SUMMARY In the absence of light, the weakly electric fish Gnathonemus
petersii detects and distinguishes objects in the environment through
active electrolocation. In order to test which features of an object the fish
use under these conditions to discriminate between differently shaped objects,
we trained eight individuals in a food-rewarded, two-alternative,
forced-choice procedure. All fish learned to discriminate between two objects
of different shapes and volumes. When new object combinations were offered in
non-rewarded test trials, fish preferred those objects that resembled the one
they had been trained to (S+) and avoided objects resembling the one that had
not been rewarded (S–). For a decision, fish paid attention to the
relative differences between the two objects they had to discriminate. For
discrimination, fish used several object features, the most important ones
being volume, material and shape. The importance of shape was demonstrated by
reducing the objects to their 3-dimensional contours, which sufficed for the
fish to distinguish differently shaped objects. Our results also showed that
fish attended strongly to the feature `volume9, because all individuals tended
to avoid the larger one of two objects. When confronted with metal
 versus plastic objects, all fish avoided metal and preferred plastic
objects, irrespective of training. In addition to volume, material and shape,
fish attended to additional parameters, such as corners or rounded edges. When
confronted with two unknown objects, fish weighed up the positive and negative
properties of these novel objects and based their decision on the outcome of
this comparison. Our results suggest that fish are able to link and assemble
local features of an electrolocation pattern to construct a representation of
an object, suggesting that some form of a feature extraction mechanism enables
them to solve a complex object recognition task.

/pdf/distance-shape-and-more-recognition-of-object-features-4rg549e9co.pdf

Distance, shape and more: recognition of object features during active electrolocation in a weakly electric fish.

To investigate the functional organization of higher brain levels in fish we test the hypothesis that the dorsal gray mantle of the telencephalon of a mormyrid fish has discrete receptive areas for several sensory modalities. Multiunit and compound field potentials evoked by auditory, visual, electrosensory, and water displacement stimuli in this weakly electric fish are recorded with multiple semimicroelectrodes placed in many tracks and depths in or near telencephalic area dorsalis pars medialis (Dm). Most responsive loci are unimodal; some respond to two or more modalities. Each modality dominates a circumscribed area, chiefly separate. Auditory and electrical responses cluster in the dorsal 500 micrometer of rostral and caudolateral Dm, respectively. Two auditory subdivisions underline specialization of this sense. Mechanoreception occupies a caudal area overlapping electroreception but centered 500 micrometer deeper. Visual responses scatter widely through ventral areas. Auditory, electrosensory, and mechanosensory responses are dominated by a negative wave within the first 50 msec, followed by 15-55 Hz oscillations and a slow positive wave with multiunit spikes lasting from 200 to 500 msec. Stimuli can induce shifts in coherence of certain frequency bands between neighboring loci. Every electric organ discharge command is followed within 3 msec by a large, mainly negative but generally biphasic, widespread corollary discharge. At certain loci large, slow ("deltaF") waves usually precede transient shifts in electric organ discharge rate. Sensory-evoked potentials in this fish pallium may be more segregated than in elasmobranchs and anurans and have some surprising similarities to those in mammals.

https://www.jneurosci.org/content/jneuro/18/18/7381.full.pdf

Sensory processing in the pallium of a mormyrid fish

Despite their diversity, vertebrate retinae are specialized to maximize either photon catch or visual acuity. Here, we describe a functional type that is optimized for neither purpose. In the retina of the elephantnose fish (Gnathonemus petersii), cone photoreceptors are grouped together within reflecting, photonic crystal-lined cups acting as macroreceptors, but rod photoreceptors are positioned behind these reflectors. This unusual arrangement matches rod and cone sensitivity for detecting color-mixed stimuli, whereas the photoreceptor grouping renders the fish insensitive to spatial noise; together, this enables more reliable flight reactions in the fish's dim and turbid habitat as compared with fish lacking this retinal specialization.

http://science.sciencemag.org/content/sci/336/6089/1700.full.pdf

Photonic crystal light collectors in fish retina improve vision in turbid water.

