Hans Straka died in the morning of December 11, 2022 at his home in Munich, unexpected and much too early. He was a dedicated biologist, loved the mountains and was connected to home (Oberammergau, active participant in the Passion Play). His scientific journey took him from Munich via Paris and New York back to Munich and his many academic accomplishments ranged from a membership of the Editorial board of the Journal of Neurophysiology and of the Journal of Neuroscience. He was associate editor for Frontiers in Neuro-otology and for the volume "The Senses" he edited the part on Vestibular Function in 2020. In 2009 he became Professor of Systemic Neurosciences at the Department of Biology in Munich. Apart from his many academic accomplishments, however, Hans was a close friend to those of us who were fortunate enough to get to know him better.

Hans Straka died in the morning of December 11, 2022 at his home in Munich, unexpected and much too early. He was a dedicated biologist, loved the mountains and was connected to home (Oberammergau, active participant in the Passion Play). His scientific journey took him from Munich via Paris and New York back to Munich and his many academic accomplishments ranged from a membership of the Editorial board of the Journal of Neurophysiology and of the Journal of Neuroscience. He was associate editor for Frontiers in Neuro-otology and for the volume "The Senses" he edited the part on Vestibular Function in 2020. In 2009 he became Professor of Systemic Neurosciences at the Department of Biology in Munich. Apart from his many academic accomplishments, however, Hans was a close friend to those of us who were fortunate enough to get to know him better.

Computational capability and connectivity are key elements for understanding how central vestibular neurons contribute to gaze-stabilizing eye movements during self-motion. In the well-characterized and segmentally distributed hindbrain oculomotor network of goldfish, we determined afferent and efferent connections along with discharge patterns of descending octaval nucleus (DO) neurons during different eye motions. Based on activity correlated with horizontal eye and head movements, DO neurons were categorized into two complementary groups that either increased discharge during both contraversive (type II) eye (e) and ipsiversive (type I) head (h) movements (e II h I ) or vice versa (e I h II ). Matching time courses of slow-phase eye velocity and corresponding firing rates during prolonged visual and head rotation suggested direct causality in generating extraocular motor commands. The axons of the dominant e II h I subgroup projected either ipsi- or contralaterally and terminated in the abducens nucleus, Area II, and Area I with additional recurrent collaterals of ipsilaterally projecting neurons within the parent nucleus. Distinct feedforward commissural pathways between bilateral DO neurons likely contribute to the generation of eye velocity signals in e I h II cells. The shared contribution of DO and Area II neurons to eye velocity storage likely represents an ancestral condition in goldfish that is clearly at variance with the task separation between mammalian medial vestibular and prepositus hypoglossi neurons. This difference in signal processing between fish and mammals might correlate with a larger repertoire of visuo-vestibular-driven eye movements in the latter species that potentially required a shift in sensitivity and connectivity within the hindbrain-cerebello-oculomotor network. NEW & NOTEWORTHY We describe the structure and function of neurons within the goldfish descending octaval nucleus. Our findings indicate that eye and head velocity signals are processed by vestibular and Area II velocity storage integrator circuitries whereas the velocity-to-position Area I neural integrator generates eye position solely. This ancestral condition differs from that of mammals, in which vestibular neurons generally lack eye position signals that are processed and stored within the nucleus prepositus hypoglossi.

ABSTRACT The vertebrate hindbrain develops as a series of well‐defined neuroepithelial segments or rhombomeres. While rhombomeres are visible in all vertebrate embryos, generally there is not any visible segmental anatomy in the brains of adults. Teleost fish are exceptional in retaining a rhombomeric pattern of reticulospinal neurons through embryonic, larval, and adult periods. We use this feature to map more precisely the segmental imprint in the reticular and motor basal hindbrain of adult goldfish. Analysis of serial sections cut in three planes and computer reconstructions of retrogradely labeled reticulospinal neurons yielded a segmental framework compatible with previous reports and more amenable to correlation with surrounding neuronal features. Cranial nerve motoneurons and octavolateral efferent neurons were aligned to the reticulospinal scaffold by mapping neurons immunopositive for choline acetyltransferase or retrogradely labeled from cranial nerve roots. The mapping corresponded well with the known ontogeny of these neurons and helps confirm the segmental territories defined by reticulospinal anatomy. Because both the reticulospinal and the motoneuronal segmental patterns persist in the hindbrain of adult goldfish, we hypothesize that a permanent “hindbrain framework” may be a general property that is retained in adult vertebrates. The establishment of a relationship between individual segments and neuronal phenotypes provides a convenient method for future studies that combine form, physiology, and function in adult vertebrates. J. Comp. Neurol. 522:2446–2464, 2014. © 2014 Wiley Periodicals, Inc.

