Tectal codification of eye movements in goldfish studied by electrical microstimulation. f

Spatial Cognition in Teleost Fish: Strategies and Mechanisms

Teleost fish have been traditionally considered primitive vertebrates compared to mammals and birds in regard to brain complexity and behavioral functions. However, an increasing amount of evidence suggests that teleosts show advanced cognitive capabilities including spatial navigation skills that parallel those of land vertebrates. Teleost fish rely on a multiplicity of sensory cues and can use a variety of spatial strategies for navigation, ranging from relatively simple body-centered orientation responses to allocentric or "external world-centered" navigation, likely based on map-like relational memory representations of the environment. These distinct spatial strategies are based on separate brain mechanisms. For example, a crucial brain center for egocentric orientation in teleost fish is the optic tectum, which can be considered an essential hub in a wider brain network responsible for the generation of egocentrically referenced actions in space. In contrast, other brain centers, such as the dorsolateral telencephalic pallium of teleost fish, considered homologue to the hippocampal pallium of land vertebrates, seem to be crucial for allocentric navigation based on map-like spatial memory. Such hypothetical relational memory representations endow fish's spatial behavior with considerable navigational flexibility, allowing them, for example, to perform shortcuts and detours.

Role of the Superior Colliculus in Guiding Movements Not Made by the Eyes

The superior colliculus (SC) has long been associated with the neural control of eye movements. Over seventy years ago, the orderly topography of saccade vectors and corresponding visual field locations was discovered in the cat SC. Since then, numerous high-impact studies have investigated and manipulated the relationship between visuotopic space and saccade vector across this topography to better understand the physiological underpinnings of the sensorimotor signal transformation. However, less attention has been paid to the other motor responses that may be associated with SC activity, ranging in complexity from concerted movements of skeletomotor muscle groups, such as arm-reaching movements, to behaviors that involve whole-body movement sequences, such as fight-or-flight responses in murine models. This review surveys these more complex movements associated with SC (optic tectum in nonmammalian species) activity and, where possible, provides phylogenetic and ethological perspective. Expected final online publication date for the Annual Review of Vision Science, Volume 7 is September 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Zebrafish dscaml1 Deficiency Impairs Retinal Patterning and Oculomotor Function

Down syndrome cell adhesion molecules (dscam and dscaml1) are essential regulators of neural circuit assembly, but their roles in vertebrate neural circuit function are still mostly unexplored. We investigated the functional consequences of dscaml1 deficiency in the larval zebrafish (sexually undifferentiated) oculomotor system, where behavior, circuit function, and neuronal activity can be precisely quantified. Genetic perturbation of dscaml1 resulted in deficits in retinal patterning and light adaptation, consistent with its known roles in mammals. Oculomotor analyses revealed specific deficits related to the dscaml1 mutation, including severe fatigue during gaze stabilization, reduced saccade amplitude and velocity in the light, greater disconjugacy, and impaired fixation. Two-photon calcium imaging of abducens neurons in control and dscaml1 mutant animals confirmed deficits in saccade-command signals (indicative of an impairment in the saccadic premotor pathway), whereas abducens activation by the pretectum-vestibular pathway was not affected. Together, we show that loss of dscaml1 resulted in impairments in specific oculomotor circuits, providing a new animal model to investigate the development of oculomotor premotor pathways and their associated human ocular disorders.SIGNIFICANCE STATEMENT Dscaml1 is a neural developmental gene with unknown behavioral significance. Using the zebrafish model, this study shows that dscaml1 mutants have a host of oculomotor (eye movement) deficits. Notably, the oculomotor phenotypes in dscaml1 mutants are reminiscent of human ocular motor apraxia, a neurodevelopmental disorder characterized by reduced saccade amplitude and gaze stabilization deficits. Population-level recording of neuronal activity further revealed potential subcircuit-specific requirements for dscaml1 during oculomotor behavior. These findings underscore the importance of dscaml1 in the development of visuomotor function and characterize a new model to investigate potential circuit deficits underlying human oculomotor disorders.

Global Jitter Motion of the Retinal Image Dynamically Alters the Receptive Field Properties of Retinal Ganglion Cells

Fixational eye movements induce aperiodic motion of the retinal image. However, it is not yet fully understood how fixational eye movements affect retinal information processing. Here we show that global jitter motion, simulating the image motion during fixation, alters the spatiotemporal receptive field properties of retinal ganglion cells. Using multi-electrode and whole-cell recording techniques, we investigated light-evoked responses from ganglion cells in the isolated goldfish retina. Ganglion cells were classified into six groups based on the filtering property of light stimulus, the membrane properties, and the cell morphology. The spatiotemporal receptive field profiles of retinal ganglion cells were estimated by the reverse correlation method, where the dense noise stimulus was applied on the dark or random-dot background. We found that the jitter motion of the random-dot background elongated the receptive filed along the rostral-caudal axis and temporally sensitized in a specific group of ganglion cells: Fast-transient ganglion cells. At the newly emerged regions of the receptive field local light stimulation evoked excitatory postsynaptic currents with large amplitude and fast kinetics without changing the properties of inhibitory postsynaptic currents. Pharmacological experiments suggested two presynaptic mechanisms underlying the receptive field alteration: (i) electrical coupling between bipolar cells, which expands the receptive field in all directions; (ii) GABAergic presynaptic inhibition from amacrine cells, which reduces the dorsal and ventral regions of the expanded receptive field, resulting in elongation along the rostral-caudal axis. Our study demonstrates that the receptive field of Fast-transient ganglion cells is not static but dynamically altered depending on the visual inputs. The receptive field elongation during fixational eye movements may contribute to prompt firing to a target in the succeeding saccade.

