Hydrodynamic image formation by the peripheral lateral line system of the Lake Michigan mottled sculpin, Cottus bairdi

A multisensory perspective on near-field detection and localization of hydroacoustic sourcesa)

This paper gives a brief synopsis of the research career of S.C. in fish bioacoustics with an emphasis on dipole near fields. The hydroacoustic nature of the dipole near field and the effective stimuli to lateral line and auditory systems combine to produce a multisensory, range-fractionated region that is critically important to many fish behaviors. The mottled sculpin and goldfish lateral lines encode the spatial complexities of the near field as spatial excitation patterns along the body surface to provide instantaneous snapshots of various source features such as distance, orientation, and direction of movement. In contrast, the pressure-sensitive channel of the goldfish auditory system [the anterior swim bladder (SB)-saccule complex] encodes the spatial complexities in a temporal fashion whenever the position or orientation of the source changes with respect to the anterior SB. A full appreciation for how these somatotopic and egocentric representations guide fish behavior requires an understanding of how multisensory information, including vision, is combined in sensorimotor regions of the brain to effect behavior. A brief overview of vertebrate brain organization indicates that behaviors directed to or away from hydroacoustic sources likely involve a variety of mechanisms, behavioral strategies, and brain regions.

Evaluating the Death and Recovery of Lateral Line Hair Cells Following Repeated Neomycin Treatments

Acute chemical ablation of lateral line hair cells is an important tool to understand lateral line-mediated behaviors in free-swimming fish larvae and adults. However, lateral line-mediated behaviors have not been described in fish larvae prior to swim bladder inflation, possibly because single doses of ototoxin do not effectively silence lateral line function at early developmental stages. To determine whether ototoxins can disrupt lateral line hair cells during early development, we repeatedly exposed zebrafish larvae to the ototoxin neomycin during a 36 h period from 3 to 4 days post-fertilization (dpf). We use simultaneous transgenic and vital dye labeling of hair cells to compare 6-h and 12-h repeated treatment timelines and neomycin concentrations between 0 and 400 µM in terms of larval survival, hair cell death, regeneration, and functional recovery. Following exposure to neomycin, we find that the emergence of newly functional hair cells outpaces cellular regeneration, likely due to the maturation of ototoxin-resistant hair cells that survive treatment. Furthermore, hair cells of 4 dpf larvae exhibit faster recovery compared to 3 dpf larvae. Our data suggest that the rapid functional maturation of ototoxin-resistant hair cells limits the effectiveness of chemical-based methods to disrupt lateral line function. Furthermore, we show that repeated neomycin treatments can continually ablate functional lateral line hair cells between 3 and 4 dpf in larval zebrafish.

Morphological malleability of the lateral line allows for surface fish (Astyanax mexicanus) adaptation to cave environments

The lateral line is the primary modality fish use to create a hydrodynamic image of their environment. These images contribute to a variety of behaviors, from rheotaxis to escape responses. Here we discern the contributions of visual and lateral line modalities in hunting behavior of larvae that have developed under different photic conditions. In particular, cave animals have a hypertrophied sense of mechanosensation, and we studied the common animal model cavefish Astyanax mexicanus and its closest related surface relative. We raised larvae in a diurnal light-dark regimen and in complete darkness. We then examined the distribution of neuromasts in their lateral lines, and their hunting performance in light and dark conditions, with and without the contribution of the lateral line. We report that all larva depend on the lateral line for success in hunting and that surface fish raised in the dark have a greater dependency on the lateral line.

Imaging dipole flow sources using an artificial lateral-line system made of biomimetic hair flow sensors

In Nature, fish have the ability to localize prey, school, navigate, etc., using the lateral-line organ. Artificial hair flow sensors arranged in a linear array shape (inspired by the lateral-line system (LSS) in fish) have been applied to measure airflow patterns at the sensor positions. Here, we take advantage of both biomimetic artificial hair-based flow sensors arranged as LSS and beamforming techniques to demonstrate dipole-source localization in air. Modelling and measurement results show the artificial lateral-line ability to image the position of dipole sources accurately with estimation error of less than 0.14 times the array length. This opens up possibilities for flow-based, near-field environment mapping that can be beneficial to, for example, biologists and robot guidance applications.

Building the posterior lateral line system in zebrafish

The posterior lateral line (pLL) in zebrafish has emerged as an excellent system to study how a sensory organ system develops. Here we review recent studies that illustrate how interactions between multiple signaling pathways coordinate cell fate,morphogenesis, and collective migration of cells in the posterior lateral line primordium. These studies also illustrate how the pLL system is contributing much more broadly to our understanding of mechanisms operating during the growth, regeneration, and self-organization of other organ systems during development and disease.

