Intrinsic processing in the mammalian superior colliculus

Spatial and single-nucleus transcriptomics decoding the molecular landscape and cellular organization of avian optic tectum

The avian optic tectum (OT) has been studied for its diverse functions, yet a comprehensive molecular landscape at the cellular level has been lacking. In this study, we applied spatial transcriptome sequencing and single-nucleus RNA sequencing (snRNA-seq) to explore the cellular organization and molecular characteristics of the avian OT from two species: Columba livia and Taeniopygia guttata. We identified precise layer structures and provided comprehensive layer-specific signatures of avian OT. Furthermore, we elucidated diverse functions in different layers, with the stratum griseum periventriculare (SGP) potentially playing a key role in advanced functions of OT, like fear response and associative learning. We characterized detailed neuronal subtypes and identified a population of FOXG1+ excitatory neurons, resembling those found in the mouse neocortex, potentially involved in neocortex-related functions and expansion of avian OT. These findings could contribute to our understanding of the architecture of OT, shedding light on visual perception and multifunctional association.

Reward Modulates Visual Responses in the Superficial Superior Colliculus of Mice

The processing of sensory input is constantly adapting to behavioral demands and internal states. The drive to obtain reward, e.g., searching for water when thirsty, is a strong behavioral demand and associating the reward with its source, a certain environment or action, is paramount for survival. Here, we show that water reward increases subsequent visual activity in the superficial layers of the superior colliculus (SC), which receive direct input from the retina and belong to the earliest stages of visual processing. We trained mice of either sex to perform a visual decision task and recorded the activity of neurons in the SC using two-photon calcium imaging and high-density electrophysiological recordings. Responses to visual stimuli in around 20% of visually responsive neurons in the superficial SC were affected by reward delivered in the previous trial. Reward mostly increased visual responses independent from modulations due to pupil size changes. The modulation of visual responses by reward could not be explained by movements like licking. It was specific to responses to the following visual stimulus, independent of slow fluctuations in neural activity and independent of how often the stimulus was previously rewarded. Electrophysiological recordings confirmed these results and revealed that reward affected the early phase of the visual response around 80 ms after stimulus onset. Modulation of visual responses by reward, but not pupil size, significantly improved the performance of a population decoder to detect visual stimuli, indicating the relevance of reward modulation for the visual performance of the animal.SIGNIFICANCE STATEMENT To learn which actions lead to food, water, or safety, it is necessary to integrate the receiving of reward with sensory stimuli related to the reward. Cortical stages of sensory processing have been shown to represent stimulus-reward associations. Here, we show, however, that reward influences neurons at a much earlier stage of sensory processing, the superior colliculus (SC), receiving direct input from the retina. Visual responses were increased shortly after the animal received the water reward, which led to an improved stimulus signal in the population of these visual neurons. Reward modulation of early visual responses may thus improve perception of visual environments predictive of reward.

Role of Rostral Superior Colliculus in Gaze Stabilization during Visual Fixation

Visual fixation (i.e., holding gaze on a specific visual object or location of interest) has been shown to be influenced by activity in the rostral pole of the intermediate layers of the superior colliculus (SCi)-a sensory-motor integration nucleus in the midbrain involved in visual fixation and saccadic eye movement generation. Neurons in the rostral SCi discharge tonically during visual fixation and pause during saccades to locations beyond their foveal visual-sensory or saccadic-motor response fields. Injection of muscimol to deactivate rostral SCi neurons also leads to an increase in fixation instability. However, the precise role of rostral SCi activity for controlling visual fixation has not been established and is actively debated. Here, we address whether this activity reflects signals related to task demands (i.e., maintaining visual fixation) or foveal visual stimulus properties. Two non-human primates performed an oculomotor task that required fixation of a central fixation point (FP) of varying luminance at the start of each trial. During this fixation period, we measured fixational saccades (≤ 2° of the FP, including microsaccades) and fixation-error saccades (> 2° from the FP) in combination with activity from the rostral SCi. Fixation of the lowest FP luminance increased the latency (onset time relative to initial FP foveation) for both fixational and fixation-error saccades. Fifty percent of the rostral SCi neurons exhibited activity that opposed the change in FP luminance and correlated with delayed fixational saccades and increased fixation-error saccades. Twenty-two percent of rostral SCi neurons exhibited activity that followed the change in FP luminance and correlated with earlier fixational saccades and decreased fixation-error saccades. This suggests the rostral SCi contains both sensory-driven and task-related motor signals related to foveal sensory stimuli and visual fixation. This evidence supports a role for the rostral SCi in gaze stabilization and can help inform artificial computational models of vision.

The role of fixation disengagement and oculomotor preparation in gap saccade task is gap-duration dependent

The influence of internal brain state on behavioral performance is well illustrated by the gap-saccade task, in which saccades might be initiated with short latency (express saccade) or with long latency (regular saccade) even though the external visual condition is identical. Accumulated evidence has demonstrated that the internal brain state is different before the initiation of an express saccade than of a regular saccade. However, the reported origin of the fluctuation of internal brain state is disputed among previous studies, e.g., the fixation disengagement theory versus the oculomotor preparation theory. In the present study, we examined these two theories by analyzing the rate and direction of fixational saccades, i.e., small amplitude saccades during fixation period, because they could be modulated by internal brain state. Since fixation disengagement is not spatially tuned, it might affect the rate but not direction of fixational saccade. In contrast, oculomotor preparation can contain the spatial information for upcoming saccade, thus, it might have a distinct effect on fixational saccade direction. We found that the different spatiotemporal characteristics of fixational saccades among tasks with different gap durations reveals different driven force to change the internal brain state. Under short gap duration (100 ms), fixation disengagement plays a primary role in switching internal brain state. Conversely, under medium (200 ms) and long (400 ms) gap durations, oculomotor preparation plays a primary role. These results suggest that both fixation disengagement and oculomotor preparation can change the internal brain state, but their relative contributions are gap-duration dependent.

Active vision at the foveal scale in the primate superior colliculus

The primate superior colliculus (SC) has recently been shown to possess both a large foveal representation as well as a varied visual processing repertoire. This structure is also known to contribute to eye movement generation. Here, we describe our current understanding of how SC visual and movement-related signals interact within the realm of small eye movements associated with the foveal scale of visuomotor behavior. Within the SC's foveal representation, there is a full spectrum of visual, visual-motor, and motor-related discharge for fixational eye movements. Moreover, a substantial number of neurons only emit movement-related discharge when microsaccades are visually guided, but not when similar movements are generated toward a blank. This represents a particularly striking example of integrating vision and action at the foveal scale. Beyond that, SC visual responses themselves are strongly modulated, and in multiple ways, by the occurrence of small eye movements. Intriguingly, this impact can extend to eccentricities well beyond the fovea, causing both sensitivity enhancement and suppression in the periphery. Because of large foveal magnification of neural tissue, such long-range eccentricity effects are neurally warped into smaller differences in anatomical space, providing a structural means for linking peripheral and foveal visual modulations around fixational eye movements. Finally, even the retinal-image visual flows associated with tiny fixational eye movements are signaled fairly faithfully by peripheral SC neurons with relatively large receptive fields. These results demonstrate how studying active vision at the foveal scale represents an opportunity for understanding primate vision during natural behaviors involving ever-present foveating eye movements.NEW & NOTEWORTHY The primate superior colliculus (SC) is ideally suited for active vision at the foveal scale: it enables detailed foveal visual analysis by accurately driving small eye movements, and it also possesses a visual processing machinery that is sensitive to active eye movement behavior. Studying active vision at the foveal scale in the primate SC is informative for broader aspects of active perception, including the overt and covert processing of peripheral extra-foveal visual scene locations.

