The brain creates illusions not just for us: sharks (Chiloscyllium griseum) can "see the magic" as well

Abstract

Bamboo sharks (Chiloscyllium griseum) were tested for their ability to perceive subjective and illusionary contours as well as line length illusions. Individuals were first trained to differentiate between squares, triangles, and rhomboids in a series of two alternative forced-choice experiments. Transfer tests then elucidated whether Kanizsa squares and triangles, grating gaps and phase shifted abutting gratings were also perceived and distinguished. The visual systems of most vertebrates and even invertebrates perceive illusionary contours despite the absence of physical luminance, color or textural differences. Sharks are no exception to the rule; all tasks were successfully mastered within 3-24 training sessions, with sharks discriminating between various sets of Kanizsa figures and alternative stimuli, as well as between subjective contours in >75% of all tests. However, in contrast to Kanizsa figures and subjective contours, sharks were not deceived by Müller-Lyer (ML) illusions. Here, two center lines of equal length are comparatively set between two arrowheads or -tails, in which case the line featuring the two arrow tails appears to be longer to most humans, primates and birds. In preparation for this experiment, lines of varying length, and lines of unequal length randomly featuring either two arrowheads or -tails on their ends, were presented first. Both sets of lines were successfully distinguished by most sharks. However, during presentation of the ML illusions sharks failed to succeed and succumbed either to side preferences or chose according to chance.

Keywords: Chiloscyllium griseum; Kanizsa; Müller-Lyer deception; elasmobranch; optical illusion; subjective contour.

Figure 1

The experimental setup located within the experimental basin, inside the white pavilion. The keyhole-shaped setup consisted of a Starting Compartment, a decision area and a frosted screen for projections, featuring a divider allowing for unambiguous choice-making (left and right). For the projections, a LED beamer was used. Sharks were placed within the SC at the start of each trial. 1 = feeders, 2 = frosted screen for projection, 3 = cable pulls to release feeders, 4a = guillotine door, 4b = cable pull to open guillotine door, 5 = ceiling mounted fluorescent tubes (above pavilion roof).

Figure 2

Experiment 1. Shown are the stimuli that were presented to each group during regular training and transfer test trials in experiments 1a and 1b. The positive, rewarded stimulus is indicated by a checkmark. (A) In group 1 an empty square was the positive, rewarded stimulus, in group 2 it was an empty triangle. During the T1 transfer tests of experiment 1a, sharks were “expected” to choose the correct Kanizsa figure. (B) During experiment 1b, group 1 was trained to recognize an empty square over an empty triangle, whereas group 2 was trained vice versa. During the T2 transfer tests, sharks were expected to choose the Kanizsa figure resembling the stimulus they had been trained on.

Figure 3

Experiment 2. Shown are the stimuli presented to each group during regular training and transfer test trials. The positive, rewarded stimulus is marked by a checkmark. All sharks were trained to choose a white square presented on diagonal lines. During T3 transfer tests (2a), sharks were expected to choose the subjective contour defining a square by using grating gaps within the white lines; during T4 transfer tests (2b), sharks were expected to choose the subjective contour defining a square by using phase-shifted abutting gratings.

Figure 4

Experiment 3. Shown are the stimuli presented to each group during the regular training and transfer test trials (3a and 3b). The positive, rewarded stimulus is marked by a checkmark. (A) In 3a, all sharks were trained to choose the longer of the two lines. In the T5 transfer tests, sharks were presented with two lines of equal length (5 vs. 5 cm). (B) In the second part (3b), sharks were trained to choose the longer of the two lines, irrespective of the orientation of the arrowheads (arrowheads or -tails). During the T6 transfer tests, sharks were presented with the Müller-Lyer deception (two lines of equal length but with differently oriented arrowheads). The gray dotted lines are only shown here to simplify the figure, but were not shown during the experiments.

Figure 5

Experiment 1. Shown is the performance of Shark 7 as % of correct choices (symbolized by triangles; left ordinate) per session as well as the average trial time in seconds (symbolized by gray bars; right ordinate) per session per phase until the learning criterion was reached.

Figure 6

Experiment 1a and b. Shown are the group results for the transfer test trials. p > 0.05 not significant, p ≤ 0.01 significant (^**), p ≤ 0.001 significant (^***). The correct choice is marked by a checkmark.

Figure 7

Experiment 2. Shown is the performance of Shark 8 as % of correct choices per session (symbolized by boxes; left ordinate) as well as the average trial time (s) per session (symbolized by gray bars; right ordinate) per phase until the learning criterion was reached.

Figure 8

Experiment 2. Shown are the group results for the transfer trials (2a and 2b). p > 0.05 not significant, p ≤ 0.001 significant (^***). The correct choice is marked by a checkmark.

Figure 9

Size pairs. Shown is the performance of Shark 3 as the percentage of correct choices per session (symbolized by boxes; left ordinate) as well as the average trial time (s) per session (symbolized by gray bars; right ordinate) per phase until the learning criterion was reached.

Figure 10

Müller-Lyer deception. Shown is the performance of Shark 5 as the percentage of correct choices per session (symbolized by boxes; left ordinate) as well as the average trial time (s) per session (symbolized by gray bars; right ordinate) per phase until the learning criterion was reached.

Figure 11

Size pairs and Müller-Lyer deception. Shown are the group results of the transfer test trials. p > 0.05 not significant, p ≤ 0.01 highly significant (^**), p ≤ 0.001 significant (^***). The correct choice is marked by a checkmark.