Expression of RING Finger Protein 38 in Serotonergic Neurons in the Brain of Nile Tilapia, Oreochromis niloticus

Serotonin (5-hydroxytryptamine, 5-HT) is one of the major neurotransmitters, modulating diverse behaviours and physiological functions. Really interesting new gene (RING) finger protein 38 (RNF38) is an E3 ubiquitin ligase whose function remains unclear. A recent study has shown a possible regulatory relationship between RNF38 and the 5-HT system. Therefore, to gain insight into the role of RNF38 in the central 5-HT system, we identified the neuroanatomical location of 5-HT positive cells and investigated the relationship between RNF38 and the 5-HT system in the brain of the Nile tilapia, Oreochromis niloticus. Immunocytochemistry revealed three neuronal populations of 5-HT in the brain of tilapia; the paraventricular organ (PVO), the dorsal and ventral periventricular pretectal nuclei (PPd and PPv), and, the superior and inferior raphe (SR and IR). The 5-HT neuronal number was highest in the raphe (90.4 in SR, 284.6 in IR), followed by the pretectal area (22.3 in PPd, 209.8 in PPv). Double-label immunocytochemistry showed that the majority of 5-HT neurons express RNF38 nuclear proteins (66.5% in PPd; 77.9% in PPv; 35.7% in SR; 49.1% in IR). These findings suggest that RNF38 could be involved in E3 ubiquitination in the central 5-HT system.

Rapid and coordinated processing of global motion images by local clusters of retinal ganglion cells

Even when the body is stationary, the whole retinal image is always in motion by fixational eye movements and saccades that move the eye between fixation points. Accumulating evidence indicates that the brain is equipped with specific mechanisms for compensating for the global motion induced by these eye movements. However, it is not yet fully understood how the retina processes global motion images during eye movements. Here we show that global motion images evoke novel coordinated firing in retinal ganglion cells (GCs). We simultaneously recorded the firing of GCs in the goldfish isolated retina using a multi-electrode array, and classified each GC based on the temporal profile of its receptive field (RF). A moving target that accompanied the global motion (simulating a saccade following a period of fixational eye movements) modulated the RF properties and evoked synchronized and correlated firing among local clusters of the specific GCs. Our findings provide a novel concept for retinal information processing during eye movements.

Relational and procedural memory systems in the goldfish brain revealed by trace and delay eyeblink-like conditioning

The presence of multiple memory systems supported by different neural substrata has been demonstrated in animal and human studies. In mammals, two variants of eyeblink classical conditioning, differing only in the temporal relationships between the conditioned stimulus (CS) and the unconditioned stimulus (US), have been widely used to study the neural substrata of these different memory systems. Delay conditioning, in which both stimuli coincide in time, depends on a non-relational memory system supported by the cerebellum and associated brainstem circuits. In contrast, trace conditioning, in which a stimulus-free time gap separates the CS and the US, requires a declarative or relational memory system, thus depending on forebrain structures in addition to the cerebellum. The distinction between the explicit or relational and the implicit or procedural memory systems that support trace and delay classical conditioning has been extensively studied in mammals, but studies in other vertebrate groups are relatively scarce. In the present experiment we analyzed the differential involvement of the cerebellum and the telencephalon in delay and trace eyeblink-like classical conditioning in goldfish. The results show that whereas the cerebellum lesion prevented the eyeblink-like conditioning in both procedures, the telencephalon ablation impaired exclusively the acquisition of the trace conditioning. These data showing that comparable neural systems support delay and trace eyeblink conditioning in teleost fish and mammals suggest that these separate memory systems and their neural bases could be a shared ancestral brain feature of the vertebrate lineage.

Goldfish morphology as a model for evolutionary developmental biology

Morphological variation of the goldfish is known to have been established by artificial selection for ornamental purposes during the domestication process. Chinese texts that date to the Song dynasty contain descriptions of goldfish breeding for ornamental purposes, indicating that the practice originated over one thousand years ago. Such a well-documented goldfish breeding process, combined with the phylogenetic and embryological proximities of this species with zebrafish, would appear to make the morphologically diverse goldfish strains suitable models for evolutionary developmental (evodevo) studies. However, few modern evodevo studies of goldfish have been conducted. In this review, we provide an overview of the historical background of goldfish breeding, and the differences between this teleost and zebrafish from an evolutionary perspective. We also summarize recent progress in the field of molecular developmental genetics, with a particular focus on the twin-tail goldfish morphology. Furthermore, we discuss unanswered questions relating to the evolution of the genome, developmental robustness, and morphologies in the goldfish lineage, with the goal of blazing a path toward an evodevo study paradigm using this teleost species as a new model species. For further resources related to this article, please visit the WIREs website.