Properties of visually guided saccadic behavior and bottom-up attention in marmoset, macaque, and human

Saccades are stereotypic behaviors whose investigation improves our understanding of how primate brains implement precise motor control. Furthermore, saccades offer an important window into the cognitive and attentional state of the brain. Historically, saccade studies have largely relied on macaques. However, the cortical network giving rise to the saccadic command is difficult to study in macaques because relevant cortical areas lie in deep sulci and are difficult to access. Recently, a New World monkey. the marmoset, has garnered attention as an alternative to macaques because of advantages including its smooth cortical surface. However, adoption of the marmoset for oculomotor research has been limited due to a lack of in-depth descriptions of marmoset saccade kinematics and their ability to perform psychophysical tasks. Here, we directly compare free-viewing and visually guided behavior of marmoset, macaque, and human engaged in identical tasks under similar conditions. In the video free-viewing task, all species exhibited qualitatively similar saccade kinematics up to 25° in amplitude although with different parameters. Furthermore, the conventional bottom-up saliency model predicted gaze targets at similar rates for all species. We further verified their visually guided behavior by training them with step and gap saccade tasks. In the step paradigm, marmosets did not show shorter saccade reaction time for upward saccades whereas macaques and humans did. In the gap paradigm, all species showed similar gap effect and express saccades. Our results suggest that the marmoset can serve as a model for oculomotor, attentional, and cognitive research while we need to be aware of their difference from macaque or human.NEW & NOTEWORTHY We directly compared the results of a video free-viewing task and visually guided saccade tasks (step and gap) among three different species: marmoset, macaque, and human. We found that all species exhibit qualitatively similar saccadic kinematics and saliency-driven saccadic behavior albeit with different parameters. Our results suggest that the marmoset possesses similar neural mechanisms to macaque and human for saccadic control, and it is an appropriate model to study neural mechanisms for active vision and attention.

Factors Influencing Saccadic Reaction Time: Effect of Task Modality, Stimulus Saliency, Spatial Congruency of Stimuli, and Pupil Size

It is often assumed that the reaction time of a saccade toward visual and/or auditory stimuli reflects the sensitivities of our oculomotor-orienting system to stimulus saliency. Endogenous factors, as well as stimulus-related factors, would also affect the saccadic reaction time (SRT). However, it was not clear how these factors interact and to what extent visual and auditory-targeting saccades are accounted for by common mechanisms. The present study examined the effect of, and the interaction between, stimulus saliency and audiovisual spatial congruency on the SRT for visual- and for auditory-target conditions. We also analyzed pre-target pupil size to examine the relationship between saccade preparation and pupil size. Pupil size is considered to reflect arousal states coupling with locus-coeruleus (LC) activity during a cognitive task. The main findings were that (1) the pattern of the examined effects on the SRT varied between visual- and auditory-auditory target conditions, (2) the effect of stimulus saliency was significant for the visual-target condition, but not significant for the auditory-target condition, (3) Pupil velocity, not absolute pupil size, was sensitive to task set (i.e., visual-targeting saccade vs. auditory-targeting saccade), and (4) there was a significant correlation between the pre-saccade absolute pupil size and the SRTs for the visual-target condition but not for the auditory-target condition. The discrepancy between target modalities for the effect of pupil velocity and between the absolute pupil size and pupil velocity for the correlation with SRT may imply that the pupil effect for the visual-target condition was caused by a modality-specific link between pupil size modulation and the SC rather than by the LC-NE (locus coeruleus-norepinephrine) system. These results support the idea that different threshold mechanisms in the SC may be involved in the initiation of saccades toward visual and auditory targets.

Sensitivity of express saccades to the expected value of the target

Express saccades, a distinct fast mode of visually guided saccades, are probably underpinned by a specific pathway that is at least partially different from the one underlying regular saccades. Whether and how this pathway deals with information on the subjective value of a saccade target is unknown. We studied the influence of varying reward expectancies and compared it with the impact of a temporal gap between the disappearance of the fixation dot and the appearance of the target on the visually guided saccades of two rhesus macaques (Macaca mulatta). We found that increasing reward expectancy increased the probability and decreased the reaction time of express saccades. The latter influence was stronger in the later parts of the reaction time distribution of express saccades, satisfactorily captured by a linear shift model of change in the saccadic reaction time distribution. Although different in strength, increasing reward expectancy and inserting a temporal gap resulted in similar effects on saccadic reaction times, suggesting that these two factors summon the same mechanism to facilitate saccadic reaction times.NEW & NOTEWORTHY Express saccades are the fastest visually driven way of shifting gaze to targets of interest. We examined whether the pathway underlying these saccades has access to information on the value of saccade targets. We found that not only regular saccades but also express saccades occur earlier in case of higher expectations of reward. Yet, the sensitivity of express saccades to reward decreases linearly when approaching the earliest possible reaction time.

Identification of retinal ganglion cell types and brain nuclei expressing the transcription factor Brn3c/Pou4f3 using a Cre recombinase knock-in allele

Members of the POU4F/Brn3 transcription factor family have an established role in the development of retinal ganglion cell (RGCs) types, the main transducers of visual information from the mammalian eye to the brain. Our previous work using sparse random recombination of a conditional knock-in reporter allele expressing alkaline phosphatase (AP) and intersectional genetics had identified three types of Brn3c positive (Brn3c⁺ ) RGCs. Here, we describe a novel Brn3c^Cre mouse allele generated by serial Dre to Cre recombination and use it to explore the expression overlap of Brn3c with Brn3a and Brn3b and the dendritic arbor morphologies and visual stimulus response properties of Brn3c⁺ RGC types. Furthermore, we explore brain nuclei that express Brn3c or receive input from Brn3c⁺ neurons. Our analysis reveals a much larger number of Brn3c⁺ RGCs and more diverse set of RGC types than previously reported. Most RGCs expressing Brn3c during development are still Brn3c positive in the adult, and all express Brn3a while only about half express Brn3b. Genetic Brn3c-Brn3b intersection reveals an area of increased RGC density, extending from dorsotemporal to ventrolateral across the retina and overlapping with the mouse binocular field of view. In addition, we report a Brn3c⁺ RGC projection to the thalamic reticular nucleus, a visual nucleus that was not previously shown to receive retinal input. Furthermore, Brn3c⁺ neurons highlight a previously unknown subdivision of the deep mesencephalic nucleus. Thus, our newly generated allele provides novel biological insights into RGC type classification, brain connectivity, and cytoarchitectonic.