Central vestibular neurons form identifiable subgroups within the boundaries of classically outlined octavolateral nuclei in primitive vertebrates that are distinct from those processing lateral line, electrosensory and auditory signals. Each vestibular subgroup exhibits a particular morpho-physiological property that receives origin-specific sensory inputs from semicircular canal and otolith organs. Behaviorally characterized phenotypes send discrete axonal projections to extraocular, spinal and cerebellar targets including other ipsi- and contralateral vestibular nuclei. The anatomical locations of vestibuloocular and vestibulospinal neurons correlate with genetically defined hindbrain compartments that are well conserved throughout vertebrate evolution though some variability exists in fossil and extant vertebrate species. The different vestibular subgroups exhibit a robust sensorimotor signal processing complemented with a high degree of vestibular and visual adaptive plasticity.

Monocular organization of the goldfish horizontal neural integrator was studied during spontaneous scanning saccadic and fixation behaviors. Analysis of neuronal firing rates revealed a population of ipsilateral (37%), conjugate (59%), and contralateral (4%) eye position neurons. When monocular optokinetic stimuli were employed to maximize disjunctive horizontal eye movements, the sampled population changed to 57, 39, and 4%. Monocular eye tracking could be elicited at different gain and phase with the integrator time constant independently modified for each eye by either centripetal (leak) or centrifugal (instability) drifting visual stimuli. Acute midline separation between the hindbrain oculomotor integrators did not affect either monocularity or time constant tuning, corroborating that left and right eye positions are independently encoded within each integrator. Together these findings suggest that the "ipsilateral" and "conjugate/contralateral" integrator neurons primarily target abducens motoneurons and internuclear neurons, respectively. The commissural pathway is proposed to select the conjugate/contralateral eye position neurons and act as a feedforward inhibition affecting null eye position, oculomotor range, and saccade pattern.

The vestibuloocular reflex (VOR) provides an amenable model of learning and memory. Neurons from the Area II nucleus of goldfish are necessary for VOR plasticity, carrying an efferent copy of the eye velocity motor command to the cerebellum. Recently we introduced a novel sensitivity analysis method [1] to quantify how dendritic morphology and intrinsic electrical parameters affect neuronal function. This approach also allows precise combinations of active conductances to be predicted that can compensate for a given morphologic change to restore normal function. Here we have extended our method to analyze interactions between explicit pairs of parameters. We reconstructed the morphology and built multi-compartment models of three distinct Area II neurons, followed by an automated parameter optimization to fit each model to a single set of physiological data. Using partial least squares regression to identify sensitivity trends globally across the space of active and morphologic parameters, neuronal outputs relevant to eye velocity integration were found to be driven largely by intracellular calcium concentration near the soma, and voltage attenuation in distal dendrites. We then analyzed the second order sensitivity matrices (Hessians) of perisomatic calcium concentration and distal voltage attenuation, for each morphologic model at two disparate points in parameter space. The results identified sensitive directions, precise combinations of electrical and morphologic parameters that varied output the most, and robust directions that varied output the least. Perturbing the model along the sensitive direction of distal voltage attenuation, which minimally changed perisomatic calcium concentration, was sufficient to induce functional changes that might underlie vestibular plasticity paradigms. Yet, even large perturbations along the robust direction of distal voltage attenuation had little effect on functional output. Our results show that specific, subtle changes of morphology and conductances in the dendrites are sufficient to alter the range of a neuron’s firing dynamics. We propose that dendritic morphology plays a significant role in determining the functional flexibility, or robustness, of a neuron.

