Concept learning and the use of three common psychophysical paradigms in the archerfish (Toxotes chatareus)

Abstract

Archerfish are well known for their specialized hunting technique of spitting water at prey located above the water line. This unique ability has made them a popular focus of study as researchers try to understand the mechanisms involved in targeting and spitting. In more recent years, archerfish have also become an increasingly popular model for studying visual discrimination and learning in general. Until now, only the alternative forced-choice (AFC) task has been used with archerfish, however, they may be capable of learning other classical discrimination tasks. As well as providing alternative, and potentially more efficient, means for testing their visual capabilities, these other tasks may also provide deeper insight into the extent to which an organism with no cortex can grasp the concepts underlying these tasks. In this paper, we consider both the matched-to-sample (MTS) and the odd-one-out (OOO) tasks as they require the subject to learn relatively sophisticated concepts rather than a straight, stimulus-reward relationship, of the kind underlying AFC tasks. A variety of line drawings displayed on a monitor were used as stimuli. We first determined if archerfish could complete the MTS and OOO test and then evaluated their ability to be retrained to new stimuli using a 4-AFC test. We found that archerfish were unable to learn the MTS and had only a limited capacity for learning the OOO task. We conclude that the MTS and OOO are impractical as paradigms for behavioral experiments with archerfish. However, the archerfish could rapidly learn to complete an AFC test and select the conditioned stimulus with a high degree of accuracy when faced with four stimuli, making this a powerful test for behavioral studies testing visual discrimination. In addition, the fish were able to learn the concept of oddity under particular training circumstances. This paper adds to the growing evidence that animals without a cortex are capable of learning some higher order concepts.

Keywords: alternative forced-choice; behavior; matched-to-sample; odd-one-out; visual discrimination.

FIGURE 1

Illustration of the stimulus presentation protocols used in the delayed and simultaneous matched-to-sample/oddity-from-sample (MTS/OFS), and the odd-one-out (OOO) task. Stimuli were a range of black line drawings (not drawn to scale in figure) on a white background, presented on a computer monitor suspended directly above the aquarium. (A) Odd-one-out. The archerfish were presented with four stimuli, three identical S- and one different S+. These stimuli could appear in any of four possible positions on the monitor. The archerfish were required to select the single reward stimulus (S+). In this case the correct response is indicated as a dashed line representing a correctly aimed spit response. (B) Delayed MTS/OFS. The archerfish were presented with the sample stimulus in the middle of the monitor, shown here as S. The archerfish were required to hit the sample stimulus in order to trigger the display of the comparison stimuli and the removal of the sample. Of the two comparison stimuli, one stimulus was identical to the sample and the second stimulus was different from the sample. The fish was required to select the matching stimulus in the MTS test or select the different stimulus in the OFS test. In the figure, an example of a correct response is indicated as a spit to the reward stimulus. (C) Simultaneous matched-to-sample/oddity-from-sample. Similarly to the delayed MTS/OFS, a sample stimulus was presented in the middle of the monitor (S). However, once the archerfish hit the sample it remained on the monitor and the two comparison stimuli (S+ and S-) were immediately presented. The archerfish then selected either S+ or S- but selection of the sample stimulus was neither rewarded nor penalized.

FIGURE 2

Discrimination performance as a function of time (binned by testing session), for four fish performing an odd-one-out task. Two stimuli were selected for each trial from a pool of four possibilities. See Table 1 for stimuli used. The dashed line at 45% indicates a statistically significant selection frequency of S+ and the dashed line at 25% indicates chance.

FIGURE 3

Results of three training procedures for matched-to-sample/oddity-from-sample (MTS/OFS) tasks. (A) Learning curve of four archerfish when presented with a delayed MTS/OFS task. A pool of 10 shapes was used as stimuli. (B) Learning curve of four archerfish given a similar delayed MTS/OFS task with the modification that the pool of stimuli used was reduced to three. (C) Learning curve of four archerfish given a simultaneous MTS/OFS task using a pool of three shapes as stimuli. The dashed line at 75% in all figures indicates a statistically significant selection frequency of S+ and the dashed line at 50% indicates chance. Filled symbols represent fish trained to an OFS task and empty symbols represent fish trained to a MTS task. See Table 1 for example stimuli.

FIGURE 4

Selection frequency of three stimuli (S1, S2, and S3) for two experiments; (A) delayed MTS/ OFS, and (B) simultaneous MTS/OFS. See Table 1 for example stimuli. The total trial number was 40 for each experiment and the selection frequency of each stimulus was tested for a selection preference using a Chi-square test.

FIGURE 5

Discrimination performance as a function of time (binned by testing session), indicating the steady improvement observed for two fish carrying out the 3-AFC task. Fish 5 and 7 were trained to select S1 and S2, respectively (Table 1).

FIGURE 6

Learning curve of four archerfish conditioned to complete a 4-AFC task. Four stimuli were presented where three stimuli were identical and were unrewarded (S-) while a single unique stimulus was rewarded (S+). In A, S+ is a cross and S- is a square. The stimuli were then replaced by a star (S+) and a triangle (S-; B). Stimuli were changed for a third time to an arrow (S+) and a crescent (S-; C). The dashed line at 45% in all figures indicates a statistically significant selection frequency of S+ and the dashed line at 25% indicates chance. See Table 1 for example stimuli.

FIGURE 7

Selection frequency (%) of S+ using a 4-AFC test where all three conditioned stimulus pairs were presented within a session. The results of two testing sessions (n = 20 trails each) are presented for four subjects. The dashed line at 45% indicates a statistically significant selection frequency of S+. All subjects achieved an S+ selection frequency above chance.