Perception of frequency, amplitude, and azimuth of a vibratory dipole source by the octavolateralis system of goldfish (Carassius auratus)

Goldfish (Carassius auratus) were conditioned to suppress respiration to a 40-Hz vibratory source and subsequently tested for stimulus generalization to frequency, stimulus amplitude, and position (azimuth). Animals completely failed to generalize to frequencies separated by octave intervals both lesser and greater than the CS. However, they did appear to generalize weakly to an aerial loudspeaker stimulus of the same frequency (40 Hz) after conditioning with an underwater vibratory source. Animals had a gradually decreasing amount of generalization to amplitude changes, suggesting a perceptual dimension of loudness. Animals generalized largely or completely to the same underwater source presented at a range of source azimuths. When these azimuths were presented at a transect of 3 cm, some animals did show decrements in generalization, while others did not. This suggests that although azimuth may be perceived more saliently at distances closer to a dipole source, perception of position is not immediately salient in conditioned vibratory source detection. Differential responding to test stimuli located toward the head or tail suggests the presence of perceptual differences between sources that are rostral or caudal with respect to the position of the animal or perhaps the head.

Progressive neurogenesis defines lateralis somatotopy

Fishes and amphibians localize hydromechanical variations along their bodies using the lateral-line sensory system. This is possible because the spatial distribution of neuromasts is represented in the hindbrain by a somatotopic organization of the lateralis afferent neurons' central projections. The mechanisms that establish lateralis somatotopy are not known. Using BAPTI and neuronal tracing in the zebrafish, we demonstrate growth anisotropy of the posterior lateralis ganglion. We characterized a new transgenic line for in vivo imaging to show that although peripheral growth-cone structure adumbrates somatotopy, the order of neurogenesis represents a more accurate predictor of the position of a neuron's central axon along the somatotopic axis in the hindbrain. We conclude that progressive neurogenesis defines lateralis somatotopy.

Using computational fluid dynamics to calculate the stimulus to the lateral line of a fish in still water

This paper presents the first computational fluid dynamics (CFD)simulations of viscous flow due to a small sphere vibrating near a fish, a configuration that is frequently used for experiments on dipole source localization by the lateral line. Both two-dimensional (2-D) and three-dimensional (3-D) meshes were constructed, reproducing a previously published account of a mottled sculpin approaching an artificial prey. Both the fish-body geometry and the sphere vibration were explicitly included in the simulations. For comparison purposes, calculations using potential flow theory (PFT) of a 3-D dipole without a fish body being present were also performed. Comparisons between the 2-D and 3-D CFD simulations showed that the 2-D calculations did not accurately represent the 3-D flow and therefore did not produce realistic results. The 3-D CFD simulations showed that the presence of the fish body perturbed the dipole source pressure field near the fish body, an effect that was obviously absent in the PFT calculations of the dipole alone. In spite of this discrepancy, the pressure-gradient patterns to the lateral line system calculated from 3-D CFD simulations and PFT were similar. Conversely, the velocity field, which acted on the superficial neuromasts (SNs), was altered by the oscillatory boundary layer that formed at the fish's skin due to the flow produced by the vibrating sphere (accounted for in CFD but not PFT). An analytical solution of an oscillatory boundary layer above a flat plate, which was validated with CFD, was used to represent the flow near the fish's skin and to calculate the detection thresholds of the SNs in terms of flow velocity and strain rate. These calculations show that the boundary layer effects can be important, especially when the height of the cupula is less than the oscillatory boundary layer's Stokes viscous length scale.

Source location encoding in the fish lateral line canal

The position of a hydrodynamic dipole source, as encoded in a linear array of mechano-detecting neuromasts in the fish lateral line canal, was electrophysiologically investigated. Measured excitation patterns along the lateral line were compared to theoretical predictions and were found to be in good agreement. The results demonstrate that information on the position of a vibrating source from a fish is linearly coded in the spatial characteristics of the excitation pattern of pressure gradients distributed along the lateral line canal. Several algorithms are discussed that could potentially be used by a fish to decode lateral line excitation patterns, in order to localise a source and its axis of vibration. Specifically, a wavelet transform of a 1-D excitation pattern is shown to reconstruct a 2-D image of dipole sources located within a distance comparable to the body length of a fish and with a close range spatial accuracy twice the inter-neuromast distance.

