Carl D. Hopkins - US grants

The proposed research is a neuroethological approach to the study of vertebrate social communication. The system of choice is the highly sophisticated electric communication system known for two groups of freshwater fishes, the Mormyridae of Africa and the Gymnotiformes of South America. Both field and laboratory studies are planned. There are two main goals: the first is to define the structure of communication signals, the second, to uncover the mechanisms of signal processing in the electrosensory system relevant to the process of species recognition. The electrosensory system is an ideal system for studying temporal processing of rapid, time-varying waveforms. The studies will have relevance not just to the electrosensory system, which is specialized for temporal processing, but also to other similar sensory systems like the auditory system. Both mormyrid and gymnotiform fishes send and receive electric signals for electrolocation and for electrical communication. Both groups have species which produce brief, pulsatile Electric Organ Discharges (EODs) which have stereotyped, often species-specific, and in some cases sex-specific waveforms. Pulses are repeated at variable rates to make up characteristic sequences of pulse intervals (SPIs) used for communication. Behavioral experiments on the mormyrids have already demonstrated that electric fish attend to the temporal characteristics of these waveforms in species- and in sex-recognition. The new research will be directed at: 1) Defining communication signals encoded in the SPIs of mormyrids and gymnotiforms, and at determining the relative importance of SPIs and EODs in species- and sex-recognition among the gymnotiformes through the use of field observation and playback experiments. 2) Uncovering a physiological basis for the fine sensitivity to temporal cues in the electrosensory system of mormyrids by the use of electrophysiological and neuroanatomical techniques. Electrophysiological studies will concentrate on temporal processing of electric signals by the Knollenorgan electroreceptors of mormyrids, putative communication sensors for electric communication. Single unit electrophysiology and neuroanatomy will be used to trace the processing of EOD-like stimuli through this central pathway to the level of the midbrain.

Sense organs provide two parallel important functions. They serve to detect and recognize particular stimuli, and to localize a stimulus in space; the animal then can make appropriate behavioral responses with a correct spatial orientation to the guided behavior. Electroreception is a special sense exploited by several fishes that use electric organs to produce a discharge used for object detection and communication. Unlike propagated sound that gives timing cues for auditory localization, electric signals act instantly; unlike straight rays of light that give sight lines for visual localization, the vectors of the electric currents are often curved. This project examines the guidance of oriented electrosen- sory behavior in electric fish using behavior, physiology, and computer modelling. Video analysis in three dimensions will be used to quantify the swimming path as a fish orients to distant electric discharges that mimic other electric fish. This behavior involves aligning the body axis parallel to the electric field vector, and swimming toward the current source. The approach track will be correlated with the electric field derived by computation that will include aquarium geometry, and both stationary and moving electrodes. Electrophysiological activity will be recorded from the nerves that innervate two classes of electroreceptors, to characterize the sensory coding of the electric field parameters. The physiological results will be compared to theoretical analyses of the electric field across the fish's skin, as a function of geometry and skin resistance. Mechanisms of stimulus localization are fundamental to sensory processing. This study uses a novel approach that will provide a substantial contribution to understanding electrosense in particular, and other sensory systems in general.

