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Abstract
Chicks (Gallus gallus) learned to run from a starting box to a target located at the end of a runway. At test, colourful and bright distractors were placed just outside the starting box. Dark incubated chicks (maintained in darkness from fertilization to hatching) stopped significantly more often, assessing more the left-side distractor than chicks hatched after late (for 42 h during the last three days before hatching) or early (for 42 h after fertilization) exposure to light. The results show that early embryonic light stimulation can modulate this particular behavioural lateralization comparably to the late application of it, though via a different route.
Highlights
It is well established that environmental light stimulation interplays with a genetic cascade of events in promoting brain specialization in two different classes of vertebrates, fish and birds.A complex chain of developmental steps leads to brain lateralization in zebrafish starting with an asymmetrical expression of a gene network that controls the development of structural left-right differences within the epithalamus, including asymmetric parapineal migration [4,5,6,7]
Fries hatched from eggs exposed to the photic input during the first week after fertilization prefer to attend to conspecifics with the left eye, whereas fries whose embryonic development happened in darkness do not display the same asymmetry [8]
Darkness during the first day results in an inversion of the reaction to a dummy predator: after normal light regimes, zebrafish avoid the predator appearing on the left side, whereas after darkness they respond more intensely to a predator coming from the right side [9]; in contrast, darkness during the third day prevents the appearance of any asymmetric response to the predator [10]
Summary
A complex chain of developmental steps leads to brain lateralization in zebrafish starting with an asymmetrical expression of a gene network that controls the development of structural left-right differences within the epithalamus, including asymmetric parapineal migration [4,5,6,7]. In the transparent eggs of the zebrafish an early action of light prompts functional brain asymmetries including motor and sensory processing. If the light fails to reach the embryos in two distinct moments within the first week post-fertilization, the normal development of some lateralized behaviours is either compromised or prevented. The role of light stimulation on brain structural asymmetry has not been conclusively clarified in zebrafish, as no effect of light has been shown on the asymmetry of molecular markers [11], and some behaviours are lateralized while some others are not, independently of the neuroanatomical asymmetries [12]
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