkB signaling during the critical period for RF refinement in SC. Activating TrkB receptors during the critical period (P33-P40) in dark reared subjects produced normally refined RFs, and blocking TrkB receptors in light-exposed animals resulted in enlarged adult RFs like those in dark reared animals. We also report here that deprivation- or TrkB blockade-induced RF enlargement in adulthood impaired fear responses to looming overhead stimuli and negatively impacted visual acuity. Thus, early TrkB activation is both necessary and sufficient to maintain visual RF refinement, robust looming responses, and visual acuity in adulthood. These findings suggest a common signaling pathway exists for the maturation of inhibition between V1 and SC.SIGNIFICANCE STATEMENT Receptive field refinement in superior colliculus differs from more commonly studied examples of critical period plasticity in visual pathways in that it does not require visual experience to occur; rather, spontaneous activity is sufficient. Maintenance of refinement beyond puberty requires a brief, early exposure to light to stabilize the lateral inhibition that shapes receptive fields. We find that TrkB activation during a critical period can substitute for visual experience in maintaining receptive field refinement into adulthood, and that this maintenance is beneficial to visual survival behaviors. Thus, as in some other types of plasticity, TrkB signaling plays a crucial role in receptive field refinement.

Searching for illusory motion

In a series of four experiments, standard visual search was used to explore whether the onset of illusory motion pre-attentively guides vision in the same way that the onset of real-motion is known to do. Participants searched for target stimuli based on Akiyoshi Kitaoka's classic illusions, configured so that they either did or did not give the subjective impression of illusory motion. Distractor items always contained the same elements as target items, but did not convey a sense of illusory motion. When target items contained illusory motion, they popped-out, with flat search slopes that were independent of set size. Search for control items without illusory motion - but with identical structural differences to distractors - was slow and serial in nature (> 200 ms/item). Using a nulling task, we estimated the speed of illusory rotation in our displays to be approximately 2 °/s. Direct comparison of illusory and real-motion targets moving with matched velocity showed that illusory motion targets were detected more quickly. Blurred target items that conveyed a weak subjective impression of illusory motion gave rise to serial but faster (< 100 ms/item) search than control items. Our behavioral findings of parallel detection across the visual field, together with previous imaging and neurophysiological studies, suggests that relatively early cortical areas play a causal role in the perception of illusory motion. Furthermore, we hope to re-emphasize the way in which visual search can be used as a flexible, objective measure of illusion strength.