“The MRI analyses have particularly allowed us to fine tune our understanding of the hormonal and social mechanisms that control these variations in volume of the nerve structures”, Jacques Balthazart explains. “The possibility of obtaining successive images of the brain of the same subject before and after an experimental manipulation is indeed a major advantage. Moreover, it allows us to greatly reduce the number of experimental animals that have to be sacrificed.”
Besides the numerous experiments that use MRI coupled with an increase in contrast through the injection of manganese, the researchers are also carrying out work based on another technique: Diffusion Tensor Imaging (DTI). Based on the polarisation of vibrations of the water in the electromagnetic field generated by the machine, this technique was essentially used to visualise the fascicules of myelinated fibres (axons), through mathematical analysis (computer reconstruction), the aim being to study the seasonal variations in the connections within the animal’s brain. This research was the subject of a publication in the European Journal of Neuroscience (3) in 2008. It demonstrated the existence of an unsuspected seasonal plasticity in the nerve connections linking the song control centres. What was even more astonishing was that this plasticity turned out to also affect many other fascicules of fibres linking the different parts of the brain to each other or to the sense organs, such as the eyes. Thus, the optic chiasm turned out to be smaller in the summer than in the spring. “Do songbirds need a better visual system during the reproductive period than outside this period?” our interviewee wonders. “In any case, the mechanisms behind the seasonal plasticity of the fascicules of fibres must now be analysed, as well as the functional significance of the phenomenon.”
(3) G. De Groof, M. Verhoye, V. Van Meir, J. Balthazart and A. Van der Linden, Seasonal rewiring of the songbird brain : an in vivo MRI study, in European Journal of Neuroscience 28 (2008), p. 2475-2485.