A new study from the laboratory of Dr. Guillermina López-Bendito, published today by Science, demonstrates in mice that the touch and sight circuits are not independent in the embryo, but rather are intermingled. It is at birth that these circuits separate and responses to sensory stimuli become independent.

In a previous study, López-Bendito’s laboratory demonstrated that tactile stimuli activate brain circuits designed to process this type of information before birth: “We had discovered, at a local level, that at the moment of birth of the embryo, the sense of touch is already developed and now we have been able to develop it on a large scale, ”explains Teresa Guillamón-Vivancos, first author of the work, to ABC.

“But we wanted to determine if they do this independently or if there is a temporary overlap with other senses. This new study provides fascinating data on how the senses are segregated in the first days of life» says Guillermina López-Bendito, who has led the research. “This study has important implications because it was not known that the senses are segregated by default,” says Guillamón-Vivancos.

In this work they have been able to verify for the first time in vivo in mice that, during embryonic development, a tactile stimulus not only triggers the expected response in the primary somatosensory cortex (one of the areas of the brain that deals with the sense of touch) but also which surprisingly also gives rise to a response in the primary visual cortex of both hemispheres.

The study has been developing for four years and although for now it has not been tested in people, due to the ethical debate of gene manipulation, they state that “it can be assumed that this information can be shared with other mammals.”

‘This multimodal response (that is, involving more than one sense) was observed in mouse embryos analyzed on the last day of gestation, but disappeared at birth. Next, we tested whether the disappearance of this multimodal response could be related to the arrival of signals from the retina to the cerebral cortex and other brain structures. Our data show that somatosensory and visual circuits do not segregate by default, but rather require the arrival of waves of activity from the retina to do so”, explains Teresa Guillamón Vivancos. «We have discovered with a blind mouse, that by not possessing the waves of the retina, it did have this multimodality. Thus, we notice that these waves of the retina arrive earlier«, Guillamón points out.

Track separation
This fundamental process of separation of sensory circuits occurs during a window of time close to birth and in a structure of the brain called the superior colliculus. Making a railway simile, at birth, in this structure the senses separate following different routes. The change of track is facilitated by the waves of activity in the retina, which act like railways that direct the stimuli of each sensory modality to the corresponding cortex, so that we can perceive them separately.

In fact, blockade of these retinal waves prolongs the multimodal (intermingled) configuration of the senses in life after birth, so that the superior colliculus retains a mixed tactile-visual identity and defects in the spatial organization of the system arise. visual.

This work extends the understanding of the role of retinal waves of activity by revealing their critical role in the acquisition of sensory modality specificity, going beyond the already known classical role in postnatal refinement of visual circuitry.

https://youtu.be/b2IT1JCvJxc

Another important contribution of this work is to have verified the existence of a limited temporal window for the segregation of the visual systems from the somatosensory ones. So any delay in this segregation will cause lasting changes in the organization of the visual circuitry.

Our results highlight the ontogenetic perspective, where the superior colliculus exerts master control during the early stages of organism development over the cortical specification and configuration of visual circuits. Therefore, we believe that a deeper understanding of the functional development of phylogenetically ancient structures is crucial to understand how the cerebral cortex is formed and its functional areas are specified.