Visual lateralization in response to familiar and unfamiliar ... - CPRG

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386 V.A. Sovrano / Behavioural Brain Research 152 (2004) 385–391 looking at their own image in a mirror is not a manifestation of a generic preference to use this eye to look at all visual stimuli because the right eye is used in preference in other tasks, for instance to inspect a predator or other potentially dangerous stimuli [8] or when deciding to perform a certain course of action on a stimulus, such as to bite it [11]. However, it is unclear what specific aspects of the mirror-image would attract the attention of the left eye and the associated neural structures. Here we have started an investigation of this issue. In the first experiment we tried to check whether the degree of familiarity with the images provided by a mirror would affect the lateral bias. For a naïve fish, its mirror-image is the image of a stranger conspecific. Do fish accustomed to a mirror would still manifest a left lateral bias in the use of the eyes? 2. Experiment 1 All evidence collected so far concerned fish that were presented for the first time with a mirror. In the present experiment fish were tested after a long exposure to a mirror. As a control, we also devised an experimental condition in which fish were exposed, for the same amount of time, to the same number of conspecifics seeing as real animals behind a glass rather than as images in a mirror. 2.1. Materials and methods 2.1.1. Subjects Females of Xenopoecilus sarasinorum (N = 24) were used. Fish were kept into vegetation rich (Ceratophyllum sp.) 120–150 l glass tanks (99 cm × 45 cm × 52.5 cm), illuminated from above by fluorescent lamps (30 W) under a 14:00-h light:10:00-h dark period. Water temperature was maintained between 22 and 25 ◦ C and fish were fed dry food twice a day. 2.1.2. Apparatus and procedure Twenty-one days before the start of the test fish were removed from the communal tank and randomly subdivided in two different rearing conditions (see Fig. 1). In one condition (N = 12) fish were placed in groups of three animals in smaller rectangular glass tanks (48 cm × 19 cm × 32 cm), covered with opaque plastic material, with one of the longer wall entirely occupied by a mirror (48 cm × 27 cm). In the other condition (N = 12) fish were placed in groups of three animals in tanks identical to those used in the other condition. However, this time there was no mirror along the longer wall, which was instead occupied by a transparent glass wall of an adjacent tank (see Fig. 1), so that the fish could be exposed to an environment similar to that of the fish exposed to the mirror (i.e. they could see three conspecifics on the other side and the mirror-image of the tank). The apparatus has been described in details elsewhere [18]. Testing was performed within a tank (44 cm ×22 cm × 30 cm), inserted into a larger tank (60 cm × 36 cm × 35 cm), with mirrors as the two longer walls and opaque screens as the shorter walls (see Fig. 2). The tank was lit from above by a neon lamp (18 W); water was 25 cm in height. Above the testing apparatus a videocamera was mounted in order to videotape the fish behaviour. Fish exposed to the mirror and fish exposed to conspecifics were tested in the same way. Each fish was tested singly, by placing it into the test apparatus and videorecording its behaviour for 15 min (for subsequent analysis, this period was split into three blocks of 5 min in order to check for variation in lateralization as a function of time). Fish positions were then scored every 2 s, and the frequency of use of the left- or right-monocular visual field was estimated on the basis of the fish angle with respect to the closest mirror (see Fig. 2 and [18] for details]. An index of eye use was calculated as: [(frequency of right-eye use)/(frequency of right-eye use + frequency of left-eye use)] × 100. Significant departures from chance level (50%) were estimated by one- or two-tailed one sam- Fig. 1. Schematic representation of the rearing conditions used in Experiment 1.
V.A. Sovrano / Behavioural Brain Research 152 (2004) 385–391 387 Fig. 2. Schematic representation of the mirror-test apparatus, showing the position of the mirrors and the angles of viewing that defined monocular vision with the right or left eye. Data were discarded when the fish was perpendicular to the mirror (binocular stimulation, dotted fish) or when it formed an angle larger than 90 ◦ with respect to the closest mirror. ple t-tests. Differences between rearing conditions (below) and time (first, second and third 5 min of test) were estimated by analysis of variance (Anova). (Normality of distribution and any need for data transformation to account for heterogeneity of variances were checked for before applying Anovas.) 2.1.3. Results and discussion The results are shown in Fig. 3. The Anova revealed that the main effect of time (first, second and third 5 min) was statistically significant (F(2, 44) = 3.307, P = 0.046), whereas the main effect of familiarity (mirror vs. conspecifics) and the familiarity × time interaction were not significant (F(1, 22) = 1.102, n.s.; F(2, 44) = 0.077, n.s., respectively). One-sample t-tests revealed that in both groups of fish there was a significant preference for using the left eye throughout the entire testing period (first 5 min: t(23) = 4.546, P = 0.0001; second 5 min: t(23) = 2.053, P = 0.052; third 5 min: t(23) = 2.295, P = 0.031), with only a minor reduction of the bias as a function of the testing time (see Fig. 3). The results showed that the degree of familiarization with the stimuli provided by a mirror is not a crucial variable for the left-eye bias to occur. The strength of lateralization determined in these experiments was basically the same as in previous experiments in which the fish had Fig. 3. Laterality index indicating preferences for left and right eye use (mean ± S.E.M. are shown) during the first, second and third 5 min of observation in fish reared with a mirror or with conspecifics.
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