Spatiotemporal transformations for gaze control

Sensorimotor transformations require spatiotemporal coordination of signals, that is, through both time and space. For example, the gaze control system employs signals that are time-locked to various sensorimotor events, but the spatial content of these signals is difficult to assess during ordinary gaze shifts. In this review, we describe the various models and methods that have been devised to test this question, and their limitations. We then describe a new method that can (a) simultaneously test between all of these models during natural, head-unrestrained conditions, and (b) track the evolving spatial continuum from target (T) to future gaze coding (G, including errors) through time. We then summarize some applications of this technique, comparing spatiotemporal coding in the primate frontal eye field (FEF) and superior colliculus (SC). The results confirm that these areas preferentially encode eye-centered, effector-independent parameters, and show-for the first time in ordinary gaze shifts-a spatial transformation between visual and motor responses from T to G coding. We introduce a new set of spatial models (T-G continuum) that revealed task-dependent timing of this transformation: progressive during a memory delay between vision and action, and almost immediate without such a delay. We synthesize the results from our studies and supplement it with previous knowledge of anatomy and physiology to propose a conceptual model where cumulative transformation noise is realized as inaccuracies in gaze behavior. We conclude that the spatiotemporal transformation for gaze is both local (observed within and across neurons in a given area) and distributed (with common signals shared across remote but interconnected structures).

Modeling Saccadic Action Selection: Cortical and Basal Ganglia Signals Coalesce in the Superior Colliculus

The distributed nature of information processing in the brain creates a complex variety of decision making behavior. Likewise, computational models of saccadic decision making behavior are numerous and diverse. Here we present a generative model of saccadic action selection in the context of competitive decision making in the superior colliculus (SC) in order to investigate how independent neural signals may converge to interact and guide saccade selection, and to test if systematic variations can better replicate the variability in responses that are part of normal human behavior. The model was tasked with performing pro- and anti-saccades in order to replicate specific attributes of healthy human saccade behavior. Participants (ages 18-39) were instructed to either look toward (pro-saccade, well-practiced automated response) or away from (anti-saccade, combination of inhibitory and voluntary responses) a peripheral visual stimulus. They generated express and regular latency saccades in the pro-saccade task. In the anti-saccade task, correct reaction times were longer and participants occasionally looked at the stimulus (direction error) at either express or regular latencies. To gain a better understanding of the underlying neural processes that lead to saccadic action selection and response inhibition, we implemented 8 inputs inspired by systems neuroscience. These inputs reflected known sensory, automated, voluntary, and inhibitory components of cortical and basal ganglia activity that coalesces in the intermediate layers of the SC (SCi). The model produced bimodal reaction time distributions, where express and regular latency saccades had distinct modes, for both correct pro-saccades and direction errors in the anti-saccade task. Importantly, express and regular latency direction errors resulted from interactions of different inputs in the model. Express latency direction errors were due to a lack of pre-emptive fixation and inhibitory activity, which aloud sensory and automated inputs to initiate a stimulus-driven saccade. Regular latency errors occurred when the automated motor signals were stronger than the voluntary motor signals. While previous models have emulated fewer aspects of these behavioral findings, the focus of the simulations here is on the interaction of a wide variety of physiologically-based information integration producing a richer set of natural behavioral variability.

Reduced Frequency of Ipsilateral Express Saccades in Cervical Dystonia: Probing the Nigro-Tectal Pathway

Background

Cervical dystonia is a hyperkinetic movement disorder of unknown cause. Symptoms of cervical dystonia have been induced in animals in which the integrity of the nigro-tectal pathway is disrupted, resulting in reduced inhibition of the deep layers of the superior colliculus. This same pathway is believed to play a critical role in saccade generation, particularly visually guided, express saccades. It was hypothesized that individuals with cervical dystonia would present with a higher frequency of express saccades and more directional errors.

Methods

Eight individuals with cervical dystonia and 11 age- and sex-matched control participants performed three saccadic paradigms: pro-saccade, gap, and anti-saccade (120 trials per task). Eye movements were recorded using electro-oculography.

Results

Mean saccadic reaction times were slower in the cervical dystonia group (only statistically significant in the anti-saccade task, F(1, 35)  =  4.76, p  =  0.036); participants with cervical dystonia produced fewer directional errors (mean 14% vs. 22%) in the anti-saccade task; and had similar frequencies of express saccades in the gap task relative to our control population (chi-square  =  1.13, p  =  0.287). All cervical dystonia participants had lower frequencies of express saccades ipsilateral to their dystonic side (the side to which their head turns), (chi-square  =  3.57, p  =  0.059).

Discussion

The finding of slower saccadic reaction times in cervical dystonia does not support the concept of reduced inhibition in the nigro-tectal pathway. Further research is required to confirm the observed relationship between the lateralization of lower frequencies of express saccades and direction of head rotation in cervical dystonia.

Integrating Brain and Biomechanical Models-A New Paradigm for Understanding Neuro-muscular Control

To date, realistic models of how the central nervous system governs behavior have been restricted in scope to the brain, brainstem or spinal column, as if these existed as disembodied organs. Further, the model is often exercised in relation to an in vivo physiological experiment with input comprising an impulse, a periodic signal or constant activation, and output as a pattern of neural activity in one or more neural populations. Any link to behavior is inferred only indirectly via these activity patterns. We argue that to discover the principles of operation of neural systems, it is necessary to express their behavior in terms of physical movements of a realistic motor system, and to supply inputs that mimic sensory experience. To do this with confidence, we must connect our brain models to neuro-muscular models and provide relevant visual and proprioceptive feedback signals, thereby closing the loop of the simulation. This paper describes an effort to develop just such an integrated brain and biomechanical system using a number of pre-existing models. It describes a model of the saccadic oculomotor system incorporating a neuromuscular model of the eye and its six extraocular muscles. The position of the eye determines how illumination of a retinotopic input population projects information about the location of a saccade target into the system. A pre-existing saccadic burst generator model was incorporated into the system, which generated motoneuron activity patterns suitable for driving the biomechanical eye. The model was demonstrated to make accurate saccades to a target luminance under a set of environmental constraints. Challenges encountered in the development of this model showed the importance of this integrated modeling approach. Thus, we exposed shortcomings in individual model components which were only apparent when these were supplied with the more plausible inputs available in a closed loop design. Consequently we were able to suggest missing functionality which the system would require to reproduce more realistic behavior. The construction of such closed-loop animal models constitutes a new paradigm of computational neurobehavior and promises a more thoroughgoing approach to our understanding of the brain's function as a controller for movement and behavior.

