Stimulus characteristics, learning bias and visual discrimination in zebrafish (Danio rerio)

Zebrafish is an emerging model in the study of brain function; however, knowledge about its behaviour and cognition is incomplete. Previous studies suggest this species has limited ability in visual learning tasks compared to other teleosts. In this study, we systematically examined zebrafish's ability to learn to discriminate colour, shape, size, and orientation of figures using an appetitive conditioning paradigm. Contrary to earlier reports, the zebrafish successfully completed all tasks. Not all discriminations were learned with the same speed and accuracy. Subjects discriminated the size of objects better than their shape or colour. In all three tasks, they were faster and more accurate when required to discriminate between outlined figures than between filled figures. With stimuli consisting of outlines, the learning performance of zebrafish was comparable to that observed in higher vertebrates. Zebrafish easily learned a horizontal-vertical discrimination task, but like many other vertebrates, they had great difficulty discriminating a figure from its mirror image. Performance was more accurate for subjects reinforced on one stimulus (green over red, triangle over circle, large over small). Unexpectedly, these stimulus biases occurred only when zebrafish were tested with filled figures, suggesting some causal relationship between stimulus preference, learning bias and performance.

The devil is in the detail: Zebrafish learn to discriminate visual stimuli only if salient

Due to their unique characteristics, the zebrafish plays a key role in the comprehension of neurobiology of cognition and its pathologies, such as neurodegenerative diseases. More and more molecular tools for this aim are being developed, but our knowledge about the cognitive abilities of zebrafish remains extremely scarce compared to other teleost fish. We aimed to investigate the complex cognitive abilities of zebrafish using a tracking-based automated conditioning chamber that allowed precise experimental control, avoided potential cueing provided by the observer (Clever Hans phenomenon), and was shown to considerably improve learning in other teleosts. A computer presented two visual stimuli in two sectors of the chamber, and zebrafish had to enter the correct sector to obtain a food reward. Zebrafish quickly learned to use the conditioning device and easily performed up to 80 trials per day. In Experiment 1, zebrafish efficiently discriminated between two differently coloured sides, reaching a 75 % accuracy in only 10 training sessions. Surprisingly, zebrafish failed to choose the correct chamber when the stimuli were two shapes, a small circle and a small triangle, even when, in Experiment 2, training on shape discrimination was prolonged for up to 30 sessions. In Experiment 3, we tested the hypothesis that simultaneously learning to use the conditioning chamber and learning discrimination imposes a too-high cognitive load. However, zebrafish that first successfully learned how the conditioning chamber functioned (in the colour discrimination) subsequently failed in the shape discrimination. Conversely, zebrafish that firstly failed the shape discrimination subsequently learned colour discrimination. In Experiment 4, zebrafish showed some evidence of learning when the stimuli were two large shapes, suggesting that zebrafish did not discriminate between the shapes of the previous experiments because they were not salient enough. Altogether, results suggest constraints in the discrimination learning abilities of zebrafish, which should be taken into account when developing cognitive tasks for this species.