Sex difference in the visual projections of young chicks: a quantitative study of the thalamofugal pathway.

We investigated quantitatively the thalamo-hyperstriatal visual projections of chickens by injecting the fluorescent dye True blue (TB) in either side of the Wulst on day 2 post hatching. The ratios resulting from the comparison of the number of cells retrogradely labelled in the nuclei dorsolateralis anterior thalami (DLA) contralateral and ipsilateral to the site of injection (C/I ratios) revealed a structural asymmetry in the thalamofugal projections of male chicks (n = 17) but not in those of females (n = 12). In the males, TB injections in the right hyperstriatum resulted in higher C/I ratios (37 +/- 14%) compared to injections in the left hyperstriatum (22 +/- 7%). Thus, the left DLA (fed by the right eye) was found to give rise to a more developed visual projection compared to the right DLA (fed by the left eye). No such difference was found in female chicks (41 +/- 13% and 38 +/- 12%, respectively): each DLA gives rise to similarly developed visual projections. We also report evidence of higher densities of cell bodies labelled in the DLA of females compared to males. These results are discussed in the light of experiments demonstrating sex differences in functional asymmetry in chicks tested with the left or right eye occluded.