Functional Organization and Dynamic Activity in the Superior Colliculus of the Echolocating Bat, Eptesicus fuscus

Sensory-guided behaviors require the transformation of sensory information into task-specific motor commands. Prior research on sensorimotor integration has emphasized visuomotor processes in the context of simplified orienting movements in controlled laboratory tasks rather than an animal's more complete, natural behavioral repertoire. Here, we conducted a series of neural recording experiments in the midbrain superior colliculus (SC) of echolocating bats engaged in a sonar target-tracking task that invoked dynamic active sensing behaviors. We hypothesized that SC activity in freely behaving animals would reveal dynamic shifts in neural firing patterns within and across sensory, sensorimotor, and premotor layers. We recorded neural activity in the SC of freely echolocating bats (three females and one male) and replicated the general trends reported in other species with sensory responses in the dorsal divisions and premotor activity in ventral divisions of the SC. However, within this coarse functional organization, we discovered that sensory and motor neurons are comingled within layers throughout the volume of the bat SC. In addition, as the bat increased pulse rate adaptively to increase resolution of the target location with closing distance, the activity of sensory and vocal premotor neurons changed such that auditory response times decreased, and vocal premotor lead times shortened. This finding demonstrates that SC activity can be modified dynamically in concert with adaptive behaviors and suggests that an integrated functional organization within SC laminae supports rapid and local integration of sensory and motor signals for natural, adaptive behaviors.SIGNIFICANCE STATEMENT Natural sensory-guided behaviors involve the rapid integration of information from the environment to direct flexible motor actions. The vast majority of research on sensorimotor integration has used artificial stimuli and simplified behaviors, leaving open questions about nervous system function in the context of natural tasks. Our work investigated mechanisms of dynamic sensorimotor feedback control by analyzing patterns of neural activity in the midbrain superior colliculus (SC) of an echolocating bat tracking and intercepting moving prey. Recordings revealed that sensory and motor neurons comingle within laminae of the SC to support rapid sensorimotor integration. Further, we discovered that neural activity in the bat SC changes with dynamic adaptations in the animal's echolocation behavior.

Prenatal cocaine exposure disrupts the dopaminergic system and its postnatal responses to cocaine

Impaired attention is the hallmark consequence of prenatal cocaine exposure (PCE), affecting brain development, learning, memory and social adaptation starting at an early age. To date, little is known about the brain structures and neurochemical processes involved in this effect. Through focusing on the visual system and employing zebrafish as a model, we show that PCE reduces expression of dopamine receptor Drd1, with levels reduced in the optic tectum and other brain regions, but not the telencephalon. Organism-wide, PCE results in a 1.7-fold reduction in the expression of the dopamine transporter (dat), at baseline. Acute cocaine administration leads to a 2-fold reduction in dat in drug-naive larvae but not PCE fish. PCE sensitizes animals to an anxiogenic-like behavioral effect of acute cocaine, bottom-dwelling, while loss of DAT due to genetic knockout (DATKO) leads to bottom-dwelling behavior at baseline. Neuronal calcium responses to visual stimuli in both PCE and DATKO fish show tolerance to acute cocaine in the principal regions of visual attention, the telencephalon and optic tectum. The zebrafish model can provide a sensitive assay by which to elucidate the molecular mechanisms and brain region-specific consequences of PCE, and facilitate the search for effective therapeutic solutions.

Circuits for Action and Cognition: A View from the Superior Colliculus

The superior colliculus is one of the most well-studied structures in the brain, and with each new report, its proposed role in behavior seems to increase in complexity. Forty years of evidence show that the colliculus is critical for reorienting an organism toward objects of interest. In monkeys, this involves saccadic eye movements. Recent work in the monkey colliculus and in the homologous optic tectum of the bird extends our understanding of the role of the colliculus in higher mental functions, such as attention and decision making. In this review, we highlight some of these recent results, as well as those capitalizing on circuit-based methodologies using transgenic mice models, to understand the contribution of the colliculus to attention and decision making. The wealth of information we have about the colliculus, together with new tools, provides a unique opportunity to obtain a detailed accounting of the neurons, circuits, and computations that underlie complex behavior.

Speeded saccadic and manual visuo-motor decisions: Distinct processes but same principles

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Core architecture of visuo-motor selection model generalises across effectors.

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Hand and eyes show very different response times, but similar decision times.

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Longer non-decision time for visuo-manual responses accounts for longer response times.

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Stronger faster transient visual inputs for saccades account for different selection dynamics.

Action decisions are considered an emergent property of competitive response activations. As such, decision mechanisms are embedded in, and therefore may differ between, different response modalities. Despite this, the saccadic eye movement system is often promoted as a model for all decisions, especially in the fields of electrophysiology and modelling. Other research traditions predominantly use manual button presses, which have different response distribution profiles and are initiated by different brain areas. Here we tested whether core concepts of action selection models (decision and non-decision times, integration of automatic and selective inputs to threshold, interference across response options, noise, etc.) generalise from saccadic to manual domains. Using two diagnostic phenomena, the remote distractor effect (RDE) and ‘saccadic inhibition', we find that manual responses are also sensitive to the interference of visual distractors but to a lesser extent than saccades and during a shorter time window. A biologically-inspired model (DINASAUR, based on non-linear input dynamics) can account for both saccadic and manual response distributions and accuracy by simply adjusting the balance and relative timings of transient and sustained inputs, and increasing the mean and variance of non-decisional delays for manual responses. This is consistent with known neurophysiological and anatomical differences between saccadic and manual networks. Thus core decision principles appear to generalise across effectors, consistent with previous work, but we also conclude that key quantitative differences underlie apparent qualitative differences in the literature, such as effects being robustly reported in one modality and unreliable in another.

Adaptation to size affects saccades with long but not short latencies

Maintained exposure to a specific stimulus property-such as size, color, or motion-induces perceptual adaptation aftereffects, usually in the opposite direction to that of the adaptor. Here we studied how adaptation to size affects perceived position and visually guided action (saccadic eye movements) to that position. Subjects saccaded to the border of a diamond-shaped object after adaptation to a smaller diamond shape. For saccades in the normal latency range, amplitudes decreased, consistent with saccading to a larger object. Short-latency saccades, however, tended to be affected less by the adaptation, suggesting that they were only partly triggered by a signal representing the illusory target position. We also tested size perception after adaptation, followed by a mask stimulus at the probe location after various delays. Similar size adaptation magnitudes were found for all probe-mask delays. In agreement with earlier studies, these results suggest that the duration of the saccade latency period determines the reference frame that codes the probe location.

