Changes in metabolic activity in the hyperstriatum of the chick before and after hatching

Development of the visual system of the chick. I. Cell differentiation and histogenesis

This review summarizes present knowledge on the embryonic development of the avian visual projections, based on the domestic chick as a model system. The reductionist goal to understand formation and function of complex neuroanatomical systems on a causal level requires a synthesis of classic developmental biology with recent advances on the molecular mechanisms of cell differentiation and histogenesis. It is the purpose of this article. We are discussing the processes underlying patterning of the anterior neural tube, when the retina and optic tectum are specified and their axial polarity is determined. Then the development of these structures is described from the molecular to the anatomical level. Following sections deal with the establishment of secondary visual connections, and the developmental interactions between compartments of the retinotectal system. Using this latter pathway, from the retina to the optic tectum, many investigations aimed at mechanisms of axonal pathfinding and connectivity have accumulated a vast body of research, which will be covered by a following review.

Light experience and lateralization of the two visual pathways in the chick

Using retrograde labelling with the fluorescent tracer rhodamine B isocyanate (RITC), we have examined the organisation of the thalamofugal and tectofugal visual projections to the forebrain of the young chick. In addition, we have investigated the influence of light exposure prior to hatching on the development of the tectofugal visual projections. Our results for the thalamofugal projections confirm those found previously; viz., that there are more projections from the left side of the thalamus to the right hyperstriatum of the forebrain than from the right side of the thalamus to the left hyperstriatum in males and females. The organisation of the tectofugal visual projections to the rotundal nuclei was more symmetrical (males only examined) although there was a trend towards a greater number of projections from the left optic tectum to its ipsilateral nucleus rotundus than from the right optic tectum to its ipsilateral nucleus rotundus. There are numerous projections from the optic tecta to their contralateral rotundal nuclei but, in contrast to reports for the pigeon, no marked asymmetry was present in these. The ratio of contralateral to ipsilateral projections revealed significant asymmetry for projections from the ventral regions of the optic tecta and symmetry from the dorsal regions. Thus both visual pathways of the chick have asymmetrical organisation but the asymmetry is much greater in the thalamofugal pathway. The slight asymmetry in the tectofugal projections may be determined by exposing the embryo to light just before hatching, as known to be the case for thalamofugal projections.

Exposure to light prior to hatching induces asymmetry of receptor binding in specific regions of the chick forebrain

This paper describes neurochemical asymmetries present in forebrain regions of the newly hatched chick that result from environmental conditions; specifically from asymmetrical exposure of the chick embryo to light prior to hatching. Quantitative autoradiography was used to determine GABA and glutamate receptor subtype binding in a number of regions of the left and right forebrain hemispheres of chicks that had either the left (LES), or the right (RES), eye system exposed to light prior to hatching. On day 19 of incubation the embryo's head was withdrawn from the egg and the left or the right eye was occulded until hatching. [3H]MK-801, [3H]AMPA and [3H]muscimol binding assays were performed on frozen sections from 2 different coronal regions of the forebrain, sampled on day-1 posthatching. Significant [3H]MK-801, [3H]AMPA and [3H]muscimol binding asymmetries were determined in forebrain regions from chicks that had their RES exposed to light prior to hatching, particularly in forebrain regions which are known to receive afferent visual input. The reverse pattern of asymmetry was found for all 3 ligands in regions such as the ectostriatum of chicks that had their LES exposed to light, while asymmetry of muscimol and AMPA binding, present in many regions in right eye system chicks was not present in chicks that had the left eye system exposed to light during incubation. Thus, the presence and pattern of experience-dependent neurochemical asymmetries in the chick forebrain are specific to both region and receptor type.

Exposure to different wavelengths of light and the development of structural and functional asymmetries in the chicken

The thalamofugal visual projections of the chick are known to develop in response to stimulation by light prior to hatching, and asymmetry in the number of projections develops as a consequence of the embryo being oriented in the egg so that it occuludes its left eye. The right eye only is stimulated by light and this causes the visual projections connected to the right eye to develop in advance of those connected to the left. We have now found that exposure of embryos, from day 19 of incubation to hatching, to red (peak transmission at 670 nm) or green (peak at 500 nm) light is as effective as broad-spectrum (white) light in establishing asymmetry in these projections. The intensities of the light to which the embryos were exposed in each case were equivalent, achieved in part by removing the air sac end of the egg shell. The thalamofugal visual projections, therefore, develop in response to light stimulation but appear to have no wavelength specificity. This result is consistent with the apparent lack of involvement of the thalamofugal visual pathway in colour vision. However, functional asymmetry, tested as left-right eye differences in categorising grain from pebbles, was found to be less marked in the chicks that had been exposed to green light compared to those that had been exposed to 'while' light, and it was absent in those exposed to red light or incubated in the dark. Thus, there is wavelength specificity for the development of the behavioural asymmetry, which suggests involvement of colour-coded neurons outside the thalamofugal visual pathway, probably in the tectofugal pathway. Exposure of the embryos to red and green light alternating at 30 min intervals is as effective as "white' light for establishing both the structural and functional asymmetry.

Behavioral, structural and neurochemical asymmetries in the avian brain: a model system for studying visual development and processing

The emphasis of this review is on the visual systems and lateralized visually guided behavior in several avian species. Lateral asymmetry is known to be present in the tectofugal visual projections to the forebrain of the pigeon and in the thalamofugal visual projections to the forebrain of the chicken. These structural asymmetries are discussed in the context of the behavioral and neurochemical asymmetries. While recognizing the need to investigate the organization of both of the visual pathways within one avian species; this review reasons inductively that the lateralized organization of the two visual pathways leads to binocular input to the right hemisphere via the thalamofugal visual system and to the left hemisphere via the tectofugal visual system. For each system, input to the other hemisphere is primarily monocular. This specialization of the hemispheres for visual processing has predictable effects on behavior. The role of asymmetrical light stimulation of the eyes of the embryo in determining the lateralizations in the visual pathways and some behaviors is discussed, as are other lateralizations generated or altered by imprinting and passive avoidance learning.