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First published online August 17, 2007
Journal of Experimental Biology 210, 3082-3095 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.005694

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

Gerhard von der Emde^* and Steffen Fetz

Institut für Zoologie, Universität Bonn, Endenicher Allee 11-13, 53115 Bonn, Germany

View larger version (9K): [in this window] [in a new window]   Fig. 1. Choice frequency for S+ of eight fishes trained to discriminate between two differently shaped objects, symbols of which (taken from Table 1) are shown below each bar along the abscissa. For each object pair, the object on the left depicts the S+ and that on the right the S–. In this and subsequent figures, choice frequency for the object on the left was determined in non-rewarded and non-punished test trials. The total number of choices recorded for each fish was always >100.

View larger version (16K): [in this window] [in a new window]   Fig. 2. In order to prove that fish used active electrolocation for object discrimination, control experiments were conducted with fish 1 (blue) and fish 2 (red), when they were prevented from using other senses. In (A), the training objects were concealed during test trials within electrically transparent agar to prevent their tactile (and visual) inspection. In (B), test trials were conducted under infrared light (invisible for the fish), which prevented vision. Both fish were trained to choose a pyramid (S+) and avoid a cube (S–). Choice performances during test conditions (Test) and during training conditions (Training) are compared. N denotes the total numbers of choices of fish 1 and fish 2, respectively. Asterisks above each graph indicate significant differences from random choice conditions; *P<0.05; **P<0.01.

View larger version (53K): [in this window] [in a new window]   Fig. 3. Results of transfer tests, during which the S– used during training was replaced by a novel object. Each bar depicts the choice frequency for the left object (S+) of each pair. Below each bar, the object replacing S– is named. Numbers in parentheses are the total number of each fish's choices. `Ratio' is the volume ratio of the two objects offered, i.e. volume of the right object (novel object, except for training conditions, shown in far right columns) divided by the volume of the left object (S+). (A) Fish 1 (blue) and fish 2 (red), both of which were trained to discriminate between a pyramid (S+) and a cube (S–). (B) Fish 3, trained with a cone (S+) and a pyramid (S–). (C) Fish 4, trained with an A (S+) and a mushroom (S–). (D) Fish 7 (yellow) and fish 8 (black), both trained with a cone (S+) and a cube (S–). Asterisks above each graph indicate significant differences from random choice conditions: *P<0.05; **P<0.01; n.s., non significant.

View larger version (54K): [in this window] [in a new window]   Fig. 4. (A–D) Choice frequencies of six fishes in transfer tests, during which the S+ used during training was replaced by a novel object. Each bar depicts the choice frequency for the left object (novel object or S+) of each pair. Below each bar, the object replacing S+ is named. `Ratio' is the volume ratio of the two objects offered, with the volume of the right object (S–) divided by the volume of the left object (novel object, except for training conditions, far right columns). Other details as in Fig. 3.

View larger version (35K): [in this window] [in a new window]   Fig. 5. Results of transfer tests with fish 1 (A) and fish 2 (B), in which the original objects were replaced by smaller but otherwise identical versions of them. The left lighter coloured bar of each pair gives the results with the small objects, the right darker coloured bar gives results with original sized objects. Same conditions as in Fig. 3, i.e. S–, which was used during training, was replaced by a novel object. Each bar depicts the choice frequency for the left object (S+) of each pair.

View larger version (14K): [in this window] [in a new window]   Fig. 6. Results of transfer tests with fish 1 (blue) and fish 2 (red), during which the size of the S+ (cone) was varied. In pair 2, the big cone was moved further away from the gate (3 cm instead of 0.5 cm) than during all other trials. Each bar depicts the choice frequency for the cone (S+). `Ratio' gives the volume ratios of the cube (S–) divided by the respective cone. Asterisks above each graph indicate significant differences from random choice conditions: **P<0.01; n.s., non significant.

View larger version (42K): [in this window] [in a new window]   Fig. 7. Results of transfer tests with fish 1 (blue) and fish 2 (red), during which the training objects (pyramid, S+; cube, S–) were replaced by so-called wire objects of the same outline as the training objects (see text). (A) Photos of the wire objects (left) and of the wire objects embedded in electrically transparent agar with interrupted vertical parts (right). (B–D) Choice frequencies for the pyramid (S+) by fish 1 (blue) and fish 2 (red). Right bars, original training objects; left bars, wire objects (B), vertically interrupted wire objects (C) and vertically and horizontally interrupted wire objects (D). Asterisks above each graph indicate significant differences from random choice conditions: **P<0.01; n.s., non significant.

View larger version (38K): [in this window] [in a new window]   Fig. 8. Results of transfer tests with fish 1 (blue) and fish 2 (red) (A,B) and fish 4 (C), during which one or two objects were replaced by plastic objects. In (A), object shapes remained unchanged, while S+ (pyramid), S– (cube) or both objects were replaced by otherwise identical plastic versions. In (B), two identically shaped objects (either two S+ or two S–) were offered, one of which was made of metal and the other one of plastic. In (C), several combinations of S+ (A) and S– (mushroom) made out of plastic or metal, respectively, were offered. Numbers in parentheses are the total number of each fish's choices. Asterisks above each graph indicate significant differences from random choice conditions: **P<0.01; n.s., non significant.

View larger version (10K): [in this window] [in a new window]   Fig. 9. Summary of the results of a variety of transfer tests conducted with fish 1 (diamonds) and fish 2 (squares) deciding between objects of different volumes. The choice frequencies for the smaller object of each combination are plotted versus the volume ratio of the two objects used, with the volume of the smaller object divided by the volume of the larger object. Colours depict the type of transfer experiment: red, exchange of S+ for a novel object (see Fig. 4); green, exchange of S– (see Fig. 3); blue, transfer tests with at least one plastic object (see Fig. 8).