Limitations of short range Mexican hat connection for driving target selection in a 2D neural field: activity suppression and deviation from input stimuli

Dynamic Neural Field models (DNF) often use a kernel of connection with short range excitation and long range inhibition. This organization has been suggested as a model for brain structures or for artificial systems involved in winner-take-all processes such as saliency localization, perceptual decision or target/action selection. A good example of such a DNF is the superior colliculus (SC), a key structure for eye movements. Recent results suggest that the superficial layers of the SC (SCs) exhibit relatively short range inhibition with a longer time constant than excitation. The aim of the present study was to further examine the properties of a DNF with such an inhibition pattern in the context of target selection. First we tested the effects of stimulus size and shape on when and where self-maintained clusters of firing neurons appeared, using three variants of the model. In each model variant, small stimuli led to rapid formation of a spiking cluster, a range of medium sizes led to the suppression of any activity on the network and hence to no target selection, while larger sizes led to delayed selection of multiple loci. Second, we tested the model with two stimuli separated by a varying distance. Again single, none, or multiple spiking clusters could occur, depending on distance and relative stimulus strength. For short distances, activity attracted toward the strongest stimulus, reminiscent of well-known behavioral data for saccadic eye movements, while for larger distances repulsion away from the second stimulus occurred. All these properties predicted by the model suggest that the SCs, or any other neural structure thought to implement a short range MH, is an imperfect winner-take-all system. Although, those properties call for systematic testing, the discussion gathers neurophysiological and behavioral data suggesting that such properties are indeed present in target selection for saccadic eye movements.

Prenatal and acute cocaine exposure affects neural responses and habituation to visual stimuli

Psychostimulants have many effects on visual function, from adverse following acute and prenatal exposure to therapeutic on attention deficit. To determine the impact of prenatal and acute cocaine exposure on visual processing, we studied neuronal responses to visual stimuli in two brain regions of a transgenic larval zebrafish expressing the calcium indicator GCaMP-HS. We found that both red light (LF) and dark (DF) flashes elicited similar responses in the optic tectum neuropil (TOn), while the dorsal telencephalon (dTe) responded only to LF. Acute cocaine (0.5 μM) reduced neuronal responses to LF in both brain regions but did not affect responses to DF. Repeated stimulus presentation (RSP) led to habituation of dTe neurons to LF. Acute cocaine prevented habituation. TOn habituated to DF, but not LF, and DF habituation was not modified by cocaine. Remarkably, prenatal cocaine exposure (PCE) prevented the effects of acute cocaine on LF response amplitude and habituation later in development in both brain regions, but did not affect DF responses. We discovered that, in spite of similar neural responses to LF and DF in the TO (superior colliculus in mammals), responses to LF are more complex, involving dTe (homologous to the cerebral cortex), and are more vulnerable to cocaine. Our results demonstrate that acute cocaine exposure affects visual processing differentially by brain region, and that PCE modifies zebrafish visual processing in multiple structures in a stimulus-dependent manner. These findings are in accordance with the major role that the optic tectum and cerebral cortex play in sustaining visual attention, and support the hypothesis that modification of these areas by PCE may be responsible for visual deficits noted in humans. This model offers new methodological approaches for studying the adverse and therapeutic effects of psychostimulants on attention, and for the development of new pharmacological interventions.

Linking express saccade occurance to stimulus properties and sensorimotor integration in the superior colliculus

Express saccades represent the fastest possible eye movements to visual targets with reaction times that approach minimum sensory-motor conduction delays. Previous work in monkeys has identified two specific neural signals in the superior colliculus (SC: a midbrain sensorimotor integration structure involved in gaze control) that are required to execute express saccades: 1) previsual activity consisting of a low-frequency increase in action potentials in sensory-motor neurons immediately before the arrival of a visual response; and 2) a transient visual-sensory response consisting of a high-frequency burst of action potentials in visually responsive neurons resulting from the appearance of a visual target stimulus. To better understand how these two neural signals interact to produce express saccades, we manipulated the arrival time and magnitude of visual responses in the SC by altering target luminance and we examined the corresponding influences on SC activity and express saccade generation. We recorded from saccade neurons with visual-, motor-, and previsual-related activity in the SC of monkeys performing the gap saccade task while target luminance was systematically varied between 0.001 and 42.5 cd/m(2) against a black background (∼0.0001 cd/m(2)). Our results demonstrated that 1) express saccade latencies were linked directly to the arrival time in the SC of visual responses produced by abruptly appearing visual stimuli; 2) express saccades were generated toward both dim and bright targets whenever sufficient previsual activity was present; and 3) target luminance altered the likelihood of producing an express saccade. When an express saccade was generated, visuomotor neurons increased their activity immediately before the arrival of the visual response in the SC and saccade initiation. Furthermore, the visual and motor responses of visuomotor neurons merged into a single burst of action potentials, while the visual response of visual-only neurons was unaffected. A linear combination model was used to test which SC signals best predicted the likelihood of producing an express saccade. In addition to visual response magnitude and previsual activity of saccade neurons, the model identified presaccadic activity (activity occurring during the 30-ms epoch immediately before saccade initiation) as a third important signal for predicting express saccades. We conclude that express saccades can be predicted by visual, previsual, and presaccadic signals recorded from visuomotor neurons in the intermediate layers of the SC.

Arm coordination in octopus crawling involves unique motor control strategies

To cope with the exceptional computational complexity that is involved in the control of its hyper-redundant arms [1], the octopus has adopted unique motor control strategies in which the central brain activates rather autonomous motor programs in the elaborated peripheral nervous system of the arms [2, 3]. How octopuses coordinate their eight long and flexible arms in locomotion is still unknown. Here, we present the first detailed kinematic analysis of octopus arm coordination in crawling. The results are surprising in several respects: (1) despite its bilaterally symmetrical body, the octopus can crawl in any direction relative to its body orientation; (2) body and crawling orientation are monotonically and independently controlled; and (3) contrasting known animal locomotion, octopus crawling lacks any apparent rhythmical patterns in limb coordination, suggesting a unique non-rhythmical output of the octopus central controller. We show that this uncommon maneuverability is derived from the radial symmetry of the arms around the body and the simple pushing-by-elongation mechanism by which the arms create the crawling thrust. These two together enable a mechanism whereby the central controller chooses in a moment-to-moment fashion which arms to recruit for pushing the body in an instantaneous direction. Our findings suggest that the soft molluscan body has affected in an embodied way [4, 5] the emergence of the adaptive motor behavior of the octopus.

Spatio-temporal response properties of local field potentials in the primate superior colliculus

Local field potentials (LFPs) are becoming increasingly popular in neurophysiological studies. However, to date, most of the knowledge about LFPs has been obtained from cortical recordings. Here, we recorded single unit activity (SUA) and LFPs simultaneously from the superior colliculus (SC) of behaving rhesus monkeys. The SC is a midbrain structure that plays a central role in the visual orienting response. Previous studies have characterised the visual and visuomotor response properties of SUA in the superficial layers of the SC and the intermediate layers of the SC, respectively. We found that the signal properties of SUA were well preserved in the LFPs recorded from the SC. The SUA and LFPs had similar spatial and temporal properties, and the response properties of LFPs differed across layers, i.e. purely visual in the superficial layers of the SC but showing significant motor responses in the intermediate layers of the SC. There were also differences between SUA and LFPs. LFPs showed a significant reversal of activity following the phasic visual response, suggesting that the neighboring neurons were suppressed. The results indicate that the LFP can be used as a reliable measure of the SC activity in lieu of SUA, and open up a new way to assess sensorimotor processing within the SC.