Semicircular canals have been sensors of angular acceleration for 450 million years. This vertebrate adaptation enhances survival by implementing postural and visual stabilization during motion in a three-dimensional environment. We used an integrated neuroethological approach in larvalXenopusto demonstrate that semicircular canal dimensions, and not the function of other elements, determines the onset of angular acceleration detection. Before angular vestibuloocular function in either the vertical or horizontal planes, at stages 47 and 48, respectively, each individual component of the vestibuloocular system was shown to be operational: extraocular muscles could be activated, central neural pathways were complete, and canal hair cells were capable of evoking graded responses. ForXenopus, a minimum semicircular canal lumen radius of 60 μm was necessary to permit endolymph displacement sufficient for sensor function at peak accelerations of 400°/s2. An intra-animal comparison demonstrated that this size is reached in the vertical canals earlier in development than in the horizontal canals, corresponding to the earlier onset of vertical canal-activated ocular motor behavior. Because size constitutes a biophysical threshold for canal-evoked behavior in other vertebrates, such as zebrafish, we suggest that the semicircular canal lumen and canal circuit radius are limiting the onset of vestibular function in all small vertebrates. Given that the onset of gravitoinertial acceleration detection precedes angular acceleration detection by up to 10 d inXenopus, these results question how the known precise spatial patterning of utricular and canal afferents in adults is achieved during development.

The macroevolutionary events leading to neural innovations for social communication, such as vocalization, are essentially unexplored. Many fish vocalize during female courtship and territorial defense, as do amphibians, birds, and mammals. Here, we map the neural circuitry for vocalization in larval fish and show that the vocal network develops in a segment-like region across the most caudal hindbrain and rostral spinal cord. Taxonomic analysis demonstrates a highly conserved pattern between fish and all major lineages of vocal tetrapods. We propose that the vocal basis for acoustic communication among vertebrates evolved from an ancestrally shared developmental compartment already present in the early fishes.

In neural integrators, transient inputs are accumulated into persistent firing rates that are a neural correlate of short-term memory. Integrators often contain two opposing cell populations that increase and decrease sustained firing as a stored parameter value rises. A leading hypothesis for the mechanism of persistence is positive feedback through mutual inhibition between these opposing populations. We tested predictions of this hypothesis in the goldfish oculomotor velocity-to-position integrator by measuring the eye position and firing rates of one population, while pharmacologically silencing the opposing one. In complementary experiments, we measured responses in a partially silenced single population. Contrary to predictions, induced drifts in neural firing were limited to half of the oculomotor range. We built network models with synaptic-input thresholds to demonstrate a new hypothesis suggested by these data: mutual inhibition between the populations does not provide positive feedback in support of integration, but rather coordinates persistent activity intrinsic to each population.

Several studies have shown that dendritic morphology and spatial distributions of active ion channels contribute significantly to neuronal firing dynamics and signal processing, however the relative importance and interactions between these mechanisms are not well understood. Within computational models these intrinsic properties are represented by parameters with different units and magnitudes that interact nonlinearly to simulate experimentally observed behaviors. Mathematical sensitivity analysis provides a tool to assess how strongly these parameters influence model output. For a given model neuron, the normalized sensitivity coefficient ('sensitivity') of its output to a particular parameter describes the percentage change in its output for a one percent increase in that parameter. The sensitivity magnitude indicates how much the measured output changes under the perturbation, while its sign indicates whether the output increases or decreases. One such sensitivity can be computed to each parameter; its sign and magnitude may vary for models represented by different points in a space defined by morphologic, active membrane, and passive cable parameters. We perform this sensitivity analysis on a compartmental model comprising a soma and cylindrical active dendrite, with output measured by firing rates under current injections and by firing rate gain. Across the parameter space, sensitivity of these output variables to perturbations of dendritic length, diameter and surface area was compared with sensitivity to active and passive conductance parameters. For spontaneous firing rate, sensitivity increased most with increasing levels of persistent sodium and A-type potassium conductance, whereas for gain, sensitivity increased most as high-threshold calcium conductance decreased. Particularly in regions of space with slow calcium removal from the cytoplasm in the model cells, sensitivities of firing rates and gain to dendritic diameter and surface area were greater than to almost all active parameters. The sensitivity analysis was extended to a model neuron from the precerebellar nucleus Area II of goldfish. Area II neurons are necessary for eye velocity storage, a mechanism that displays persistent activity after extinguishing visual or vestibular stimuli. Parameter optimization identified sets of active and passive model parameters consistent with Area II electrophysiology for a morphology obtained in vivo and traced in 3D. Sensitivities to perturbations of dendritic length, diameter and surface area were compared with those of active and passive parameters. These data indicate that, as in the simple model, there are regions of parameter space where dendritic morphology influences firing rate and gain more strongly than active conductances or passive cable parameters.

