Active Electric Imaging: Body-Object Interplay and Object's “Electric Texture”

This article deals with the role of fish's body and object's geometry on determining the image spatial shape in pulse Gymnotiforms. This problem was explored by measuring local electric fields along a line on the skin in the presence and absence of objects. We depicted object's electric images at different regions of the electrosensory mosaic, paying particular attention to the perioral region where a fovea has been described. When sensory surface curvature increases relative to the object's curvature, the image details depending on object's shape are blurred and finally disappear. The remaining effect of the object on the stimulus profile depends on the strength of its global polarization. This depends on the length of the object's axis aligned with the field, in turn depending on fish body geometry. Thus, fish's body and self-generated electric field geometries are embodied in this “global effect” of the object. The presence of edges or local changes in impedance at the nearest surface of closely located objects adds peaks to the image profiles (“local effect” or “object's electric texture”). It is concluded that two cues for object recognition may be used by active electroreceptive animals: global effects (informing on object's dimension along the field lines, conductance, and position) and local effects (informing on object's surface). Since the field has fish's centered coordinates, and electrosensory fovea is used for exploration of surfaces, fish fine movements are essential to perform electric perception. We conclude that fish may explore adjacent objects combining active movements and electrogenesis to represent them using electrosensory information

Electric images of a cube on the side of the fish.

<p>Images of a cube (20 millimeters side placed 4 millimeters away from the skin) evaluated following the 3D-LEOD procedure. Images consisted of a Mexican hat profiles. Top: A copper cube causes a center-increase surround-decrease of the <i>rms</i>LEOD pattern. Bottom: A plastic cube causes the opposite effect. Shadow area indicates the orthogonal projection of the cube. Dashed lines correspond to nulls Δ<i>rms</i>LEODs.</p

Methods.

<p>A) Local fields (LEOD) were recorded along three orthogonal directions. We measured the drop of voltage between a reference point placed adjacent to the skin and each three other points placed along orthogonal directions (longitudinal, transverse and vertical) and the fields components were estimated dividing the recorded drop of voltage by the inter-electrode distance. At the fovea the field vector moved along a line, while at the trunk the field showed wobbles due to the different origin of the EOD components. B) Changes in the amplitude of field components at the fovea due to the respiratory movements.</p

Image profiles depend on objects position.

<p>A) <i>rms</i>LEOD stimulus patterns in basal (black), object centered (blue) and object lateralized (red) are compared. The main axis of the object was aligned with the largest field component in each case. Note that when the object was on the side facing a peak of the basal profile the change was greater at the region facing the object and was reduced below the control line on the other side. B) Modulation profiles showed a larger peak and a contra-lateral through when the object was on the side. C) Superimposed normalized profiles obtained from six experiments in which the spheroid was differently oriented but with the closest point facing the same point at the skin (three placed at the middle cold color traces, and three on the side, warm color traces).</p

Edge effects on the image profiles.

<p>A and B) Raw data and fitting curve showing the raw stimulus profiles in the absence (black) and presence of a “nail shaped” object (3 millimeters diameter except for the conical end, 25 millimeters total length) oriented with the tip (red) or the back (blue) towards the fovea. C) Modulation profiles show a sharp peak in the tip facing condition and two peaks on top of the global effect profile corresponding to the limits of the circular base of the object. D) Similar effects when the objects were on the side of the head. Note the Mexican hat effect on the contra-lateral side.</p

Effects of fish's body at the fovea.

<p>A) Field profiles recorded along a trajectory following the surface of the jaw of a fish's cadaver when a sinusoidal field was applied between the rostral and caudal walls of the tank. Red symbols represent the local field measured in the presence of a copper cube facing the fish's cadaver. Black symbols represent local field measured in the presence of the same object but in the absence of the fish's body. Blue line represents the effect of the fish's body calculated as the difference between the two fitted curves.</p

Amplitude of the global effect increases with the length of the field line covered by the object footprint.

<p>A and B) Images of cylinders of 10 millimeters diameter base and three different lengths with their main axis oriented along the field line. As the normalized profiles are the same the amplitude of the image grows linearly with the length of the cylinder. C and D) Image of three steel spheres showing that image amplitude increases linearly with the diameter but the normalized profile is the same. E) Image amplitude vs. diameter showing data from 8 spheres. r² = 0.96 N = 8.</p

The effect of the fish body on the electric image.

<p>At the fovea, images are similar. A) Image of the same sphere at two distances (16 millimeters diameter, 1 and 4 millimeters away from the skin) showing a reduction in amplitude. B) Normalized profiles showing that the shapes of the profiles are identical. Arrows indicate the transition between the top and the trough of the Mexican hat profile. On the side of the fish, images increase in width when the object is moved away. C) The same sphere placed at two distances (16 millimeters diameter, 2.5 and 5 millimeters away from the skin) showing a reduction in amplitude. D) Normalized profiles showing that the image increased in width when the object was moved away.</p

Importance of the alignment of the object with the field lines.

<p>Images of a prolate-spheroid-shaped copper object placed at the same distance but differently oriented with respect to the field. A to C: raw data (dots) and fitted stimulus profiles (lines). Object main axis orientation: A) longitudinal (red), B) vertical (blue), C) transverse (green). D) Modulation profiles show that in the transverse orientation (shorter dimension aligned with the field) the image of the smallest amplitude.</p