Electric imaging through active electrolocation: implication for the analysis of complex scenes

Abstract

The electric sense of mormyrids is often regarded as an adaptation to conditions unfavourable for vision and in these fish it has become the dominant sense for active orientation and communication tasks. With this sense, fish can detect and distinguish the electrical properties of the close environment, measure distance, perceive the 3-D shape of objects and discriminate objects according to distance or size and shape, irrespective of conductivity, thus showing a degree of abstraction regarding the interpretation of sensory stimuli. The physical properties of images projected on the sensory surface by the fish's own discharge reveal a "Mexican hat" opposing centre-surround profile. It is likely that computation of the image amplitude to slope ratio is used to measure distance, while peak width and slope give measures of shape and contrast. Modelling has been used to explore how the images of multiple objects superimpose in a complex manner. While electric images are by nature distributed, or 'blurred', behavioural strategies orienting sensory surfaces and the neural architecture of sensory processing networks both contribute to resolving potential ambiguities. Rostral amplification is produced by current funnelling in the head and chin appendage regions, where high density electroreceptor distributions constitute foveal regions. Central magnification of electroreceptive pathways from these regions particularly favours the detection of capacitive properties intrinsic to potential living prey. Swimming movements alter the amplitude and contrast of pre-receptor object-images but image modulation is normalised by central gain-control mechanisms that maintain excitatory and inhibitory balance, removing the contrast-ambiguity introduced by self-motion in much the same way that contrast gain-control is achieved in vision.

Motor patterns during active electrosensory acquisition

“…Although electric images can be ambiguous (Engelmann et al, 2008), conditioning experiments in Gnathonemus have shown that these ambiguities can be overcome (Fechler et al, 2012). This indicates that the animals make use of a spatiotemporal sequence of electric images, i.e., electric flow.…”

Sensory flow shaped by active sensing: sensorimotor strategies in electric fish

“…(C,D)The superposition effect as modeled for a small pair of objects (3×3×3mm). Depending on the gap size, the static electric images (EIs) sum in a non-linear manner and will possibly be non-discriminable (modified from Engelmann et al, 2008). (E)Measured EIs (positive modulation values are normalized to the maximum modulation) for a 20×20×20mm object with a 20mm gap (see blue trace in B).…”

“…During this behavior, fish stay stationary next to an object while the tail and trunk are bent towards the object. As a consequence, the EI is subject to large (predictive) distortions influencing both intensity and contrast (Bacher, 1983;Caputi, 2004;Chen et al, 2005;Engelmann et al, 2008;Heiligenberg, 1975;von der Emde et al, 2008). The analysis of these distortions over time could be used to determine the lateral distance to an object (Sim and Kim, 2011).…”

Electrolocation based on tail-bending movements in weakly electric fish

“…Weakly electric fish can also experience changes in transdermal potential during tail bending and other body movements, and it has been reported that tail-bending movements of electric fish are strongly related to prey capture (Heiligenberg, 1975;Lannoo and Lannoo, 1993;MacIver, 2001). Tail-bending movements cause large distortions of the electric image (Bastian, 1995;Bastian, 1999, Caputi, 2004Engelmann et al, 2008). However, no study has investigated whether the tail-bending action of weakly electric fish can serve as a localization procedure for a target object near the fish body.…”

“…The electrolocation rule of the slope-to-amplitude rate can be applied to the two-dimensional electric image. In threedimensional space, weakly electric fish can initially identify the location of a target object in the rostrocaudal-dorsoventral plane (Engelmann et al, 2008), and the sensory image has peak amplitude at the sensor closest to the object. The lateral distance as another dimension in three-dimensional space is not simply determined but it may be estimated with the relative slope measure described above, that is, the slope-to-amplitude rate of the electric image (von der Emde et al, 1998;von der Emde, 1999;Schwarz and von der Emde, 2001).…”