Extra-Visual Systems in the Spatial Reorientation of Cavefish

Abstract

Disoriented humans and animals are able to reorient themselves using environmental geometry ("metric properties" and "sense") and local features, also relating geometric to non-geometric information. Here we investigated the presence of these reorientation spatial skills in two species of blind cavefish (Astyanax mexicanus and Phreatichthys andruzzii), in order to understand the possible role of extra-visual senses in similar spatial tasks. In a rectangular apparatus, with all homogeneous walls (geometric condition) or in presence of a tactilely different wall (feature condition), cavefish were required to reorient themselves after passive disorientation. We provided the first evidence that blind cavefish, using extra-visual systems, were able i) to use geometric cues, provided by the shape of the tank, in order to recognize two geometric equivalent corners on the diagonal, and ii) to integrate the geometric information with the salient cue (wall with a different surface structure), in order to recover a specific corner. These findings suggest the ecological salience of the environmental geometry for spatial orientation in animals and, despite the different niches of adaptation, a potential shared background for spatial navigation. The geometric spatial encoding seems to constitute a common cognitive tool needed when the environment poses similar requirements to living organisms.

Figure 1

Results of Experiment 1. The schematic representation of the apparatus on the right shows the percentages of choices for corners of completed training procedure for all cavefish together tested in the geometric condition: C1 and C2 = correct corners on the rewarded diagonal; X1 = near the correct corner C1; X2 = near the correct corner C2. X1 and X2 were the geometrically incorrect corners on the not rewarded diagonal. All walls of the apparatus were uniform. The learning criterion was based on correct choices for corner C1 plus corner C2 (as reported in the bars graph: group means with 95% confidence intervals). Cavefish were able to use the environmental geometry, thus not distinguishing geometrically equivalent corners C1 and C2 and choosing them significantly more than X1 and X2.

Figure 2

Results of Experiment 2. The bars graph and the schematic representation of the apparatus on the right show the percentages of choices for each corner of completed training procedure (group means with 95% confidence intervals in the graph) for all cavefish together tested in the feature condition: C = correct corner; X1 = near the correct corner, it has correct feature, but incorrect geometry; D = diagonal corner, it has correct geometry, but incorrect feature; X2 = far from the correct corner, it has incorrect feature and geometry. The learning criterion was based on correct choices for corner C. The wall (C-X1) of the apparatus with a different surface structure has been indicated with grey small bars in the schematic representation. Cavefish were able to integrate geometric and non-geometric information available, choosing predominantly the correct corner C over corners X1, D and X2.

Figure 3

Experimental apparatus. Photographs of the experimental apparatus inserted in a larger tank, without (a) and with outer opaque black cover (b): under the black cover there was a dark and comfortable environment for cavefish, becoming an incentive to get out of the experimental arena. Details of small corridors, embedded in the walls, and transparent plastic doors, at the end of each corridor, are visible in (c) - view from inside - and (d) - view from outside.