Cerebral cognitive lateralisation was once thought to be confined to humans. During the last 20 years it was shown that rats, chicks and other animals also exhibit cognitive lateralisation that is best displayed by their asymmetric perceptual processing of various classes of stimuli. If fish showed behavioural lateralisation, asymmetrical functioning could be an inherited feature of the vertebrate brain. We observed the body postures of zebrafish in the course of their approach to novel and familiar stimuli in a long runway. All trials were videotaped and analyzed later. A computer generated cursor was placed over the image of the fish on the video screen and the angle of vision in relation to the object was determined. Analysis was confined to the periods when the fish was within 25 cm or less of the end of the swimway at which the test object. Since fish have no mobile neck the position of the body also defines the position of the eyes that in turn define the viewing angle of the retina. Five such measurements were taken per 1 second. In the analysis postures were grouped into bins of 10 degrees. For the analysis of viewing corresponding right and left sectors were compared.

In most trials we found that body positions between 0-20 degrees and 170-180 degrees were the most common which suggests that these body positions allow particularly high acuity vision. Strange objects were viewed at first exposure chiefly with the right frontal field (at 0-20 degrees); so was a complex and unfamiliar scene made up of familiar components. In the second trial using the same stimulus or scene, left frontal viewing tended to be used. A familiar heterospecific fish was viewed left frontally.

We suppose that similarly to other vertebrates (e.g. chick) right eye is used when it is necessary to inhibit premature response and left is used when it is necessary to keep an eye on a familiar scene.