https://phys.org/news/2018-11-hemispheric-dominance-brain.html

The two hemispheres are connected via thick nerve fibre bundles, so-called commissures. "In the past, it had been assumed that the dominant hemisphere transmits inhibitory signals to the other hemisphere via the commissures, thus suppressing specific functions in that region," explains Onur Güntürkün. However, the interactions that take place between the two hemispheres are excitatory, as well as inhibitory. "This is why it has remained a mystery where, exactly, functional brain asymmetries stem from," says Güntürkün.

In the biopsychology lab in Bochum, the researchers therefore approached this question using a new method. They had pigeons perform a colour differentiation test and extrapolated the activity of individual cells in the birds' visuomotoric forebrain. That brain region processes information provided by the visual sense and controls motor functions based on visual input. In birds, the left hemisphere is the dominant one for these tasks.