Whole-body prepulse inhibition protocol to test sensorymotor gating mechanisms in monkeys

Prepulse inhibition (PPI) is the decrease of startle reflex amplitude when a slight stimulus is previously generated. This paradigm may provide valuable information about sensorimotor gating functionality. Here we aimed at determining the inhibited and uninhibited startle response of capuchin monkeys (Sapajus spp.), and to evaluate the role of the superior colliculus in PPI. Capuchin monkeys were tested in a whole-body protocol, to determine the best startle amplitude and interstimuli interval. Additionally we tested two subjects with bilateral superior colliculus damage in this protocol. Results show that 115 dB auditory pulse has induced the best startle response. In contrast to reports in other species, no habituation to the auditory stimuli was observed here in capuchins. Also, startle reflex inhibition was optimal after 120 msec interstimuli interval. Finally, there was a downward tendency of percentage inhibition in superior colliculus-lesioned monkeys. Our data provides the possibility of further studies with whole-body protocol in capuchin monkeys and reinforces the importance of the superior colliculus in PPI.

Saccade learning with concurrent cortical and subcortical basal ganglia loops

The Basal Ganglia (BG) is a central structure involved in multiple cortical and subcortical loops. Some of these loops are believed to be responsible for saccade target selection. We study here how the very specific structural relationships of these saccadic loops can affect the ability of learning spatial and feature-based tasks. We propose a model of saccade generation with reinforcement learning capabilities based on our previous BG and superior colliculus models. It is structured around the interactions of two parallel cortico-basal loops and one tecto-basal loop. The two cortical loops separately deal with spatial and non-spatial information to select targets in a concurrent way. The subcortical loop is used to make the final target selection leading to the production of the saccade. These different loops may work in concert or disturb each other regarding reward maximization. Interactions between these loops and their learning capabilities are tested on different saccade tasks. The results show the ability of this model to correctly learn basic target selection based on different criteria (spatial or not). Moreover the model reproduces and explains training dependent express saccades toward targets based on a spatial criterion. Finally, the model predicts that in absence of prefrontal control, the spatial loop should dominate.

Cervical dystonia: a disorder of the midbrain network for covert attentional orienting

While the pathogenesis of cervical dystonia remains unknown, recent animal and clinical experimental studies have indicated its probable mechanisms. Abnormal temporal discrimination is a mediational endophenotype of cervical dystonia and informs new concepts of disease pathogenesis. Our hypothesis is that both abnormal temporal discrimination and cervical dystonia are due to a disorder of the midbrain network for covert attentional orienting caused by reduced gamma-aminobutyric acid (GABA) inhibition, resulting, in turn, from as yet undetermined, genetic mutations. Such disinhibition is (a) subclinically manifested by abnormal temporal discrimination due to prolonged duration firing of the visual sensory neurons in the superficial laminae of the superior colliculus and (b) clinically manifested by cervical dystonia due to disinhibited burst activity of the cephalomotor neurons of the intermediate and deep laminae of the superior colliculus. Abnormal temporal discrimination in unaffected first-degree relatives of patients with cervical dystonia represents a subclinical manifestation of defective GABA activity both within the superior colliculus and from the substantia nigra pars reticulata. A number of experiments are required to prove or disprove this hypothesis.

Neuronal responses to face-like and facial stimuli in the monkey superior colliculus

The superficial layers of the superior colliculus (sSC) appear to function as a subcortical visual pathway that bypasses the striate cortex for the rapid processing of coarse facial information. We investigated the responses of neurons in the monkey sSC during a delayed non-matching-to-sample (DNMS) task in which monkeys were required to discriminate among five categories of visual stimuli [photos of faces with different gaze directions, line drawings of faces, face-like patterns (three dark blobs on a bright oval), eye-like patterns, and simple geometric patterns]. Of the 605 sSC neurons recorded, 216 neurons responded to the visual stimuli. Among the stimuli, face-like patterns elicited responses with the shortest latencies. Low-pass filtering of the images did not influence the responses. However, scrambling of the images increased the responses in the late phase, and this was consistent with a feedback influence from upstream areas. A multidimensional scaling (MDS) analysis of the population data indicated that the sSC neurons could separately encode face-like patterns during the first 25-ms period after stimulus onset, and stimulus categorization developed in the next three 25-ms periods. The amount of stimulus information conveyed by the sSC neurons and the number of stimulus-differentiating neurons were consistently higher during the 2nd to 4th 25-ms periods than during the first 25-ms period. These results suggested that population activity of the sSC neurons preferentially filtered face-like patterns with short latencies to allow for the rapid processing of coarse facial information and developed categorization of the stimuli in later phases through feedback from upstream areas.

Changes in Otx2 and parvalbumin immunoreactivity in the superior colliculus in the platelet-derived growth factor receptor-β knockout mice

The superior colliculus (SC), a relay nucleus in the subcortical visual pathways, is implicated in socioemotional behaviors. Homeoprotein Otx2 and β subunit of receptors of platelet-derived growth factor (PDGFR-β) have been suggested to play an important role in development of the visual system and development and maturation of GABAergic neurons. Although PDGFR-β-knockout (KO) mice displayed socio-emotional deficits associated with parvalbumin (PV-)immunoreactive (IR) neurons, their anatomical bases in the SC were unknown. In the present study, Otx2 and PV-immunolabeling in the adult mouse SC were investigated in the PDGFR-β KO mice. Although there were no differences in distribution patterns of Otx2 and PV-IR cells between the wild type and PDGFR-β KO mice, the mean numbers of both of the Otx2- and PV-IR cells were significantly reduced in the PDGFR-β KO mice. Furthermore, average diameters of Otx2- and PV-IR cells were significantly reduced in the PDGFR-β KO mice. These findings suggest that PDGFR-β plays a critical role in the functional development of the SC through its effects on Otx2- and PV-IR cells, provided specific roles of Otx2 protein and PV-IR cells in the development of SC neurons and visual information processing, respectively.

Parietal cortical neuronal activity is selective for express saccades

Saccadic eye movements are central to primate behavior and serve to move the eyes to visual objects of interest. Express saccades, unlike regular saccades, occur with very short reaction times, a behavior necessary for speeded reactions in goal-directed behavior. Previous studies have shown that introduction of a blank interval (gap) between the fixation point offset and the saccadic target onset leads to an increase in the number of express saccades and that the superior colliculus plays a crucial role in the generation of express saccades. A longstanding hypothesis asserted that express saccades are mediated largely by a subcortical circuit, circumventing extrastriate visual cortex. An alternative "posterior pathway" hypothesis proposed the involvement of posterior parietal cortex. In the present study, using a gap saccade task, we investigated the role of nonhuman primate's lateral intraparietal cortex (LIP) in generation of express saccades. We show that roughly half of recorded LIP neurons were modulated during the gap interval. Moreover, a group of neurons with persistent activity in a memory-guided saccade task enhanced their activity during express saccades relative to that during regular saccades. After reducing the target's certainty by increasing the potential target locations, neuronal activity remained in the similar level during express saccades but markedly reduced during regular saccades that correlated with the increase of saccadic reaction time in the regular saccade. Our results suggest that area LIP is directly involved in generating saccades in express mode.