Visuomotor transformations underlying hunting behavior in zebrafish

Visuomotor circuits filter visual information and determine whether or not to engage downstream motor modules to produce behavioral outputs. However, the circuit mechanisms that mediate and link perception of salient stimuli to execution of an adaptive response are poorly understood. We combined a virtual hunting assay for tethered larval zebrafish with two-photon functional calcium imaging to simultaneously monitor neuronal activity in the optic tectum during naturalistic behavior. Hunting responses showed mixed selectivity for combinations of visual features, specifically stimulus size, speed, and contrast polarity. We identified a subset of tectal neurons with similar highly selective tuning, which show non-linear mixed selectivity for visual features and are likely to mediate the perceptual recognition of prey. By comparing neural dynamics in the optic tectum during response versus non-response trials, we discovered premotor population activity that specifically preceded initiation of hunting behavior and exhibited anatomical localization that correlated with motor variables. In summary, the optic tectum contains non-linear mixed selectivity neurons that are likely to mediate reliable detection of ethologically relevant sensory stimuli. Recruitment of small tectal assemblies appears to link perception to action by providing the premotor commands that release hunting responses. These findings allow us to propose a model circuit for the visuomotor transformations underlying a natural behavior.

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Zebrafish hunting responses are triggered by conjunctions of visual features

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Tectal neurons show non-linear mixed selectivity for prey-like visual stimuli

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Tectal assemblies show premotor activity specifically preceding hunting responses

The lamprey pallium provides a blueprint of the mammalian motor projections from cortex

BACKGROUND

The frontal lobe control of movement in mammals has been thought to be a specific function primarily related to the layered neocortex with its efferent connections. In contrast, we now show that the same basic organization is present even in one of the phylogenetically oldest vertebrates, the lamprey.

RESULTS

Stimulation of specific sites in the pallium/cortex evokes eye, trunk, locomotor, or oral movements. The pallial projection neurons target brainstem motor centers and basal ganglia subnuclei and have prominent dendrites extending into the outer molecular layer. They exhibit the characteristic features of pyramidal neurons and elicit monosynaptic glutamatergic excitatory postsynaptic potentials in output neurons of the optic tectum, reticulospinal neurons, and, as shown earlier, basal ganglia neurons.

CONCLUSIONS

Our results demonstrate marked similarities in the efferent functional connectivity and control of motor behavior between the lamprey pallium and mammalian neocortex. Thus, the lamprey motor pallium/cortex represents an evolutionary blueprint of the corresponding mammalian system.

Prey capture in zebrafish larvae serves as a model to study cognitive functions

Prey capture in zebrafish larvae is an innate behavior which can be observed as early as 4~days postfertilization, the day when they start to swim. This simple behavior apparently involves several neural processes including visual perception, recognition, decision-making, and motor control, and, therefore, serves as a good model system to study cognitive functions underlying natural behaviors in vertebrates. Recent progresses in imaging techniques provided us with a unique opportunity to image neuronal activity in the brain of an intact fish in real-time while the fish perceives a natural prey, paramecium. By expanding this approach, it would be possible to image entire brain areas at a single-cell resolution in real-time during prey capture, and identify neuronal circuits important for cognitive functions. Further, activation or inhibition of those neuronal circuits with recently developed optogenetic tools or neurotoxins should shed light on their roles. Thus, we will be able to explore the prey capture in zebrafish larvae more thoroughly at cellular levels, which should establish a basis of understanding of the cognitive function in vertebrates.

Control of a specific motor program by a small brain area in zebrafish

Complex motor behaviors are thought to be coordinated by networks of brain nuclei that may control different elementary motor programs. Transparent zebrafish larvae offer the opportunity to analyze the functional organization of motor control networks by optical manipulations of neuronal activity during behavior. We examined motor behavior in transgenic larvae expressing channelrhodopsin-2 throughout many neurons in the brain. Wide-field optical stimulation triggered backward and rotating movements caused by the repeated execution of J-turns, a specific motor program that normally occurs during prey capture. Although optically-evoked activity was widespread, behavioral responses were highly coordinated and lateralized. 3-D mapping of behavioral responses to local optical stimuli revealed that J-turns can be triggered specifically in the anterior-ventral optic tectum (avOT) and/or the adjacent pretectum. These results suggest that the execution of J-turns is controlled by a small group of neurons in the midbrain that may act as a command center. The identification of a brain area controlling a defined motor program involved in prey capture is a step toward a comprehensive analysis of neuronal circuits mediating sensorimotor behaviors of zebrafish.

Prey capture behavior evoked by simple visual stimuli in larval zebrafish

Understanding how the nervous system recognizes salient stimuli in the environment and selects and executes the appropriate behavioral responses is a fundamental question in systems neuroscience. To facilitate the neuroethological study of visually guided behavior in larval zebrafish, we developed “virtual reality” assays in which precisely controlled visual cues can be presented to larvae whilst their behavior is automatically monitored using machine vision algorithms. Freely swimming larvae responded to moving stimuli in a size-dependent manner: they directed multiple low amplitude orienting turns (∼20°) toward small moving spots (1°) but reacted to larger spots (10°) with high-amplitude aversive turns (∼60°). The tracking of small spots led us to examine how larvae respond to prey during hunting routines. By analyzing movie sequences of larvae hunting paramecia, we discovered that all prey capture routines commence with eye convergence and larvae maintain their eyes in a highly converged position for the duration of the prey-tracking and capture swim phases. We adapted our virtual reality assay to deliver artificial visual cues to partially restrained larvae and found that small moving spots evoked convergent eye movements and J-turns of the tail, which are defining features of natural hunting. We propose that eye convergence represents the engagement of a predatory mode of behavior in larval fish and serves to increase the region of binocular visual space to enable stereoscopic targeting of prey.