Elucidating the causal role of head and eye movement signaling during cerebellar-dependent oculomotor behavior and plasticity is contingent on knowledge of precerebellar structure and function. To address this question, single-unit extracellular recordings were made from hindbrain Area II neurons that provide a major mossy fiber projection to the goldfish vestibulolateral cerebellum. During spontaneous behavior, Area II neurons exhibited minimal eye position and saccadic sensitivity. Sinusoidal visual and vestibular stimulation over a broad frequency range (0.1-4.0 Hz) demonstrated that firing rate mirrored the amplitude and phase of eye or head velocity, respectively. Table frequencies >1.0 Hz resulted in decreased firing rate relative to eye velocity gain, while phase was unchanged. During visual steps, neuronal discharge paralleled eye velocity latency (approximately 90 ms) and matched both the build-up and the time course of the decay (approximately 19 s) in eye velocity storage. Latency of neuronal discharge to table steps (40 ms) was significantly longer than for eye movement (17 ms), but firing rate rose faster than eye velocity to steady-state levels. The velocity sensitivity of Area II neurons was shown to equal (+/- 10%) the sum of eye- and head-velocity firing rates as has been observed in cerebellar Purkinje cells. These results demonstrate that Area II neuronal firing closely emulates oculomotor performance. Conjoint signaling of head and eye velocity together with the termination pattern of each Area II neuron in the vestibulolateral lobe presents a unique eye-velocity brain stem-cerebellar pathway, eliminating the conceptual requirement of motor error signaling.

Intracellular recording and single-cell labeling were combined to investigate the oculomotor circuitry of the goldfish cerebellar vestibulolateral lobe, consisting of the eminentia granularis (Egr) and caudal lobe. Purkinje cells exhibiting highly conserved vertebrate electrophysiological and morphological properties provide the direct output from the caudal lobe to the vestibular nuclei. Biocytin labeling of the Egr distinguished numerous hindbrain precerebellar sources that could be divided into either putative mechano- or vestibulosensitive nuclei based on cellular location and axon trajectories. Precerebellar neurons in a hindbrain nucleus, called Area II, were electrophysiologically characterized after antidromic activation from the Egr (>50% bilateral) and their morphology analyzed after intracellular biocytin labeling (n = 28). Bipolar spindle-shaped somas ranged widely in size with comparably scaled dendritic arbors exhibiting largely closed field configuration. Area II neurons (85%) projected to the ipsilateral Egr with most (93%) sending a collateral through the cerebellar commissure to the contralateral Egr; however, 15% projected to the contralateral Egr by crossing in the ventral hindbrain. Axon terminals in the vestibular nucleus were the only collaterals within the hindbrain. Every Area II neuron received a disynaptic EPSP after contralateral horizontal canal nerve stimulation and a disynaptic IPSP, preceded by a small EPSP (>50%), after ipsilateral activation. Vestibular synaptic potentials were of varying shape/amplitude, unrelated to neuron location in the nucleus, and thus likely a correlate of somadendritic size. The exceptional separation of eye position and eye velocity signals into two separate hindbrain nuclei represents an ideal model for understanding the precerebellar projection to the vestibulocerebellum.