Cholinergic responses in GABAergic and non-GABAergic neurons in the intermediate gray layer of mouse superior colliculus

Neurons in the intermediate gray layer (SGI) of the mammalian superior colliculus (SC) receive dense cholinergic innervations from the brainstem parabrachial region. Such cholinergic inputs may influence execution of orienting behaviors. To obtain deeper insights into how the cholinergic inputs modulate the SC local circuits, we analysed the cholinergic responses in identified γ-aminobutyric acid (GABA)ergic and non-GABAergic neurons using SC slices obtained from GAD67-GFP knock-in mice. The responses of SGI neurons to cholinergic agonists were various combinations of fast inward currents mediated mainly via α4β2 and partly by α7 nicotinic receptors (nIN), slow inward currents caused by activation of M1 plus M3 muscarinic receptors (mIN), and slow outward currents caused by activation of M2 muscarinic receptors (mOUT). The most common cholinergic responses in non-GABAergic neurons was nIN + mIN + mOUT (38/68), followed by nIN + mIN (16/68), nIN + mOUT (11/68), nIN only (2/68), and no response (1/68). On the other hand, the major response pattern in GABAergic neurons was either nIN only (26/54) or nIN + mIN (21/54), followed by nIN + mOUT (4/54), mOUT only (2/54), and no response (1/54). Thus, major effects of cholinergic inputs to both SGI GABAergic and non-GABAergic neurons are excitatory, but the response patterns in these two types of SGI neurons are different. Thus, actions of the cholinergic inputs to non-GABAergic and GABAergic SGI neurons are not simple push-pull mechanisms, like excitation vs inhibition, but might cooperate to balance the level of excitation and inhibition for setting the state of the response property of the local circuit.

Covert attention regulates saccadic reaction time by routing between different visual-oculomotor pathways

Covert attention modulates saccadic performance, e.g., the abrupt onset of a task-irrelevant visual stimulus grabs attention as measured by a decrease in saccadic reaction time (SRT). The attentional advantage bestowed by the task-irrelevant stimulus is short-lived: SRT is actually longer ∼200 ms after the onset of a stimulus than it is when no stimulus appears, known as inhibition of return. The mechanism by which attention modulates saccadic reaction is not well-understood. Here, we propose two possible mechanisms: by selective routing of the visuomotor signal through different pathways (routing hypothesis) or by general modulation of the speed of visuomotor transformation (shifting hypothesis). To test them, we designed a cue gap paradigm in which a 100-ms gap was introduced between the fixation point disappearance and the target appearance to the conventional cued visual reaction time paradigm. The cue manipulated the location of covert attention, and the gap interval resulted in a bimodal distribution of SRT, with an early mode (express saccade) and a late mode (regular saccade). The routing hypothesis predicts changes in the proportion of express saccades vs. regular saccades, whereas the shifting hypothesis predicts a shift of SRT distribution. The addition of the cue had no effect on mean reaction time of express and regular saccades, but it changed the relative proportion of two modes. These results demonstrate that the covert attention modification of the mean SRT is largely attributed to selective routing between visuomotor pathways rather than general modulation of the speed of visuomotor transformation.

Cortico-basal ganglia mechanisms for overcoming innate, habitual and motivational behaviors

Most of the human behaviors are executed automatically under familiar circumstances. These behaviors are prepotent in that they take precedence over any other potential alternatives. Yet, humans are also capable of engaging cognitive resources to inhibit such a prepotent behavior and replace it with an alternative controlled behavior in response to an unforeseen situation. This remarkable capability to switch behaviors in a short period of time is the hallmark of executive functions. In this article, we first argue that the prepotent automaticity could emerge at least in three different domains - innate, habitual and motivational. We then review neurophysiological findings on how the brain might realize its switching functions in each domain, primarily by focusing on the monkey oculomotor system as the experimental model. Emerging evidence now suggests that multiple neuronal populations in the shared cortico-basal ganglia network contribute to overriding prepotent eye movement, be its origin innate, habitual or motivational. This consideration suggests the general versatility of the cortico-basal ganglia network as the neural mechanism whereby humans and other animals keep themselves from becoming subservient to reflex, habit and motivational impulses.

Organization of GABAergic inhibition in the motor output layer of the superior colliculus

The direction and amplitude of saccadic eye movements are determined by the location of the center of gravity of burst activity over a neuronal population on the spatial map of the intermediate gray layer (SGI) of the superior colliculus (SC). GABAergic interneurons might play critical roles in shaping the activation field on the topographical map but, to understand the mechanism, basic information on the organization of inhibitory circuits is essential. In the present study, we investigated the electrophysiological and morphological properties of GABAergic neurons in SGI by whole-cell patch-clamp recordings and intracellular staining using biocytin in GAD67-GFP knock-in mice (PND17-22), in which GABAergic neurons specifically express GFP fluorescence. The most common firing properties among these GABAergic neurons (n=231) were fast spiking (58%), followed by burst spiking (29%), late spiking (8%) and, the least common, regular spiking (2%) and rapid spike inactivation (3%). Morphological analysis of axonal trajectories of intracellularly-labeled GABAergic neurons revealed three major subclasses: (i) intralaminar interneurons, which were further divided into two subclasses, local and horizontal interneurons; (ii) interlaminar interneurons; and (iii) commissural and tectofugal neurons. These results reveal distinct subsets of GABAergic neurons including neurons that mediate local and long-range inhibition in the SC, neurons that potentially modulate visual and other sensory inputs to the SC, and neurons that project to nuclei outside the SC.

Filtering of visual information in the tectum by an identified neural circuit

The optic tectum of zebrafish is involved in behavioral responses that require the detection of small objects. The superficial layers of the tectal neuropil receive input from retinal axons, while its deeper layers convey the processed information to premotor areas. Imaging with a genetically encoded calcium indicator revealed that the deep layers, as well as the dendrites of single tectal neurons, are preferentially activated by small visual stimuli. This spatial filtering relies on GABAergic interneurons (using the neurotransmitter γ-aminobutyric acid) that are located in the superficial input layer and respond only to large visual stimuli. Photo-ablation of these cells with KillerRed, or silencing of their synaptic transmission, eliminates the size tuning of deeper layers and impairs the capture of prey.

