Brief articleModularity and spatial reorientation in a simple mind: encoding of geometric and nongeometric properties of a spatial environment by fish
Introduction
Developmental research on spatial reorientation mechanisms has shown that geometric features are spontaneously taken into account by young children and predominate over local, nongeometric cues, even when the latter would allow the organisms to make the distinction between geometrically similar places (Hermer & Spelke, 1996). For instance, when disoriented in a familiar rectangular room, perfectly homogeneous and without distinctive featural information, young children rely on the large-scale geometry of the room to reorient themselves (Hermer & Spelke, 1994). Similar results have been reported previously for several other vertebrate species (Cheng, 1986, Kelly et al., 1998, Vallortigara et al., 1990). Much more surprisingly, however, young children (Hermer & Spelke, 1994) failed to reorient by nongeometric information, such as a distinctive differently coloured wall in the rectangular cage, in spite of the fact that this featural information would have allowed fully successful reorientation. Rats also have been proved to rely almost exclusively on geometric cues in a working memory version of the reorientation task in the rectangular environment (Cheng, 1986). In a reference memory version of the task rats eventually used featural information to distinguish between geometrically equivalent locations, but geometric shape still dominated over features because rats did not follow the correct feature when it was moved to a geometrically incorrect corner (Cheng, 1986). Given that rats have been proved able to use nongeometric information for solving spatial tasks that do not involve spatial disorientation (e.g. Morris, 1981, Suzuki et al., 1980), these findings have been interpreted to suggest that spatial reorientation depends on an encapsulated, task-specific mechanism, a “geometric module” (Cheng, 1986, Cheng and Gallistel, 1984; see also Fodor, 1983). The module would encode only the geometric properties in the arrangement of surfaces as surfaces: in the case of the spatial reorientation task in the rectangular environment, for instance, the geometric module would use only “metric properties” (i.e. distinction between a long and a short wall) and what is known in geometry as “sense” (i.e. distinction between right and left).
Human adults, in contrast to young children and rats, readily solved the blue-wall version of the reorientation task in the rectangular environment (Hermer & Spelke, 1994), suggesting that the most striking limitations of the geometric module are overcome during human development. Hermer and Spelke, 1994, Hermer and Spelke, 1996 also went on with a more specific and strong hypothesis: namely that the performance of human adults, when compared with that of rats and young children, would suggest that some representational systems become more accessible and flexible over development and evolution. Research has suggested that language could be necessary to human beings for combining geometric and nongeometric information (Hermer-Vasquez, Spelke, & Katsnelson, 1999).
The aim of this paper is twofold. Firstly, we want to check whether reliance on purely geometric information for spatial reorientation could be observed even in a vertebrate species which is very distantly related to humans, such as fish. If so, that would provide quite convincing evidence for an ancient evolutionary origin of the geometric module in vertebrates. Secondly, we want to check whether the combined use of geometric and nongeometric information is indeed out of reach for (supposed-to-be) less advanced species (see e.g. Hodos & Campbell, 1969 for the difficulties associated with comparing cognitive abilities and phylogenetic histories in different, current-living organisms).
We tested fish (Xenotoca eiseni), a species that live in shallow, transparent water with pebbles and rich vegetation (Meyer, Wischnath, & Foerster, 1985), in the same task used with humans. In the first experiment fish were tested in a closed rectangular tank, lacking any distinctive landmark, with uniform white-coloured walls. Fish could escape from the tank by pressing small flexible opaque doors of similar appearance, located at the corners (Fig. 1, top). We were interested to check whether fish proved able to discriminate between the two geometrically equivalent locations, A–C, and the other two, geometrically different, locations B–D. Such a behavioural performance requires the combined use of “metric properties” and “sense” (above), which are the distinctive computations performed by the geometric module. To check that fish were orienting using only geometric information, and therefore chose geometrically equivalent corners with the same frequency, two testing conditions were devised. For some fish only one door (e.g. at corner A) and its geometric equivalent (e.g. at corner C) could be opened, the other two doors being blocked; for some other fish only one door (e.g. at corner A) could be opened, the other three doors being blocked.
In the second experiment another group of fish was tested in a similar apparatus, but this time with one wall with a distinctive colour, i.e. blue. Only one door could be opened (A in Fig. 1, bottom), the others being blocked. We were interested to check whether in this case fish could distinguish between the two geometrically equivalent corners, A and C, choosing correctly corner A. This would demonstrate that fish conjoined geometric and nongeometric information to reorient themselves.
Section snippets
Methods
Subjects were 18 mature fish (ranging 3–5 cm in length) of the species X. eiseni from a stock maintained in our laboratory within vegetation rich (Ceratophillum sp.) large tanks (55–120 l) provided with artificial illumination 16 h per day.
The apparatus consisted of a rectangular tank (31 cm long, 14 cm wide and 16 cm high), with uniform white walls in Experiment 1 (Fig. 1, top) and a distinctive blue wall in Experiment 2 (Fig. 1, bottom), covered with a one-way screen to eliminate extra-tank
Results
Frequencies of escape attempts in the white-walls task (Experiment 1) are shown in Fig. 2a,b. Data were analyzed by analysis of variance (ANOVA) with testing conditions (two doors reinforced vs. one door reinforced) as a between-subjects factor, and geometry (AC vs. BD) and sessions as within-subjects factors. The ANOVA revealed significant effects of geometry (F(1,8)=263.30, P=0.0001), testing conditions (F(1,8)=98.152, P=0.0001), sessions (F(4,32)=3.902, P=0.011) and a geometry×testing
Discussion
In Experiment 1 geometric information alone could not specify unambiguously single locations, but was sufficient for a partial disambiguation of the reorientation task. Fish chose the two geometrically equivalent locations (A and C) with equal frequency, even when only one of them was reinforced (Fig. 2b); this proves that fish did not have access to some other means of orientation. Results showed that fish could distinguish between locations A–C and locations B–D, thus revealing their ability
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