Brienomyrus niger fires its electric organ in short pulses of approximately 0.3 ms duration, at a rather irregular rate, which rapidly increases in novel situations. The animal electrolocates objects in its near field by evaluating electroreceptive feedback from its ownelectricorgandischarges (EODs). Electrolocation performance is measured by the critical distance at which an approaching nonconducting object of standard size and velocity is detected (Figs. 1, 3).

Electrolocation and jamming avoidance in the mormyrid fishBrienomyrus

In this work the methods of the communication theory for binary detection, multiple detection and extraction are applied to biological systems. It is the objective of this investigation to compare the performance of optimal and biological systems in receiving signals superimposed by noise. The required mathematical relations and methods of measurements are derived. In the second part of this work pattern recognition experiments (multiple detection) at the human visual system with stationary and time variant patterns are described. The comparison of the performance between optimal and biological system shows that the human visual system acts in a suboptimal way. From some other detection experiments it can be concluded that the recognition process is describable by spatial cross-correlation.

Methods for comparison of the efficiency of biological and technical systems

Weakly electric fish generate an electric field, called electric organ discharge (EOD), that they use for active electrosensation. This system is used for both object localisation and electrocommunication. Both, objects that are close to the fish and the EODs of other nearby electric fish, modulate the amplitude of a fish’s EOD. Localisation signals are low in amplitude and frequency whereas electrocommunication signals are large amplitude signals with higher frequencies. Electroreceptor neurons are tuned to the frequency of the fish’s own EOD. This tuning, however, is rather broad to allow for the reception of EODs of other fish with different frequencies. This is the basis for electrocommunication. Spike trains of electroreceptor afferents are surprisingly noisy even in the absence of any external signal. From theoretical studies it is known that in populations of spiking neurons such internal noise can improve the information carried about a common input signal in comparison to the noiseless case. In particular, the processing of high-frequency signals benefits from internal noise and the convergence of large populations of neurons. The target neurons of the electroreceptor afferents, the pyramidal cells in the electrosensory lateral line lobe, are organised in three distinct maps of the electroreceptive body surface that are characterised by different receptive field sizes, i.e. the number of afferents that converge on them, and frequency tuning. The properties of these three maps can be understood based on the differential impact of the noise in the electroreceptor afferent spike trains on the processing of the distinct types of signals arising in localisation and communication contexts. Further, the noise in the electroreceptors allows for the discrimination of synchronous spikes from all spikes fired by the afferent population. The level of synchrony seems particularly important for encoding high-frequency communication signals. The electrosensory system is thus a showcase for demonstrating how neural systems actually use noise to enhance processing of signals.

Neural Noise in Electrocommunication: From Burden to Benefits

Weakly electric fish such as mormyrids and gymnotids possess a well developed sensory system which enables them to locate objects by means of their own electric discharge (EOD). IVIormyrids have special receptors (mormyromast) which act as intensity coders. The electrolocating behaviour is characterized by a particular EOD pattern at which the sense organ functions at optimal level. On the other hand, \"communication\" between individuals is by means of frequency variations in their EOD. The receptors involved are in this case the tuberous (or Knollen) organs.

Elektrische Sinneswahrnehmungen und Verhalten bei elektrischen Fischen

A short description of the transmitting and receiving organ of Gnathonemus petersii is followed by results of experiments carried out to measure the sensitivity of the receptors for external signals (communication) and for distortions of the own electric field (active object location) with and without additive noise. Possible principles of reception are synchronous, asynchronous and threshold receivers; further experiments indicate the threshold receiver to be the most probable structure. Treating the active object location as a problem of statistical communication theory yields for small disturbances to a generalized cross-correlator, which in this case behaves like a threshold receiver.

Empfang gestörter Signale durch den schwachelektrischen Fisch Gnathonemus petersii

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Empfang gestörter Signale durch den schwachelektrischen Fisch Gnathonemus petersii