To address these questions, we tracked changes in neural activity and characterized its timing properties along the entire visual pathway in healthy and plasticity-inducing animal models. First, we developed a novel set-up to display contextual information in animal scanners, and a set-up to acquire simultaneously fMRI and calcium signals (Objective 1). Second, we will harness our unique setup to characterize brain topography and cortical circuitry in the healthy rodent visual pathway (Objective 2). Third, we tracked plasticity changes (brain remapping and changes in function specialization) over several months in a chronic model of visual deprivation (Objective 3).
We used a model in which rodents are born and raised in the dark until adulthood, well past the critical period of plasticity. Consequently, the brains of these animals had not yet undergone the key processes required for visual specialisation. The animals were then exposed to light for the first time inside the MRI scanner. This allowed us to observe the brain’s response to its first encounter with visual stimuli, but also to study how it might adapt to this delayed exposure, yielding two pivotal insights. First, when the animals were exposed to light for the first time during the initial MRI scan, their brains displayed no organised response to visual information. Instead, their nerve cells across different areas reacted to a broad range of visual details, from fine to coarse. Moreover, the receptive field sizes of neurons – the specific area of the visual field that they respond to – was also larger in visually deprived rats compared to the control group. Together, these findings suggested that the visual pathway in the light-deprived rats lacked specialisation. Second, after exposure to light, the animals’ brains began to change. Even
within a week, visual responses became more organised, such that neighbouring neurons began to respond to nearby positions in the visual field, and the cells started to react more to specific visual characteristics. The receptive fields of the neurons also became smaller and more spatially selective. After a month, the animals’ brains looked much like those of healthy controls. In less than a month, the structure and function of the visual system in the visually deprived animals became similar to the controls. While plasticity has been observed in humans, interpreting it remains very difficult. What we are seeing here in rodents, which offer insights into brain mechanisms unattainable in human studies, is a phenomenon that has not been observed before: large-scale plasticity in the adult brain across the entire visual pathway, not just localised to a specific brain area as shown in previous studies.
