A JAR of Chirps: The Gymnotiform Chirp Can Function as Both a Communication Signal and a Jamming Avoidance Response.

The weakly electric gymnotiform fish produce a rhythmic electric organ discharge (EOD) used for communication and active electrolocation. The EOD frequency is entrained to a medullary pacemaker nucleus. During communication and exploration, this rate can be modulated by a pre-pacemaker network, resulting in specific patterns of rate modulation, including stereotyped communication signals and dynamic interactions with conspecifics known as a Jamming Avoidance Response (JAR). One well-known stereotyped signal is the chirp, a brief upward frequency sweep usually lasting less than 500 ms. The abrupt change in frequency has dramatic effects on phase precession between two signalers. We report here on chirping in Brachyhypopmus cf. sullivani, Microsternarchus cf. bilineatus Lineage C, and Steatogenys cf. elegans during conspecific playback experiments. Microsternarchus also exhibits two behaviors that include chirp-like extreme frequency modulations, EOD interruptions with hushing silence and tumultuous rises, and these are described in terms of receiver impact. These behaviors all have substantial impact on interference caused by conspecifics and may be a component of the JAR in some species. Chirps are widely used in electronic communications systems, sonar, and other man-made active sensing systems. The brevity of the chirp, and the phase disruption it causes, makes chirps effective as attention-grabbing or readiness signals. This conforms to the varied assigned functions across gymnotiforms, including pre-combat aggressive or submissive signals or during courtship and mating. The specific behavioral contexts of chirp expression vary across species, but the physical structure of the chirp makes it extremely salient to conspecifics. Chirps may be expected in a wide range of behavioral contexts where their function depends on being noticeable and salient. Further, in pulse gymnotiforms, the chirp is well structured to comprise a robust jamming signal to a conspecific receiver if specifically timed to the receiver's EOD cycle. Microsternarchus and Steatogenys exploit this feature and include chirps in dynamic jamming avoidance behaviors. This may be an evolutionary re-use of a circuitry for a specific signal in another context.

Signals and noise in the octavolateralis systems: what is the impact of human activities on fish sensory function?

The octavolateralis systems of fishes include the vestibular, auditory, lateral line and electrosensory systems. They are united by common developmental and neuro-computational features, including hair cell sensors and computations based on cross-neuron analyses of differential hair cell stimulation patterns. These systems also all use both spectral and temporal filters to separate signals from each other and from noise, and the distributed senses (lateral line and electroreception) add spatial filters as well. Like all sensory systems, these sensors must provide the animal with guidance for adaptive behavior within a sensory scene composed of multiple stimuli and varying levels of ambient noise, including that created by human activities. In the extreme, anthropogenic activities impact the octavolateralis systems by destroying or degrading the habitats that provide ecological resources and sensory inputs. At slightly lesser levels of effect, anthropogenic pollutants can be damaging to fish tissues, with sensory organs often the most vulnerable. The exposed sensory cells of the lateral line and electrosensory systems are especially sensitive to aquatic pollution. At still lesser levels of impact, anthropogenic activities can act as both acute and chronic stressors, activating hormonal changes that may affect behavioral and sensory function. Finally, human activities are now a nearly ubiquitous presence in aquatic habitats, often with no obvious effects on the animals exposed to them. Ship noise, indigenous and industrial fishing techniques, and all the ancillary noises of human civilization form a major part of the soundscape of fishes. How fish use these new sources of information about their habitat is a new and burgeoning field of study.

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.

Vibratory sources as compound stimuli for the octavolateralis systems: dissection of specific stimulation channels using multiple behavioral approaches.

Underwater vibratory sources simultaneously present acoustic and hydrodynamic disturbances. Because vibratory dipole sources are poor sonic projectors, most researchers have assumed that such sources are of greatest relevance to the lateral line system. Both hydroacoustic principles and empirical studies have shown that dipole sources are also a potent stimulus to the inner ear. Responses to vibratory sources in mottled sculpin (Cottus bairdi) were assessed using unconditioned orienting, differential and nondifferential conditioning. Orienting responses are dominated by lateral line inputs and eliminated by lateral line inactivation. Simple conditioning depends on inputs from other systems and was not affected by lateral line inactivation. Differential conditioning alters behavioral control, and sculpin could be conditioned to ignore substrate-borne vibrations and respond only to hydroacoustic stimulation of the ear. The lateral line and inner ear of mottled sculpin do not necessarily exhibit range fractionation, as both systems operate over a similar distance (within 1.5 body lengths) and respond to many of the same sources. Vibratory dipole sources generate compound stimuli that simultaneously activate multiple octavolateralis systems, and sculpin make use of the channels differentially under different behavioral tasks.

The detection of pressure fluctuations, sonic audition, is the dominant mode of dipole-source detection in goldfish (Carassius auratus).