The serotonergic system in fish

Neurons using serotonin (5-HT) as neurotransmitter and/or modulator have been identified in the central nervous system in representatives from all vertebrate clades, including jawless, cartilaginous and ray-finned fishes. The aim of this review is to summarize our current knowledge about the anatomical organization of the central serotonergic system in fishes. Furthermore, selected key functions of 5-HT will be described. The main focus will be the adult brain of teleosts, in particular zebrafish, which is increasingly used as a model organism. It is used to answer not only genetic and developmental biology questions, but also issues concerning physiology, behavior and the underlying neuronal networks. The many evolutionary conserved features of zebrafish combined with the ever increasing number of genetic tools and its practical advantages promise great possibilities to increase our understanding of the serotonergic system. Further, comparative studies including several vertebrate species will provide us with interesting insights into the evolution of this important neurotransmitter system.

Focusing on optic tectum circuitry through the lens of genetics

The visual pathway is tasked with processing incoming signals from the retina and converting this information into adaptive behavior. Recent studies of the larval zebrafish tectum have begun to clarify how the 'micro-circuitry' of this highly organized midbrain structure filters visual input, which arrives in the superficial layers and directs motor output through efferent projections from its deep layers. The new emphasis has been on the specific function of neuronal cell types, which can now be reproducibly labeled, imaged and manipulated using genetic and optical techniques. Here, we discuss recent advances and emerging experimental approaches for studying tectal circuits as models for visual processing and sensorimotor transformation by the vertebrate brain.

Spatial distribution of a fusiform cell in the optic tectum of Pantodon buchholzi, the African butterfly fish (Teleostei, Osteoglossomorpha)

Pantodon buchholzi, the African butterfly fish, inhabits an ecological niche just below the water surface. At that position, each eye necessarily views into the air through the surface of the water and into the water. Since Pantodon is an obligatory surface feeder, the ventral retina viewing the aerial environment provides all visual information for prey acquisition. The visual pathway of this fish reflects the divided visual field with structural differences in the retina and brain corresponding to the different views. In this study, we describe a specific type of neuron in the tectum that, due to its intrinsic structure, likely integrates visual and mechanoreceptor inputs. Because of its spatial distribution, this type of neuron is a candidate as a basic element in a network involved with prey acquisition.

Neural substrates of sensory-guided locomotor decisions in the rat superior colliculus

Deciding in which direction to move is a ubiquitous feature of animal behavior, but the neural substrates of locomotor choices are not well understood. The superior colliculus (SC) is a midbrain structure known to be important for controlling the direction of gaze, particularly when guided by visual or auditory cues, but which may play a more general role in behavior involving spatial orienting. To test this idea, we recorded and manipulated activity in the SC of freely moving rats performing an odor-guided spatial choice task. In this context, not only did a substantial majority of SC neurons encode choice direction during goal-directed locomotion, but many also predicted the upcoming choice and maintained selectivity for it after movement completion. Unilateral inactivation of SC activity profoundly altered spatial choices. These results indicate that the SC processes information necessary for spatial locomotion, suggesting a broad role for this structure in sensory-guided orienting and navigation.

Morphology and spatial arrangement of large retinal ganglion cells projecting to the optic tectum in the perciform fish Pholidapus dybowskii

Using retrograde HRP labeling from the optic nerve (ON) or optic tectum (OT), we have visualized large ganglion cells (LGCs) in wholemounted retinas of the teleost Pholidapus dybowskii and studied their morphology and spatial properties. In all, three LGC types were distinguished. In a previous paper, detailed data were provided on one type, biplexiform cells [Pushchin, I. I., & Kondrashev, S. L. (2003). Biplexiform ganglion cells in the retina of the perciform fish Pholidapus dybowskii revealed by HRP labeling from the optic nerve and optic tectum. Vision Research, 43, 1117-1133]. Here, we present data on the other two confirmed types, alpha(a) and alpha(ab) cells. The types differed in the level of dendrite stratification, dendrite arborization pattern, dendritic field size, and other features, and formed in the retina significantly non-random, spatially independent mosaics. Both types were labeled from the OT, indicating their participation in OT-mediated visual reactions. The comparison of spatial properties of alpha(a) and alpha(ab) mosaics labeled from the ON and OT suggests that the OT is the major or one of the major projection areas of both types. We also describe the morphology of cells resembling alpha(c) cells of other fishes, which were only labeled from the ON. The LGC types presently revealed were similar in their morphology to LGCs found in other teleosts supporting the hypothesis of LGC homology across the teleost lineage.

