Are critical periods reversible in the adult brain? Insights on cortical specializations based on sensory deprivation studies

Abstract

We review here studies with visual and auditory deprived/recovery populations to argue for the need of a redefinition of the crucial role of unisensory-specific experiences during critical periods (CPs) on the emergence of sensory specializations. Specifically, we highlight that these studies, with emphasis on results with congenitally blind adults using visual sensory-substitution devices, consistently document that typical specializations (e.g., in visual cortex) could arise also in adulthood via other sensory modalities (e.g., audition), even after relatively short (tailored) trainings. Altogether, these studies suggest that 1) brain specializations are driven by sensory-independent computations rather than by unisensory-specific inputs and that 2) specific computationoriented trainings, even if executed during adulthood, can guide the sensory brain to display/recover, core properties of brain specializations. We thus introduce here the concept of a reversible plasticity gradient, namely that brain plasticity spontaneously decreases with age in line with CPs theory, but it nonetheless can be reignited across the lifespan, even without any exposure to unisensory (e.g., visual) experiences during childhood, thus diverging dramatically from CPs assumptions.

A self-training program for sensory substitution devices

“…Implementing multisensory training for blind individuals would have entailed the involvement of audio-tactile inputs, requiring an additional hardware component (i.e., to deliver tactile SSD stimulations), which both raises costs and is harder to adapt to an online platform as it would require constant maintenance. It is important though to note, that multisensory audio-visual approaches can potentially impact the rehabilitative aspects of visually impaired individuals, for instance, individuals with residual vision, or with degenerative visual loss (see for instance [ 49 , 50 ] suggesting the coupling of SSDs with sight restoration approaches). Our current results on the sighted population show that for this initial stage of SSD training, the unisensory and multisensory, perceptual and descriptive training methods were equally efficient.…”

Brain-Machine Interfaces to Assist the Blind

“…One could therefore argue that once this critical period is over, the brain is less likely to adapt itself to a new condition. Nonetheless, a number of studies have reported neuroanatomical differences between CB and LB, and LB and subjects with normal vision, which challenges the rigidity of critical periods in the brain ( Heimler and Amedi, 2020 ). For example, cross-modal plastic processes have been usually found in CB whose visual cortex is activated by other senses like audition, touch and even smell [reviewed in Kupers and Ptito (2014) ].…”

Metamodal Coupling of Vibrotactile and Auditory Speech Processing Systems Through Matched Stimulus Representations

“…In contrast, our study provides novel evidence for metamodal engagement of intermediate auditory areas. This is particularly noteworthy given the sparse evidence for metamodal engagement of intermediate sensory areas (Heimler and Amedi, 2020). Studying metamodal engagement in intermediate sensory areas has been difficult because it is difficult to determine what cross-modal congruences might exist in a cognitive space – thereby highlighting the importance of focusing on congruences between neural codes when attempting cross-modal coupling of sensory processing hierarchies.…”