(a) Schematic representation of the impact of light (off/on) on the lateralization of projections from the left thalamus (nucleus geniculatus lateralis pars dorsalis, Gld) to the right Wulst during the sensitive window (E17–18). (b) Typical longitudinal relaxation time (T1) map of a chick’s brain after administration of manganese. Mn2+ is a paramagnetic ion and shortens the T1 of nuclear spins in brain tissue in a concentration-dependent manner. Blue-darker regions correspond to higher intracellular Mn2+ accumulation, and hence to increased neuronal activity. Left-right front-posterior coordinates (L, R, F, P) are indicated with respect to the sagittal brain axis of embryo chick brain inside the egg. (c) Brain activity is reported as lateralization index (1=activity only in the left hemisphere; –1=only in the right). Regions of the thalamofugal pathway: Wulst and thalamus (Gld); region of the tectofugal pathway: optic tectum. Asterisk between groups indicates a significant difference between experimental conditions (Dark n=8, Light n=10). Asterisk above single group indicates lateralization significantly different from chance (0, dotted line). * indicates p<0.05, *** indicates p≤0.001. To best represent data, we used a raincloud plot that combines for each region and for each group: boxplot, raw data (circles) and data distribution (half violin on the left).