Visual lateralization in fish: Responses to familiar and unfamiliar stimuli

Left- and right-monocular viewing during inspection of their own mirror-image was measured in fish (Xeno- poecilus sarasinorum) that had been kept for 20 days in a tank with a mirror or in a tank in which conspecifics were visible behind a transparent glass partition. Results revealed a preferential use of the monocular visual field of the left eye in both conditions. The asymmetry was stronger during the first 5 min of observation and tended to fade slightly thereafter. In a second experiment left- and right-monocular viewing was measured in presence of artificial stimuli. Fish were kept for 20 days in a tank with either horizontal or vertical stripes positioned along one wall and then tested for eye use in a tank with a familiar (same orientation) or an unfa- miliar (different orientation) pattern of stripes. Fish showed a preferential use of the monocular field of the left eye when presented with the familiar pattern and a slight preferential use of the right eye with the unfamiliar pattern. The former bias was stronger in the first minutes of test, after which it tended first to reverse and then to fade away; the latter bias, in contrast, appeared only after some minutes of observation. It is argued that the preferential use of the monocular visual field of the left eye (mainly serving structures located to the right side of the encephalon) is probably part of a more general specialization to establish identity, i.e. that an apparently familiar stimulus is indeed identical with one previously experienced. Preferential use of the monocular field of the right eye, in contrast, is argued to be associated with visual control of response.