Behavioural Brain Research 159 (2005) 63–71 Research report Corticosterone treatment of the chick embryo affects light-stimulated development of the thalamofugal visual pathway L.J. Rogers ∗ , C. Deng 1 Centre for Neuroscience and Animal Behaviour, University of New England, Armidale, NSW 2351, Australia Received 14 July 2004; received in revised form 6 October 2004; accepted 7 October 2004 Available online 11 November 2004 Abstract By injecting a single 60 g dose of corticosterone into the eggs of domestic chicks on day 18 of incubation, we have shown that elevated levels of this hormone affect the development of asymmetry in the visual projections from the thalamus to the Wulst regions in the left and right hemispheres of the forebrain. In vehicle-treated (control) embryos this visual pathway develops asymmetry in response to light stimulation during the final stages of incubation, when the embryo is oriented so that its left eye is occluded by its body and its right eye can be stimulated by light entering through the egg shell. Pre-hatching exposure to light leads to more projections from the left side of the thalamus to the right Wulst than from the right side of the thalamus to the left Wulst, as confirmed here by injection of the tracers Fluorogold and Rhodamine into the left and right Wulst followed by counting the number of labelled cell bodies in the thalamus (asymmetry greater in males than females). The chicks injected with corticosterone pre-hatching did not develop any group bias for asymmetry in response to light exposure before hatching. They were random with respect to presence/absence of lateralization and, when present, the lateralization was not as strong as in the controls and its direction was random. The corticosterone-treated group had fewer projections from the left side of the thalamus to the right Wulst than did the controls. The results are considered with respect to maternal deposits of the hormone in the yolk and pre-hatching stress of the embryo. © 2004 Elsevier B.V. All rights reserved. Keywords: Corticosterone; Visual pathway; Lateralization; Experience; Development; Embryo; Chick; Stress 1. Introduction The thalamofugal visual projections to the forebrain of the young chick are characterised by an asymmetry that de- velops in response to exposure of the embryo to light for a brief period during the final stages of embryonic develop- ment [31]. It is at this stage of development that the embryo is turned in the egg so that its right eye only is positioned to permit stimulation by light entering through the eggshell and membranes, and the visual connections to the forebrain become functional [27,28]. The left eye is occluded by the embryo’s body. As a consequence of this asymmetrical stim- ulation of the eyes by light, the thalamofugal projections that ∗ Corresponding author. Tel.: +61 267 73 3969; fax: +61 267 73 3452. E-mail address: lrogers@une.edu.au (L.J. Rogers). 1 Present address: Department of Biomedical Science, University of Wol- longong, Wollongong, NSW 2522, Australia. receive input from the right eye (i.e. those on the left side of the thalamus) develop to a greater extent than their counter- parts receiving input from the left eye [30]. The asymmetry is present in both the ipsilateral and contralateral projections to the Wulst regions in the hemispheres but it is most obvious in the latter ([19,31]; Fig. 1). Asymmetry in the thalamofugal visual pathways is not in- consequential to visual behaviour. It underlies lateralization of foraging behaviour in the chick: grains of food are discrim- inated from small pebbles when the chick uses its right eye but not when it uses its left eye [29]. It also underlies lateraliza- tion of attack responses: testosterone-treated chicks exhibit elevated levels of attack when they are using their left eye but not when they are using their right eye [36]. The association between the structural asymmetry in the visual pathways and the behavioural lateralization has been shown by the absence of both forms of asymmetry in chicks hatched from eggs in- 0166-4328/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.bbr.2004.10.003