The proposed research is a neuroethological study of social communication among electroreceptive fishes of South America (the Gymnotiformes) and Africa (the Mormyridae). Electrical fish are seen as a model system for a mechanistic study of communication in a vertebrate. The system is favorable because it includes many of the rich complexities that we associate with a vertebrate social communication system while employing signals that are relatively simple to describe and to reproduce, and a sensory modality whose characteristics can be carefully documented. This research will examine the mechanisms of electric signal production and evolution, and will explore the neuronal basis for electric signal recognition among pulse-type electric fish. Many of the African and South American electric fish produce pulsatile electric organ discharges (EODs) which serve in species- and sex-recognition. In addition, they discharge these pulses according to patterned sequences of pulse intervals (SPIs) which serve in display functions including courtship, alarm, appeasement and threat. The first goal of this study is to describe the variation in the structure of the communication signals among the pulse gymnotiform fishes of the genus, Hypopomus from South America. Members of this genus differ in the waveform, duration, spectral characteristics, and amplitude of their species-typical EODs; males and females also differ with regard to these parameters. Animals will be collected in their natural habitat order to catalogue the variation in EODs. The amplitudes of discharges will be measured in order to determine the distance over which signals are effective in their own natural habitats, and under conditions of artificially-altered water conductivity. These comparative studies will provide a quantitative database for understanding signal variation within populations of animals, and a basis for understanding how signals evolve. Behavioral observations will be conducted to record the electrical display repertoire of Hypopomus both under field conditions, and laboratory conditions. Playback experiments will be conducted to study the ability of Hypopomus to recognize electric discharges and to discriminate between them. The experiments will explore species and sex recognition in Hypopomus. Electrophysiological studies will be run in parallel to describe the sensory coding of natural EOD stimuli by peripheral electroreceptors in Hypopomus. The electric stimuli will be presented to the animals in a naturalistic way. Electrophysiological studies will be conducted in mormyrids at sensory processing of electric discharges at the level of the midbrain.

[unreadable] DESCRIPTION (provided by applicant): This research aims to understand how the auditory system, and other time-based senses, encode and recognize complex features of natural stimuli. In the auditory system, temporal coding is important for localization and recognition of speech and other sounds. In electroreception in fishes, independently evolved neural pathways have converged upon similar anatomical and physiological solutions to temporal coding requirements. This project focuses on electroreception in mormyrid fishes because these fish offer several significant research opportunities: an unusual but potentially important mechanism of temporal comparison involving timed inhibition; a system with short, simple signals that invite neurobiological analysis; and a conveniently exposed anatomy that permits observation of neural activity by optical imaging, and electrophysiological exploration by single cell analysis with microelectrodes. The researchers, based at Cornell University will test four related hypotheses about temporal coding in electroreception. To test the hypothesis that recognition of species-specific signals is based on temporal analysis, the researchers will conduct playback experiments designed to identify key features of stimuli eliciting approach and avoidance. A quantitative multi-factorial survey of species and sex differences in Electric Organ Discharge (EOD) waveforms will be undertaken to examine the importance of temporal codes in signal diversity. To test the hypothesis that neural pathways sharpen the timing of spikes responding to EODs, the researchers will record spike jitter at different levels of a sensory hierarchy from periphery to midbrain. To test the hypothesis that temporal analysis is based on a delay-line anticoincidence detector in the midbrain, the researchers will record evoked potentials in response to different stimulus geometries, and also employ optical imaging of calcium-activated fluorescence to monitor activity of time-comparison cells which are too small to record from physiologically. The researchers will also test with optical imaging the presence or absence of a map of stimulus duration in the midbrain predicted to be absent. Finally, the researchers will test the hypothesis that multidimensional stimulus features including stimulus location, waveform duration, and stimulus repetition rate are preserved and analyzed by higher centers in the time-coding pathway. [unreadable] [unreadable]

This award provides partial support for purchase of a modern capillary DNA sequencer and supporting computer equipment for the Cornell Evolutionary Genetics Core Facility (EGCF). The instrument will be equipped for both sequence determination and fragment analysis. The facility supports research by evolutionary biologists, population biologists, behavioral ecologists, and systematists. Research in these areas increasingly depends on variation in the DNA sequences of individual organisms to provide information on mating systems, population structure, genealogy, and phylogeny. Automated DNA sequencing and genotyping are critical to studies employing such variation. The facility is used by 19 faculty in seven departments across the Cornell campus, and contributes significantly to their research programs and the educational experience of their graduate and undergraduate students. Their research programs address various aspects of biodiversity, including the origins and maintenance of plant and animal diversity, and the management of taxa of conservation concern or of economic importance. Aside from the impact of each individual user, the facility plays an important role in teaching and training on the Cornell campus. The new equipment will increase throughput and data quality for all users, improve the training of students, and catalyze interactions among evolutionary biologists and ecologists with diverse taxonomic and disciplinary interests.