Estimating position and velocity of a submerged moving object by the clawed frog Xenopus and by fish--a cybernetic approach

The lateral-line system is a unique facility of aquatic animals to locate predator, prey, or conspecifics. We present a detailed model of how the clawed frog Xenopus, or fish, can localize submerged moving objects in three dimensions by using their lateral-line system. In so doing we develop two models of a slightly different nature. First, we exploit the characteristic properties of the velocity field, such as zeros and maxima or minima, that a moving object generates at the lateral-line organs and that are directly accessible neuronally, in the context of a simplified geometry. In addition, we show that the associated neuronal model is robust with respect to noise. Though we focus on the superficial neuromasts of Xenopus the same arguments apply mutatis mutandis to the canal lateral-line system of fish. Second, we present a full-blown three-dimensional reconstruction of the source on the basis of a maximum likelihood argument.

Information-processing demands in electrosensory and mechanosensory lateral line systems

The electrosensory and mechanosensory lateral line systems of fish exhibit many common features in their structural and functional organization, both at the sensory periphery as well as in central processing pathways. These two sensory systems also appear to play similar roles in many behavioral tasks such as prey capture, orientation with respect to external environmental cues, navigation in low-light conditions, and mediation of interactions with nearby animals. In this paper, we briefly review key morphological, physiological, and behavioral aspects of these two closely related sensory systems. We present arguments that the information processing demands associated with spatial processing are likely to be quite similar, due largely to the spatial organization of both systems and the predominantly dipolar nature of many electrosensory and mechanosensory stimulus fields. Demands associated with temporal processing may be quite different, however, due primarily to differences in the physical bases of electrosensory and mechanosensory stimuli (e.g. speed of transmission). With a better sense of the information processing requirements, we turn our attention to an analysis of the functional organization of the associated first-order sensory nuclei in the hindbrain, including the medial octavolateral nucleus (MON), dorsal octavolateral nucleus (DON), and electrosensory lateral line lobe (ELL). One common feature of these systems is a set of neural mechanisms for improving signal-to-noise ratios, including mechanisms for adaptive suppression of reafferent signals. This comparative analysis provides new insights into how the nervous system extracts biologically significant information from dipolar stimulus fields in order to solve a variety of behaviorally relevant problems faced by aquatic animals.

3-D-orientation with the octavolateralis system

Fish detect and localize a sound source with inner ear receptors and with the mechanosensory lateral line. The inner ear of fish is sensitive to the water displacements caused by sound waves through a direct, inertial response by hair cell epithelia of the ear. Hearing specialists, such as goldfish and herring, have accessory peripheral structures that provide additional sensitivity to the pressure component of a sound wave. While the inner ear of fish responds to the whole body motions caused by sound waves and--in case of hearing specialists--to sound pressure, the lateral line is only sensitive to water motions relative to the surface of the fish and to local pressure gradients. Using lateral line and/or acoustic input, some fish can determine the direction and the distance to a sound source. Most likely they do so by exploiting some of the mechanisms described in this paper. Piscivorous fish may use lateral line input to detect the wakes caused by swimming fish. Even in the absence of light catfish, for instance, can follow a 10 s old, three-dimensional wake left by a prey fish over distances up to 55 prey-body length.

Minimal model of prey localization through the lateral-line system

The clawed frog Xenopus is an aquatic predator catching prey at night by detecting water movements caused by its prey. We present a general method, a "minimal model" based on a minimum-variance estimator, to explain prey detection through the frog's many lateral-line organs, even in case several of them are defunct. We show how waveform reconstruction allows Xenopus' neuronal system to determine both the direction and the character of the prey and even to distinguish two simultaneous wave sources. The results can be applied to many aquatic amphibians, fish, or reptiles such as crocodiles.

Directional selectivity and frequency tuning of midbrain cells in the oyster toadfish, Opsanus tau

Single-unit recordings were made from areas in the midbrain (torus semicircularis) of the oyster toadfish. We evaluated frequency tuning and directional responses using whole-body oscillation to simulate auditory stimulation by particle motion along axes in the horizontal and mid-sagittal planes. We also tested for bimodality in responses to auditory and hydrodynamic stimuli. One recording location in each animal was marked by a neurobiotin injection to confirm the recording site. Recordings were made in nucleus centralis, nucleus ventrolateralis, and the deep cell layer. Most units were frequency-selective with best frequencies between 50 and 141 Hz. Suppression of activity was apparent in 10% of the cells. Bimodality was common, including inhibition and suppression of background activity by auditory or hydrodynamic stimulation. The majority of the cells were directionally selective with directional response patterns that were sharpened compared with those of primary saccular afferents. The best directional axes were arrayed widely in spherical space, covering most azimuths and elevations. This representation is adequate for the computation of the motional axis of an auditory stimulus for sound source localization.