Genetic single-cell mosaic analysis implicates ephrinB2 reverse signaling in projections from the posterior tectum to the hindbrain in zebrafish

The optic tectum is a visual center in vertebrates. It receives topographically ordered visual inputs from the retina in the superficial layers and then sends motor outputs from the deeper layers to the premotor reticulospinal system in the hindbrain. Although the topographic patterns of the retinotectal projection are well known, it is not yet well understood how tectal efferents in the tectobulbar tract project to the hindbrain. The retinotectal and the tectobulbar projections were visualized in a zebrafish stable transgenic line Tg(brn3a-hsp70:GFP). Using a single-neuron labeling system in combination with the cre/loxP and Gal4/UAS systems, we showed that the tectal neurons that projected to rhombomeres 2 and 6 were distributed with distinctive patterns along the anterior-posterior axis. Furthermore, we found that ephrinB2a was critically involved in increasing the probability of neurons projecting to rhombomere 2 through a reverse signaling mechanism. These results may provide a neuroanatomical and molecular basis for the motor command map in the tectum.

Tectal control of locomotion, steering, and eye movements in lamprey

The intrinsic function of the brain stem-spinal cord networks eliciting the locomotor synergy is well described in the lamprey-a vertebrate model system. This study addresses the role of tectum in integrating eye, body orientation, and locomotor movements as in steering and goal-directed behavior. Electrical stimuli were applied to different areas within the optic tectum in head-restrained semi-intact lampreys (n = 40). Motions of the eyes and body were recorded simultaneously (videotaped). Brief pulse trains (<0.5 s) elicited only eye movements, but with longer stimuli (>0.5 s) lateral bending movements of the body (orientation movements) were added, and with even longer stimuli locomotor movements were initiated. Depending on the tectal area stimulated, four characteristic response patterns were observed. In a lateral area conjugate horizontal eye movements combined with lateral bending movements of the body and locomotor movements were elicited, depending on stimulus duration. The amplitude of the eye movement and bending movements was site specific within this region. In a rostromedial area, bilateral downward vertical eye movements occurred. In a caudomedial tectal area, large-amplitude undulatory body movements akin to struggling behavior were elicited, combined with large-amplitude eye movements that were antiphasic to the body movements. The alternating eye movements were not dependent on vestibuloocular reflexes. Finally, in a caudolateral area locomotor movements without eye or bending movements could be elicited. These results show that tectum can provide integrated motor responses of eye, body orientation, and locomotion of the type that would be required in goal-directed locomotion.

Kinematics and eye-head coordination of gaze shifts evoked from different sites in the superior colliculus of the cat

Shifting gaze requires precise coordination of eye and head movements. It is clear that the superior colliculus (SC) is involved with saccadic gaze shifts. Here we investigate its role in controlling both eye and head movements during gaze shifts. Gaze shifts of the same amplitude can be evoked from different SC sites by controlled electrical microstimulation. To describe how the SC coordinates the eye and the head, we compare the characteristics of these amplitude-matched gaze shifts evoked from different SC sites. We show that matched amplitude gaze shifts elicited from progressively more caudal sites are progressively slower and associated with a greater head contribution. Stimulation at more caudal SC sites decreased the peak velocity of the eye but not of the head, suggesting that the lower peak gaze velocity for the caudal sites is due to the increased contribution of the slower-moving head. Eye-head coordination across the SC motor map is also indicated by the relative latencies of the eye and head movements. For some amplitudes of gaze shift, rostral stimulation evoked eye movement before head movement, whereas this reversed with caudal stimulation, which caused the head to move before the eyes. These results show that gaze shifts of similar amplitude evoked from different SC sites are produced with different kinematics and coordination of eye and head movements. In other words, gaze shifts evoked from different SC sites follow different amplitude-velocity curves, with different eye-head contributions. These findings shed light on mechanisms used by the central nervous system to translate a high-level motor representation (a desired gaze displacement on the SC map) into motor commands appropriate for the involved body segments (the eye and the head).

Eye movements evoked by electrical microstimulation of the mesencephalic reticular formation in goldfish

Anatomical studies in goldfish show that the tectofugal axons provide a large number of boutons within the mesencephalic reticular formation. Electrical stimulation, reversible inactivation and cell recording in the primate central mesencephalic reticular formation have suggested that it participates in the control of rapid eye movements (saccades). Moreover, the role of this tecto-recipient area in the generation of saccadic eye movements in fish is unknown. In this study we show that the electrical microstimulation of the mesencephalic reticular formation of goldfish evoked short latency saccadic eye movements in any direction (contraversive or ipsiversive, upward or downward). Movements of the eyes were usually disjunctive. Based on the location of the sites from which eye movements were evoked and the preferred saccade direction, eye movements were divided into different groups: pure vertical saccades were mainly elicited from the rostral mesencephalic reticular formation, while oblique and pure horizontal were largely evoked from middle and caudal mesencephalic reticular formation zones. The direction and amplitude of pure vertical and horizontal saccades were unaffected by initial eye position. However the amplitude, but not the direction of most oblique saccades was systematically modified by initial eye position. At the same time, the amplitude of elicited saccades did not vary in any consistent manner along either the anteroposterior, dorsoventral or mediolateral axes (i.e. there was no topographic organization of the mesencephalic reticular formation with respect to amplitude). In addition to these groups of movements, we found convergent and goal-directed saccades evoked primarily from the anterior and posterior mesencephalic reticular formation, respectively. Finally, the metric and kinetic characteristics of saccades could be manipulated by changes in the stimulation parameters. We conclude that the mesencephalic reticular formation in goldfish shares physiological functions that correspond closely with those found in mammals.