Weakly electric fish use active electrolocation for object detection and orientation in their environment even in complete darkness The African mormyrid Gnathonemus petersii can detect object parameters, such as material, size, shape and distance Here, we tested whether individuals of this species can learn to identify 3-dimensional objects independently of the training conditions and independently of the object’s position in space (rotation-invariance; size-constancy) Individual G petersii were trained in a two-alternative forced-choice procedure to electrically discriminate between a 3-dimensional object (S+) and several alternative objects (S-) Fish were then tested whether they could identify the S+ among novel objects and whether single components of S+ were sufficient for recognition Size-constancy was investigated by presenting the S+ together with a larger version at different distances Rotation-invariance was tested by rotating S+ and/or S- in 3D Our results show that electrolocating G petersii could (1) recognize an object independently of the S- used during training When only single components of a complex S+ were offered, recognition of S+ was more or less affected depending on which part was used (2) Object-size was detected independently of object distance, ie fish showed size-constancy (3) The majority of the fishes tested recognized their S+ even if it was rotated in space, ie, these fishes showed rotation-invariance (4) Object recognition was restricted to the near field around the fish and failed when objects were moved more than about 4 cm away from the animals Our results indicate that even in complete darkness our G petersii were capable of complex 3-dimensional scene perception using active electrolocation

/pdf/3-dimensional-scene-perception-during-active-electrolocation-4tq5sv6nbl.pdf

3-Dimensional scene perception during active electrolocation in a weakly electric pulse fish

Weakly electric fish generate electric fields with an electric organ and perceive them with cutaneous electroreceptors. During active electrolocation, nearby objects are detected by the distortions they cause in the electric field. The electrical properties of objects, their form and their distance, can be analysed and distinguished. Here we focus on Gnathonemus petersii (Gunther 1862), an African fish of the family Mormyridae with a characteristic chin appendix, the Schnauzenorgan. Behavioural and anatomical results suggest that the mobile Schnauzenorgan and the nasal region serve special functions in electroreception, and can therefore be considered as electric foveae. We investigated passive pre-receptor mechanisms that shape and enhance the signal carrier. These mechanisms allow the fish to focus the electric field at the tip of its Schnauzenorgan where the density of electroreceptors is highest (tip-effect). Currents are funnelled by the open mouth (funnelling-effect), which leads to a homogenous voltage distribution in the nasal region. Field vectors at the trunk, the nasal region and the Schnauzenorgan are collimated but differ in the angle at which they are directed onto the sensory surface. To investigate the role of those pre-receptor effects on electrolocation, we recorded electric images of objects at the foveal regions. Furthermore, we used a behavioural response (novelty response) to assess the sensitivity of different skin areas to electrolocation stimuli and determined the receptor densities of these regions. Our results imply that both regions - the Schnauzenorgan and the nasal region - can be termed electric fovea but they serve separate functions during active electrolocation.

/pdf/active-sensing-in-a-mormyrid-fish-electric-images-and-3wlrhtbnor.pdf

Active sensing in a mormyrid fish: electric images and peripheral modifications of the signal carrier give evidence of dual foveation.

The spectral sensitivity of the weakly electric mormyrid fish Gnathonemus petersi was investigated under dark- and light-adapted conditions using a transient change (startle) in its electric organ discharge (EOD) rate as response measure. The startle was resistant to habituation and graded with light intensity. Under both lighting conditions, the fish responded optimally to a monochromatic light of 525 nm. A porphyropsin pigment (520–5402) appears to mediate spectral sensitivity over most of the visible spectrum. However, G. petersi responded more strongly to 625- and 675-nm lights (dark- and light-adapted fish) and a 725-nm light (light-adapted fish only) than predicted by the presence of a single rod pigment. These data suggest that at least one additional visual pigment (most likely of cone cells) maximally absorbing long wavelength light (600 nm or longer) is present. The spectral sensitivity data are consistent with the sensitivity hypothesis in that heightened sensitivity to long wavelength light is predicted for fish living in blackwater habitats which are characterized typically by low light levels and transmission of predominantly long wavelengths. Histology of the retina showed photoreceptors grouped into bundles and ensheathed by pigment epithelial cells. Our results demonstrated a functional visual sense in a species of fish much better known and studied for its electrosensory and electromotor abilities.

Spectral sensitivity of the weakly discharging electric fish Gnathonemus petersi using its electric organ discharges as the response measure

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Spectral sensitivity of the weakly discharging electric fish Gnathonemus petersi using its electric organ discharges as the response measure