Research reportMonocular sleep in male domestic chicks
Introduction
Several studies pointed out that domestic chicks (Gallus gallus) show remarkable lateralization of brain function, which also exhibits an age-dependent pattern [1], [2], [4], [9], [13], [17], [18], [20], [25]. Overall, the left hemisphere seems to be mainly involved in visual learning and stimulus categorization, whilst the right hemisphere seems to be involved in spatial analysis and response to novelty [12], [14], [22], [23], [24].
Together with aquatic mammals and other bird species, domestic chicks also exhibit a unique behavioural and electrophysiological state called monocular or unihemispheric sleep [6], [10], [11], [19]. Briefly, during normal sleep, chicks have brief periods of time in which one eye is opened and the contra-lateral hemisphere shows an EEG pattern typical of wakefulness (fast and low-voltage waves), whereas the other eye remains closed and an EEG pattern of slow wave sleep (slow and high-voltage waves) can be recorded in the contralateral hemisphere [6].
Mascetti et al. [10] reported that female chicks show an age-dependent pattern of monocular sleep which seems to be associated with brain lateralization and strongly affected by the rearing conditions. During the first week post-hatching, female chicks reared with an imprinting object showed a pattern of monocular sleep with either right- or left-eye closure occurring with approximately the same frequency, except on day 5 in which right-eye closure predominated. During the second week, there was a clear bias towards more pronounced monocular sleep with left-eye closure [11]. By contrast, monocular sleep in female chicks reared without any imprinting object showed a predominant left-eye closure pattern in both weeks, except on day 8 in which monocular sleep was recorded equally from each eye. Comparing the two rearing conditions, it was apparent that during the first week imprinted chicks showed more right-eye closure compared to chicks reared without the imprinting object; this effect was tentatively associated with consolidation of imprinting memories in the left hemisphere (see, e.g. [3]). During the second week, both imprinted and non-imprinted chicks showed a preference for left-eye closure. Finally, the removal or the change in colour of the imprinting object on day 8 caused a striking shift towards a predominant right-eye closure during monocular sleep [10]. This was tentatively associated with right hemisphere involvement in response to novelty (see, e.g. [22]).
Domestic chicks show also remarkable sex differences in brain lateralization. Anatomically, the crossed thalamofugal pathway (dorsal supraoptic decussation) is larger towards the right hyperstriatum than towards the left one [7], [16], but this asymmetry is more pronounced in males than in females [17]. As a result of the asymmetry in the thalamofugal pathway, the right hemisphere receives a larger amount of binocular input than left one and this difference is larger in males than in females. Rogers and Erlich [15] claimed that the age-dependent shifts in standing bias of behavioural control by one or other hemisphere are different in male and female chicks. Moreover, social attachment [23], social recognition [21] and response to novelty [13], tasks all seem to produce different responses in male and female chicks. Females seem to have a tendency of to use one hemisphere at a time which would take the overall control of attentional strategies [4]; in other words, the hemisphere that is in control can impose its own attentional strategies to the partner thus reducing the behavioural lateralization (see, e.g. [4]). On the contrary, this effect seems not to be present in males and therefore they show more pronounced asymmetries in the visual control of behaviour (see, e.g. [4]).
The main aim of this study was to investigate the effects of different rearing conditions (with or without an imprinting object) on the pattern of monocular sleep in male domestic chicks during the first 2 weeks post-hatching. Considering that males and females seem to show somewhat different patterns of lateralization in virtually all tests (review in [4]), a secondary aim of our study was to provide a comparison with previously published work on monocular sleep in female chicks.
Section snippets
Experiment 1
In this experiment, monocular sleep was investigated in two groups of male chicks: chicks in one group were allowed to develop normal social attachment to an imprinting object (I-chicks), whilst chicks of another group were reared in social (visual) isolation (NI-chicks).
Subjects and apparatus
The subjects were male Hybro (a local hybrid variety derived from the White Leghorn breed) domestic chicks (G. gallus). Chicks arrived in the laboratory in from a commercial hatchery in the early morning of day 1 after hatching and were immediately placed singly into glass cages ( cm) with semi-transparent cloths along the walls that served as one-way screens. Each cage was illuminated continuously from above and contained two identical small transparent glass containers (5 cm in
Results
The percentage of time spent in binocular sleep is shown in Fig. 1A and revealed a slow decrease during the 2 weeks of observation. The ANOVA revealed that the main effect of age was significant (F(9,126)=6.070, P=0.0001). There were no other statistically significant effects.
The mean number of episodes of binocular sleep is shown in Fig. 1B. The ANOVA revealed that the main effect of age (F(9,126)=2.444, P=0.0134) and the interaction age×rearing conditions (F(9,126)=1.977, P=0.0472) were
Experiment 2
The aim of this experiment was to study whether the removal or the change in colour of the imprinting object would affect the pattern of monocular sleep of male chicks.
Subjects
The subjects were 14 male Hybro chicks. Apart from the changes of imprinting object, the rearing conditions, general procedure and apparatus were the same as in the previous experiment.
Apparatus and procedure
The apparatus and general procedure was the same as in the previous experiment. This time, in one group of chicks (n=8), the red ball, which was placed from day 1, was exchanged at the early morning of day 8 by a yellow ball (YE-chicks), otherwise identical in shape and size which remained in the cage for the
Results
The change in colour of the imprinting object on day 8 produced mild effects on monocular sleep (Fig. 3). The scores of YE-chicks were compared with those of I-chicks (data of I-chicks from experiment 1 can be considered as a control condition of absence of change in the imprinting object). As to percentages of time spent in monocular sleep, only the interaction change×age was significant (F(4,56)=2.94, P=0.0282), whilst there were no significant effects in the number of episodes. However,
Discussion
Results of the present experiments showed that binocular sleep of male chicks tended to decrease progressively with age. The time spent in binocular sleep decreased similarly in both I-chicks and NI-chicks, whilst there was a reduction of the number of episodes in NI-chicks, but not in I-chicks. In a previously published work on female chicks [10], binocular sleep decreased with age, but the number of episodes declined similarly under both rearing conditions, whilst the time spent decreased
Acknowledgements
This study was supported by Grant PRIN n. 2002118144_002 of the Italian Ministry of Education, University and Research (MIUR).
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