Connections of eye-saccade-related areas within mesencephalic reticular formation with the optic tectum in goldfish

Physiological studies demonstrate that separate sites within the mesencephalic reticular formation (MRF) can evoke eye saccades with different preferred directions. Furthermore, anatomical research suggests that a tectoreticulotectal circuit organized in accordance with the tectal eye movement map is present. However, whether the reticulotectal projection shifts with the gaze map present in the MRF is unknown. We explored this question in goldfish, by injecting biotin dextran amine within MRF sites that evoked upward, downward, oblique, and horizontal eye saccades. Then, we analyzed the labeling in the optic tectum. The main findings can be summarized as follows. 1) The MRF and the optic tectum were connected by separate axons of the tectobulbar tract. 2) The MRF was reciprocally connected mainly with the ipsilateral tectal lobe, but also with the contralateral one. 3) The MRF received projections chiefly from neurons located within intermediate and deep tectal layers. In addition, the MRF projections terminated primarily within the intermediate tectal layer. 4) The distribution of labeled neurons in the tectum shifted with the different MRF sites in a manner consistent with the tectal motor map. The area containing these cells was targeted by a high-density reticulotectal projection. In addition to this high-density topographic projection, there was a low-density one spread throughout the tectum. 5) Occasionally, boutons were observed adjacent to tectal labeled neurons. We conclude that the organization of the reticulotectal circuit is consistent with the functional topography of the MRF and that the MRF participates in a tectoreticulotectal feedback circuit.

Visual orienting response in goldfish: a multidisciplinary study

The neural basis underlying the orienting response has been thoroughly studied in frontal-eyed mammals. However, in non-mammalian species, including fish, it remains almost unknown. Therefore, we studied the contribution of the optic tectum and the mesencephalic reticular formation to the performance of the orienting response in goldfish, using behavioural, physiological, and anatomical tracer techniques. The appearance of a visual stimulus (a pellet of food) in the environment of a goldfish evoked a turn of the body to reorient the line of sight. Left-tectal lobe ablation abolished the orienting turn response towards the contralateral hemifield. Electrical microstimulation of the optic tectum suggested the presence of a motor map, which is in correspondence with the overlying visual representation, as previously reported in other vertebrates. The tracer biotin-dextran amine was injected into different functionally identified tectal zones. The results showed that rostral and caudal poles of the mesencephalic reticular formation receive outflow mainly from the rostral and caudal tectal poles, respectively. This suggests that the tectal wiring with downstream structures is site-dependent. Furthermore, the electrical activation of rostral and caudal mesencephalic reticular formation revealed a different contribution to vertical and horizontal orienting eye movements. We conclude that the basic neural system coding the orienting response appears early in phylogenesis, although some specific characteristics are selected by adaptive pressure.

Involvement of the optic tectum and mesencephalic reticular formation in the generation of saccadic eye movements in goldfish

The circuitry and physiological properties underlying saccadic eye movement generation have been studied mainly in monkeys and cats. By contrast, current knowledge in nonmammalian species is rather scarce. We review here some of our recent findings about the involvement of the optic tectum and mesencephalic reticular formation in the generation of saccades in goldfish. Electrical microstimulation of the optic tectum evokes contraversive saccadic eye movements. In goldfish, as in mammals, the amplitude and direction of saccades are encoded in a spatial topographical map. In addition, there are some areas that have evolved, such as the extreme anteromedial tectal zone, whose activation yields eye convergence. Injections of the bidirectional tracer biotin dextran amine within functionally identified sites of the tectum provide reciprocal, site-dependent connectivity with different downstream structures. Of these structures, the major tectofugal target is the mesencephalic reticular formation. In goldfish, as in mammals, the mesencephalic reticular formation and optic tectum establish reciprocal connections at regional and neuronal levels which support the presence of feedback circuits. Electrical microstimulation demonstrates that the mesencephalic reticular formation can be functionally parceled-the rostral part is linked to vertical saccades, while the caudal part is related with horizontal ones. Finally, these zones are also differently connected to the optic tectum. From these data, we conclude that the involvement of the optic tectum and mesencephalic reticular formation in eye movement generation in goldfish is similar to that reported in cats and monkeys.

Afferent connectivity to different functional zones of the optic tectum in goldfish

This work studies the afferent connectivity to different functionally identified tectal zones in goldfish. The sources of afferents contributed to different degrees to the functionally defined zones. The dorsocentral area of the telencephalon was connected mainly with the ipsilateral anteromedial tectal zone. At diencephalic levels, neurons were found in three different regions: preoptic, thalamic, and pretectal. Preoptic structures (suprachiasmatic and preoptic nuclei) projected mainly to the anteromedial tectal zone, whereas thalamic (ventral and dorsal) and pretectal (central, superficial, and posterior commissure) nuclei projected to all divisions of the tectum. In the mesencephalon, the mesencephalic reticular formation, torus longitudinalis, torus semicircularis, and nucleus isthmi were, in the anteroposterior axis, topographically connected with the tectum. In addition, neurons in the contralateral tectum projected to the injected zones in a symmetrical point-to-point correspondence. At rhombencephalic levels, the superior reticular formation was connected to all studied tectal zones, whereas medial and inferior reticular formations were connected with medial and posterior tectal zones. The present results support a different quantitative afferent connectivity to each tectal zone, possibly based on the sensorimotor transformations that the optic tectum carries out to generate orienting responses.

