Precise endogenous control of involvement of right and left visual structures in assessment by zebrafish.

Birds and fish use monocular viewing to bring to bear appropriate lateralised specialisations. In larval zebrafish, persistent left eye (LE) viewing of their own reflection is interrupted by a series of brief ‘events’, in which the right eye has equal use. These recur with precise 160 s periodicity, with the first beginning after 40 s, and allow right and left mechanisms to assess the reflection. In a strain in which persistent LE bias ceased after about 2 min, events were still marked by a brief period of frontal viewing, immediately after the time of each event (‘after-effects’). Such viewing allows simultaneous access by both right and leftvisual systems, and may be associated therefore with interaction, perhaps involving linkage of initially independent traces. Events appeared to cease after the maximal after-effect following the third event. Older fish show changes in overt behaviour immediately after event times; again the third event ends this. Processes associated with events thus progressively change what is available to recall. Domestic chicks show enhanced evocation of response (e.g. sex, defense) to fellows, with the LE but not the right, if exposed to light during embryonic development. In zebrafish (where light may act by an epiphyseal route) we found that after dark development, viewing bias peaked at the time of each event, with LE use at the first and right at the second. The main factor determining the outcome of right/left competition appears here to be recency of access to viewing.