Behavioral detection of a low-frequency (40 Hz) vibratory dipole at source distances of 1.5-24 cm was measured by classically conditioned respiratory suppression in goldfish (Carassius auratus). Detection thresholds were compared across distances and before and after ablation of individual octavolateralis sensory channels. Detection thresholds, expressed in units of pressure (SPL), remained roughly constant as distance between the stimulus source and animal increased. Lateral line inactivation, using CoCl2, had no measurable effect on sensitivity, although some other results can be construed as weak evidence for a small contribution of the lateral line to dipole detection when source distances are

Objective threshold estimation and measurement of the residual background noise in auditory evoked potentials of goldfish.

A survey of papers using auditory evoked potentials (AEPs) published over the last 10 years (Table I) demonstrates that most AEP studies in animals have used subjective methods for auditory threshold determination. Subjective methods greatly reduce the value of statistical hypothesis testing and jeopardize tests of hypothetical experimental group differences in hearing sensitivity. Correspondingly, many attempts have been made to develop objective threshold determination methods, but these have not been used widely. Further, they seldom include an appreciation of the effects of residual noise in the AEP. In this study, AEPs evoked by tonal and noise stimuli in goldfish (Carassius auratus) were recorded and the residual background noise was measured and analyzed in detail. High variability was found in residual noise, but can be effectively controlled with a simple modification of averaging routines. Considerable interobserver disagreements were found using subjective threshold estimation. An objective method of threshold determination was developed based on comparison between AEP amplitude and controlled residual noise, using a signal detection theory approach to set specific threshold criteria. The usefulness of AEP in hypothesis testing for auditory function requires more control over residual background noise amplitudes and the use of objective threshold determination techniques.

The hydrodynamic footprint of a benthic, sedentary fish in unidirectional flow.

Mottled sculpin (Cottus bairdi) are small, benthic fish that avoid being swept downstream by orienting their bodies upstream and extending their large pectoral fins laterally to generate negative lift. Digital particle image velocimetry was used to determine the effects of these behaviors on the spatial and temporal characteristics of the near-body flow field as a function of current velocity. Flow around the fish's head was typical for that around the leading end of a rigid body. Flow separated around the edges of pectoral fin, forming a wake similar to that observed for a flat plate perpendicular to the flow. A recirculation region formed behind the pectoral fin and extended caudally along the trunk to the approximate position of the caudal peduncle. In this region, the time-averaged velocity was approximately one order of magnitude lower than that in the freestream region and flow direction varied over time, resembling the periodic shedding of vortices from the edge of a flat plate. These results show that the mottled sculpin pectoral fin significantly alters the ambient flow noise in the vicinity of trunk lateral line sensors, while simultaneously creating a hydrodynamic footprint of the fish's presence that may be detected by the lateral line of nearby fish.

The use of anesthesia during evoked potential audiometry in goldfish (Carassius auratus).

Auditory-evoked potentials (AEPs) have become a widely utilized measure of hearing sensitivity. Most investigators use pharmacological paralysis to reduce myogenic noise and immobilize the animal for stable electrical recordings, but additional anesthesia is generally not used because the most commonly available fish anesthetic, the cholinergic antagonist tricaine methanosulfate (MS222), is known to disrupt hair cell and primary afferent physiology. Anesthetic agents that do not interfere with auditory function would be a useful adjunct to paralytic immobilization and would reduce any possible distress incurred by prolonged immobilization. In this report we tested the opiate anesthetic fentanyl and compared hearing thresholds in immobilized versus immobilized and anesthetized animals. Short-term effects of mild MS222 anesthesia were also measured via evoked potential audiometry. Animals were tested before and after fentanyl injection (100, 500 and 2500 microg g(-1) fish body-weight) using standard evoked potential audiometry. Tone pips, 0.2-3 kHz, from an aerial loudspeaker served as stimuli. Fentanyl altered evoked potential waveforms slightly but did not alter estimated threshold sensitivity. These results suggest fentanyl be considered as a possible addition to AEP techniques in goldfish (Carassius auratus) and poikilothermic vertebrates generally.

What is the nature of multisensory interaction between octavolateralis sub-systems?

The octavolateralis system consists of several submodalities, including the inertial-sensitive inner ear, the pressure-sensitive ear/air cavity complex (when present), and acceleration- and velocity-sensitive components of the lateral line system (canal and superficial neuromasts, respectively). All four of these channels are responsive to many of the same stimulus sources, particularly moving or vibrating objects within a short distance from the receiver. We therefore argue that the octavolateralis system is an excellent model for the study of multisensory interactions. We focus on the possible ways in which these channels may contribute to source localization mechanisms and to the multisensory guidance of behaviors with strong directional components (e.g., predator avoidance, prey capture and mate attraction). Finally, we define four ways in which information from multiple senses might interact. These include fractionation, synergy, accessory stimulation, and complementation. Although evidence for all types of octavolateralis interactions can be found, the primary modes of interaction appear to be complementation and fractionation. For example, the inertial and pressure-sensitive submodalities of the auditory system provide complementary pieces of information about the direction (e.g., left/right) and polarity (advancing or receding) of a moving source. In contrast, the lateral line canal system subserves short-range localization tasks, whereas the auditory system may subserve longer-range detection and localization tasks.