Connectivity of the goldfish optic tectum with the mesencephalic and rhombencephalic reticular formation

The optic tectum of goldfish, as in other vertebrates, plays a major role in the generation of orienting movements, including eye saccades. To perform these movements, the optic tectum sends a motor command through the mesencephalic and rhombencephalic reticular formation, to the extraocular motoneurons. Furthermore, the tectal command is adjusted by a feedback signal arising from the reticular targets. Since the features of the motor command change with respect to the tectal site, the present work was devoted to determining, quantitatively, the particular reciprocal connectivity between the reticular regions and tectal sites having different motor properties. With this aim, the bidirectional tracer, biotin dextran amine, was injected into anteromedial tectal sites, where eye movements with small horizontal and large vertical components were evoked, or into posteromedial tectal sites, where eye movements with large horizontal and small vertical components were evoked. Labeled boutons and somas were then located and counted in the reticular formation. Both were more numerous in the mesencephalon than in the rhombencephalon, and ipsilaterally than contralaterally, with respect to the injection site. Furthermore, the somas showed a tendency to be located in the area containing the most dense labeling of synaptic endings. In addition, labeled boutons were often observed in close association with retrogradely stained neurons, suggesting the presence of a tectoreticular feedback circuit. Following the injection in the anteromedial tectum, most of the boutons and labeled neurons were found in the reticular formation rostral to the oculomotor nucleus. Conversely, following the injection in the posteromedial tectum, most of the boutons and neurons were also located in the caudal mesencephalic reticular formation. Finally, boutons and neurons were found in the rhombencephalic reticular formation surrounding the abducens nucleus. They were more numerous following the injection in the posteromedial tectum. These results demonstrate characteristic patterns of reciprocal connectivity between physiologically different tectal sites and the mesencephalic and rhombencephalic reticular formation. These patterns are discussed in the framework of the neural substratum that underlies the codification of orienting movements in goldfish.

Visuomotor behaviors in larval zebrafish after GFP-guided laser ablation of the optic tectum

The optic tectum is the largest visual center in most vertebrates and the main target for retinal ganglion cells (RGCs) conveying visual information from the eye to the brain. The retinotectal projection has served as an important model in many areas of developmental neuroscience. However, knowledge of the function of the tectum is limited. We began to address this issue using laser ablations and subsequent behavioral testing in zebrafish. We used a transgenic zebrafish line that expresses green-fluorescent protein in RGCs projecting to the tectum. By aiming a laser beam at the labeled retinal fibers demarcating the tectal neuropil, the larval tectum could be selectively destroyed. We tested whether tectum-ablated zebrafish larvae, when presented with large-field movements in their surroundings, displayed optokinetic responses (OKR) or optomotor responses (OMR), two distinct visuomotor behaviors that compensate for self-motion. Neither OKR nor OMR were found to be dependent on intact retinotectal connections. Also, visual acuity remained unaffected. Tectum ablation, however, slowed down the OKR by reducing the frequency of saccades but left tracking velocity, gain, and saccade amplitude unaffected. Removal of the tectum had no effect on the processing of second-order motion, to which zebrafish show both OKR and OMR, suggesting that the tectum is not an integral part of the circuit that extracts higher-order cues in the motion pathway.

Neural substrata underlying tectal eye movement codification in goldfish

The optic tectum encodes orienting eye saccades in a spatially ordered map. To investigate whether the functional properties of each tectal site are related to a particular pattern of connectivity with downward structures in the brainstem, two sets of experiments were carried out. First, biotinylated dextran amine (BDA) was injected at different tectal sites along the anteroposterior axis. Electrical stimulation at these sites evoked saccades whose horizontal component amplitudes increased with the distance to the rostral pole. In the second experiment, BDA and fluoro-ruby (FR) were injected at different tectal sites along the mediolateral axis. Electrical stimulation here evoked saccades with different upward and downward directions, but similar horizontal component amplitudes. A major finding of the first experiment was that a topographic link of the tectum exists with the mesencephalic reticular formation, but that such a connection was absent or very attenuated for the rhombencephalic reticular formation. In the second set of experiments, the clusters of BDA and FR boutons left by the mediolateral tectal sites were separated in the rostral mesencephalon, at the level of the nucleus of the medial longitudinal fasciculus, but overlapped in the caudal mesencephalon and rhombencephalon. These data provide evidence that decodification of tectal motor commands is based, at least in part, on the connectivity of each tectal locus on downward structures with the brainstem.

The decade 1989-1998 in Spanish psychology: an analysis of research in psychobiology

In this paper, we present an analysis of the research published during the 1989-1998 decade by tenured Spanish faculty members from the area of psychobiology. Database search and direct correspondence with the 110 faculty members rendered a list of 904 psychobiological papers. Classification and analysis of these papers led to the definition of at least 70 different research trends. Topics are grouped into several specific research areas: Learning and Memory; Development and Neural Plasticity; Emotion and Stress; Ethology; Neuropsychology; Sensory Processing; and Psychopharmacology. The international dissemination of this research, published in journals of high impact index, and the increasing number of papers are two noteworthy features.