We used a comparative genomic approach to identify putative cis-acting regulatory sequences of the zebrafish hoxb3a and hoxb4a genes. We aligned genomic sequences spanning the clustered Hoxb1 to Hoxb5 genes from pufferfish, mice, and humans with the zebrafish hoxba and hoxbb cluster sequences. We identified multiple blocks of conserved sequences in non-coding regions within and surrounding the Hoxb3/b4 gene locus; a subset of these blocks are conserved in the zebrafish hoxbb cluster, despite loss of hoxb3/b4 genes. Overall, we find that the architecture of the Hoxb3/b4 loci and of the conserved sequence elements is very similar in teleosts and mammals. Our analyses also revealed two alternative transcripts of the zebrafish hoxb3a gene and an exon sequence unusually located 10 kb upstream of adjacent hoxb4a; an equivalent murine Hoxb3 exon has not yet been confirmed. We show that many of the Hoxb3/b4 conserved non-coding sequences correlate with functional neural enhancers previously described in the mouse. Further, within the conserved non-coding sequences we have identified binding sites for transcription factors, including Kreisler/Valentino, Krox20, Hox, and Pbx, some of which had not been previously described for the mouse. Finally, we demonstrate that the regulatory sequences of zebrafish hoxa3a are divergent with respect to the mouse ortholog Hoxa3, or the paralog hoxb3a. Despite limited conservation of regulatory sequences, zebrafish hoxa3a and hoxb3a genes share very similar expression profiles.

The morphology of secondary vertical vestibular neurons was investigated by injection of horseradish peroxidase (HRP) into cells connected to the posterior canal system in rabbits (lateral-eyed animals) and cats (frontal-eyed animals). Vestibular neurons were identified by stimulation with bipolar electrodes implanted into the ampullae of the anterior and posterior (PC) semicircular canals of pigmented rabbits; in the cat, these cells were identified by natural and electrical stimulation. Axons monosynaptically activated by PC stimulation were injected with HRP in the medial longitudinal fasciculus (MLF). These were later reconstructed by light microscopy after the brains had been processed with a DAB-CoCl2 method.

To study the genetic and developmental basis of sensorimotor processing, behavioral output must be quantified before the neural circuit dynamics can be investigated. The oculomotor system is ideal as it has been extensively utilized for quantitative analysis; however, no complete apparatus exists to both elicit and measure the eye movements in small genetic model organisms in real time. Instrumentation designed for much larger animals must be scaled to accommodate animals only a few millimeters in length, while accurately quantifying the motion of the minuscule eyes. To this end, a video microscope and optokinetic drum were mounted on a miniature, motorized vestibular turntable, servo-controlled with velocity and position feedback and capable of producing sinusoidal motion or position triangles with latencies less than 0.10 s and accelerations greater than 1000 degrees/s(2). The optokinetic drum, also feedback controlled, accommodated a wide range of spatial frequencies that could be nested concentrically to provide visual stimuli for both monocular and binocular testing. Larval and juvenile Xenopus, zebrafish, goldfish, and medaka were embedded in agarose with the head free, allowing unrestricted eye movements and normal respiration. Infrared transillumination permitted video imaging of eye movements in either light or dark. Video images were computer processed in real-time (60 Hz), producing accurate (+/- 0.1 degrees) eye position measurements that, in turn, could be utilized in real-time for visuo-motor plasticity paradigms. This instrumentation permits high resolution ontogenetic analysis of oculomotor function in small animals as illustrated for larval zebrafish (5 to 35 dpf).

In a companion paper, we reported that the goldfish oculomotor neural integrator could be trained to instability or leak by rotating the visual surround with a velocity proportional to +/- horizontal eye position, respectively. Here we analyze changes in the firing rate behavior of neurons in area I in the caudal brainstem, a central component of the oculomotor neural integrator. Persistent firing could be detuned to instability and leak, respectively, along with fixation behavior. Prolonged training could reduce the time constant of persistent firing of some cells by more than an order of magnitude, to <1 s. Normal visual feedback gradually retuned persistent firing of integrator neurons toward stability, along with fixation behavior. In animals with unstable fixations, approximately half of the eye position-related cells had upward or unstable firing rate drift. In animals with leaky fixations, two-thirds of the eye position-related cells showed leaky firing drift. The remaining eye position-related cells, generally those with lower eye position thresholds, showed a more complex pattern of history-dependent/predictive firing rate drift in relation to eye drift. These complex drift cells often showed a drop in maximum persistent firing rate after training to leak. Despite this diversity, firing drift and the degree of instability or leak in firing rates were broadly correlated with fixation performance. The presence, strength, and reversibility of this plasticity demonstrate that, in this system, visual feedback plays a vital role in gradually tuning the time course of persistent neural firing.