First trial outcome but not training difficulty predicts performance in goldfish visual discrimination

The easy-to-hard effect in perceptual learning shows that training with easier examples can facilitate initially difficult or impossible distinctions between very similar stimuli. This effect has been reported in humans and other species. We tested whether easy-to-hard training could facilitate visual discrimination in common goldfish (Carassius auratus). Fish (n = 6) performed a two-alternative forced choice discrimination task, which consisted of simultaneously presenting two striped patterns at a constant distance away on the outside of the tank. Fish were required to approach and bite a porthole corresponding to one of the stimuli for a food reward. Half of the fish were randomly assigned to a training schedule where stimuli became more similar as training progressed. The rest were trained only on the most difficult to distinguish version of the stimuli. All fish received a similar total amount of training regardless of the assigned schedule. We also examined whether performance on the first training trial for a given day was related to overall performance. Contrary to our hypothesis, fish in the easy-to-hard group did not perform significantly better than those in the constant-hard group. However, performance was found to be significantly higher on days when the first trial was correct compared to days on which it was incorrect, regardless of the type of training schedule. The current results contribute to understanding individual differences in perceptual learning in fish, and are consistent with research in humans, and other species, reporting better learning after initial reward.