Gaze shifts evoked by electrical stimulation of the superior colliculus in the head-unrestrained cat. I. Effect of the locus and of the parameters of stimulation

Several studies have suggested that the pattern of neuronal activity in the superior colliculus (SC) interacts with the well-known topographical coding of saccades (motor map). To further describe this interaction, we recorded gaze saccades evoked by electrical microstimulation of SC deeper layers in the head-unrestrained cat and systematically varied the collicular locus (25 sites) and parameters (intensity, frequency) of the stimulation. Long stimulation trains were used to avoid saccade truncation. We found that the direction and amplitude of evoked gaze shifts were related to the stimulation locus, describing a gaze shift map. For 18 out of 20 sites the amplitude, but not the direction, also strongly depended on stimulation strength. Indeed, gaze amplitude continuously increased when raising current intensity up to several times the threshold value T (the largest intensity tested was 6 x T), whereas varying pulse frequency from 150 to 750 pulses per second (p.p.s.) revealed an optimal frequency range (300 and 500 p.p.s.) eliciting the largest gaze shifts. Moreover, the intensity effect on amplitude increased in an orderly fashion with the rostro-caudal stimulation locus. Gaze shift amplitude was not related to the number of delivered stimulation pulses. Concerning movement initiation, increasing either intensity or frequency led to an exponential decrease in gaze latency until minimal values near 30 ms were reached, but the number of pulses delivered during the corresponding latency period remained constant within a 300-500 p.p.s. frequency range. These findings indicate that the pattern of collicular discharge evoked by electrical stimulation strongly interacts with the gaze shift map and provide evidence for a summation of collicular activities by downstream premotor neurons.

Eye-movement recording in freely moving animals

A new method is described for precise recording of eye movements in freely moving animals using Hall-effect devices. This inexpensive system, of small size and low weight, allows the analysis of horizontal and vertical components of saccadic eye movements, optokinetic nystagmus, slow tracking movements, eye vergence, etc., in unrestrained animals. A set of Hall-effect devices mounted in the skull is used to sense variations in the position of high-power miniature magnets fixed to the eye sclera. The output of the Hall-effect devices is amplified by operational amplifiers and collected through an analog-to-digital converter to be displayed on-line in a personal computer and stored for later analysis by specific software. Some examples of simultaneous body- and eye-movement recordings obtained in freely moving goldfish in different experimental situations are presented. This method would be useful in the recording of eye and gaze movements under natural conditions and for behavioural studies in freely moving animals.

Dissociation of place and cue learning by telencephalic ablation in goldfish

This study examined the spatial strategies used by goldfish (Carassius auratus) to find a goal in a 4-arm maze and the involvement of the telencephalon in this spatial learning. Intact and telencephalon-ablated goldfish were trained to find food in an arm placed in a constant room location and signaled by a local visual cue (mixed place-cue procedure). Both groups learned the task, but they used different learning strategies. Telencephalon-ablated goldfish learned the task more quickly and made fewer errors to criterion than controls. Probe trials revealed that intact goldfish could use either a place or a cue strategy, whereas telencephalon-ablated goldfish learned only a cue strategy. The results offer additional evidence that place and cue learning in fish are subserved by different neural substrates and that the telencephalon of the teleost fish, or some unspecified structure within it, is important for spatial learning and memory in a manner similar to the hippocampus of mammals and birds.

Tectotectal connectivity in goldfish

The vertebrate optic tectum is a functionally coupled bilateral structure which plays a major role in the generation of motor commands for orienting responses. However, the characteristics of the tectotectal connectivity are unknown in fish, and have been reported only to a limited extent in other vertebrates. The purpose of the present study was to determine the anatomical basis underlying the functional coupling between tecta in goldfish, and to identify both similarities and differences to those features reported in other vertebrate species. The present experiments used the bidirectional tracer biotinylated dextran amine to map the distribution of labeled cells and synaptic boutons in the contralateral tectum following injections into identified tectal sites. Fibers that interconnect both tecta coursed through the tectal commissure. The cells of origin of these fibers, the tectotectal cells, and their synaptic endings were located in the deep layers, mainly in the strata periventricular and griseum central, respectively. Corresponding sites throughout the two tecta were interconnected in a symmetrical point-to-point fashion. The tectal commissure was composed of at least two distinct bundles of axons, which differed in their dorsoventral location, fiber diameter, and projection targets. The dorsal axons were tectotectal axons, they were thinner in diameter and profusely branched, and gave off en passant and terminal boutons in the deep layers of the contralateral tectum. The ventral axons were thicker in diameter, and formed the contralateral tectofugal-descending tract. Such fibers had few axon collaterals and boutons in the contralateral tectum. Boutons adjacent to retrogradely labeled tectotectal cells were very scarce. The data are discussed in terms of the coupling between tecta generating the motor commands required for orienting movements.

A method for measuring eye movements using Hall-effect devices

A system for precise recording of eye position and movements in laboratory animals, by means of Hall-effect devices, is described. The system, useful in neurophysiological and neurobehavioral studies, allows the analysis of saccadic eye movements, optokinetic- and vestibular-induced nystagmus, slow tracking movements, eye vergences, and so forth. This small, light-weight, and inexpensive system uses a set of Hall-effect devices and associated electronics to sense variations in the position of high-power magnets fixed in the eye sclera or in scleral contact lenses. The output of the Hall-effect devices is amplified by operational amplifiers, collected through an A/D converter, and analyzed in a PC computer by specific software.