Intralaminar and interlaminar activity within the rodent superior colliculus visualized with voltage imaging

The superior colliculus (SC) is a midbrain structure that plays a role in converting sensation into action. Most SC research focuses on either in vivo extracellular recordings from behaving monkeys or patch-clamp recordings from smaller mammals in vitro. However, the activity of neuronal circuits is necessary to generate behavior, and neither of these approaches measures the simultaneous activity of large populations of neurons that make up circuits. Here, we describe experiments in which we measured changes in membrane potential across the SC map using voltage imaging of the rat SC in vitro. Our results provide the first high temporal and spatial resolution images of activity within the SC. Electrical stimulation of the SC evoked a characteristic two-component optical response containing a short latency initial-spike and a longer latency after-depolarization. Single-pulse stimulation in the superficial SC evoked a pattern of intralaminar and interlaminar spread that was distinct from the spread evoked by the same stimulus applied to the intermediate SC. Intermediate layer stimulation produced a more extensive and more ventrally located activation of the superficial layers than did stimulation in the superficial SC. Together, these results indicate the recruitment of dissimilar subpopulations of circuitry depending on the layer stimulated. Field potential recordings, pharmacological manipulations, and timing analyses indicate that the patterns of activity were physiologically relevant and largely synaptically driven. Therefore, voltage imaging is a powerful technique for the study of spatiotemporal dynamics of electrical signaling across neuronal populations, providing insight into neural circuits that underlie behavior.

Spatial updating in monkey superior colliculus in the absence of the forebrain commissures: dissociation between superficial and intermediate layers

In previous studies, we demonstrated that the forebrain commissures are the primary pathway for remapping from one hemifield to the other. Nonetheless, remapping in lateral intraparietal cortex (LIP) across hemifield is still present in split brain monkeys. This finding indicates that a subcortical structure must contribute to remapping. The primary goal of the current study was to characterize remapping activity in the superior colliculus in intact and split brain monkeys. We recorded neurons in both the superficial and intermediate layers of the SC. We found that across-hemifield remapping was reduced in magnitude and delayed compared with within-hemifield remapping in the intermediate layers of the SC in split brain monkeys. These results mirror our previous findings in area LIP. In contrast, we found no difference in the magnitude or latency for within- compared with across-hemifield remapping in the superficial layers. At the behavioral level, we compared the performance of the monkeys on two conditions of a double-step task. When the second target remained within a single hemifield, performance remained accurate. When the second target had to be updated across hemifields, the split brain monkeys' performance was impaired. Remapping activity in the intermediate layers was correlated with the accuracy and latency of the second saccade during the across-hemifield trials. Remapping in the superficial layers was correlated with latency of the second saccade during the within- and across-hemifield trials. The differences between the layers suggest that different circuits underlie remapping in the superficial and intermediate layers of the superior colliculus.

In vivo spike-timing-dependent plasticity in the optic tectum of Xenopus laevis

Spike-timing-dependent plasticity (STDP) is found in vivo in a variety of systems and species, but the first demonstrations of in vivo STDP were carried out in the optic tectum of Xenopus laevis embryos. Since then, the optic tectum has served as an excellent experimental model for studying STDP in sensory systems, allowing researchers to probe the developmental consequences of this form of synaptic plasticity during early development. In this review, we will describe what is known about the role of STDP in shaping feed-forward and recurrent circuits in the optic tectum with a focus on the functional implications for vision. We will discuss both the similarities and differences between the optic tectum and mammalian sensory systems that are relevant to STDP. Finally, we will highlight the unique properties of the embryonic tectum that make it an important system for researchers who are interested in how STDP contributes to activity-dependent development of sensory computations.

Saccade control after V1 lesion revisited

A number of previous articles on blindsight have stressed that even after the lesion of the primary visual cortex (V1), subjects can perform visually guided saccades toward the targets in the blind field and that the extrageniculate visual pathway which bypasses the V1 can control the saccades by itself. However, in monkey model of V1 lesion, about two months of time is needed for recovery, suggesting that the extrageniculate visual pathway cannot immediately take over the function of the geniculo-striatal pathway, and on close look at the dynamics of saccades, saccades became ballistic and online corrections of trajectories and velocities were impaired, and that decision threshold for saccade initiation was lowered. On the basis of these findings, we propose that the visual signal through V1 is necessary for deliberate control of saccades.

Exploring the superior colliculus in vitro

The superior colliculus plays an important role in the translation of sensory signals that encode the location of objects in space into motor signals that encode vectors of the shifts in gaze direction called saccades. Since the late 1990s, our two laboratories have been applying whole cell patch-clamp techniques to in vitro slice preparations of rodent superior colliculus to analyze the structure and function of its circuitry at the cellular level. This review describes the results of these experiments and discusses their contributions to our understanding of the mechanisms responsible for sensorimotor integration in the superior colliculus. The experiments analyze vertical interactions between its superficial visuosensory and intermediate premotor layers and propose how they might contribute to express saccades and to saccadic suppression. They also compare and contrast the circuitry within each of these layers and propose how this circuitry might contribute to the selection of the targets for saccades and to the build-up of the premotor commands that precede saccades. Experiments also explore in vitro the roles of extrinsic inputs to the superior colliculus, including cholinergic inputs from the parabigeminal and parabrachial nuclei and GABAergic inputs from the substantia nigra pars reticulata, in modulating the activity of the collicular circuitry. The results extend and clarify our understanding of the multiple roles the superior colliculus plays in sensorimotor integration.

Oculomotor distraction by signals invisible to the retinotectal and magnocellular pathways

Irrelevant stimulus onsets interfere with saccade planning to other stimuli, prolonging saccadic latency (the oculomotor distractor effect) or eliciting directional errors (saccadic capture). Such stimulus-driven interference has been associated with the retinotectal pathway, the direct pathway from retina to superior colliculus. Consistent with this theory, the distractor effect has not been found for stimuli visible only to the short-wave cones in the retina (S cones), which are thought not to contribute to the retinotectal pathway. However, S-cone signals are generally slower than luminance signals and such differences in temporal dynamics have not been taken into account when investigating the saccadic distractor effect. Here, by varying the delay between target and distractor, we found that S-cone stimuli do in fact produce a distractor effect, but the optimal delay is generally different from that for luminance distractors. The temporal dynamics of the distractor effect conform to a general framework of saccadic competition that takes sensory transmission time into account. Additionally, we observe that S-cone stimuli are able to produce saccadic capture in our paradigm. We conclude that stimulus-driven oculomotor interference does not rely on the retinotectal pathway, or indeed the magnocellular pathway, which is also blind to our S-cone stimuli.

Impaired control of the oculomotor reflexes in Parkinson's disease

To investigate the role of the basal ganglia in integrating voluntary and reflexive behaviour, the current study examined the ability of patients with Parkinson's disease to voluntarily control oculomotor reflexes. We measured the size of the fixation offset effect (the reduction in saccadic reaction time when a fixation point is removed) during a block of pro- and a block of anti-saccades. Healthy controls showed the expected reduction of the FOE during the anti-saccades, which results from efforts to suppress reflexive eye movements (a preparatory set characterized by increased internal control and reduced external control). However, there was no reduction of the FOE in the anti-saccade task in Parkinson's patients, indicating that they are impaired in exerting control over oculomotor reflexes.