Social isolation can bias judgement of ambiguous cues in adult zebrafish

Cognitive bias in decision-making under ambiguity occurs in animals and humans, with individuals interpreting ambiguous stimuli positively (“optimistic”) and others negatively (“pessimists”). Here, we investigated whether social isolation affects judgement of ambiguous stimuli in adult zebrafish (Danio rerio, 6-8 months). We implemented a Go/No-Go task in a color-coordinated half-radial maze where fish learned to discriminate between positive (P) and negative (N) arms, and were tested in an ambiguous arm sharing elements of P and N arms. During habituation and training, distances traveled and latencies were measured to characterize locomotor and exploratory behaviour. Over testing, discrimination learning was assessed by the latencies to enter these arms, from which later a Judgment Bias Score was calculated to classify fish on a pessimistic/optimistic axis. Control social fish showed individual variability and two behavioral phenotypes: active animals that explored the maze extensively and more static animals that froze or made brief excursions. A week of social isolation increases the proportion of pessimistic vs. optimistic animals, shifting judgment of the ambiguous stimulus toward negative valence. Isolated fish also showed increased locomotor activity and longer latencies to explore unfamiliar maze areas. These results indicate that cognitive bias is individual and plastic, hence recent social interactions can rapidly alter the valence assigned to ambiguous stimuli.