Editorial: Brain plasticity following sensory loss: from basic mechanisms to therapy

The majority of the papers in this special issue deal with altera@ons in the visual system. Arend and co-workers report that blind individuals have changes in cor@cal gyrifica@on, an anatomical measure that has not been previously reported in this context. The authors show an increase in gyrifica@on in several brain areas of CB individuals and, importantly, a nega@ve correla@on between gyrifica@on and cor@cal thickness in several different cor@cal areas. The authors discuss the impact of their results in rela@on to brain development and plas@city. Yizhar and colleagues used fMRI to study the role of the extrastriate body area (EBA) in ac@on-related func@ons in CB individuals. Their findings indicate that the absence of visual experience does not favorize the development of ac@on-related responses in the EBA. Moreover, CB par@cipants showed a decrease in func@onal connec@vity of the EBA with sensorimotor cor@ces, whereas connec@vity with percep@on-related visual occipital cor@ces remained high. The authors further demonstrated that ac@on-related func@ons and connec@vity of the visual cortex are dependent on visuomotor experience. Bleau and co-workers present a meta-analysis of the neural substrates of spa@al processing and naviga@on in blind individuals through touch and audi@on. The meta-analysis reveals that most studies agree that CB individuals recruit the same neural pathways as sighted controls when processing non-visual spa@al informa@on. The meta-analysis further shows that the primary visual cortex and associa@ve occipital areas are involved in visuospa@al processing via cross-modal plas@city mechanisms. The authors discuss the results in terms of the amodality hypothesis of spa@al representa@ons. Arbel and co-workers present novel data on face recogni@on in CB individuals. The authors trained a group of CB par@cipants to use a visual-to-auditory sensory subs@tu@on device to recognize faces, whereaUer they par@cipated in an fMRI study. The results showed ac@va@on of the fusiform gyrus and other face-responsiveregions of the ventral visual stream. The authors concluded that there is a predisposi@on for sensory-independent and computa@on-specific processing in specific cor@cal regions that is independent of previous perceptual experience and that is pertained following sensory depriva@on. Nadvar and colleagues studied res@ng state func@onal connec@vity (rsFC) of area V1 following sight restora@on in pa@ents with re@ni@s pigmentosa who were implanted with the Argus II re@nal prosthesis which par@ally restores vision. The aim was to test whether sight restora@on with this treatment would reverse, in full or partly, the plas@c changes induced by the vision loss. Their results showed that the decrease in rsFC between V1 and the post-central gyrus in CB par@cipants was par@ally reversed by vision restora@on. The authors suggest that rsFC between the occipital and somatosensory cor@ces could provide a biomarker for func@onal plas@c changes following vision recovery. Maimon and colleagues report on visual percep@on in a small but unique group of children who had undergone vision-restoring cataract removal surgery as part of the Himalayan Cataract Project. Some of the children in the study were born with cataracts and gained a sense of sight for the first @me, whereas others suffered late-onset blindness in one eye alone. The authors discuss their findings in the context of Molyneux's problem, i.e. the ability to correlate vision with touch quicky following sight restora@on in blind individuals, and Hubel and Wiesel's theory of cri@cal periods.Two papers relate to plas@city following auditory depriva@on, one in humans and a second one in animals. Grégoire and co-workers performed a meta-analysis of the literature on brain plas@c changes following hearing loss at birth or later in life. Hearing loss is a growing problem in modern Western socie@es due to an aging popula@on. Moreover, knowledge of brain neuroplas@c changes could help to understand some disappoin@ng results with cochlear implants, and therefore could improve hearing rehabilita@on. The literature research revealed that the most consistent finding in deaf individuals was a volumetric decrease in gray maZer around the auditory cortex. In deaf children, an addi@onal volumetric decrease was reported in both gray and white maZer at the level of the visual cortex. Grégoire et al. further discuss the role of confounding factors that could affect brain plas@city in deaf individuals such as the use of sign language and hearing aids, and frequently observed associated ves@bular dysfunc@on or neurocogni@ve impairments. Using kiZens rendered deaf, Mitzetlfelt and colleagues inves@gated the s@mulus-driven neural ac@vity associated with visual localiza@on. The researchers recorded visual evoked poten@als (VEPs) in response to visual s@muli presented at various eccentrici@es in the visual field. Their results showed no significant changes in VEPs in deaf cats that could explain the previously observed behavioral advantage. The authors concluded that cross-modal plas@city in deafness does not play a major role in cor@cal processing of the peripheral visual field.Two studies report on pa@ent groups with either central or peripheral lesions. Araneda and associates used diffusion MRI to study changes in white maZer (WM) architecture in the geniculo-striate pathway in 40 children with unilateral spas@c cerebral palsy (USCP). The authors report several altera@ons in diffusion imaging parameters of the op@c radia@ons on the lesional compared with the non-lesional hemisphere. Both the nature and the side of the lesion (leU or right hemisphere) had an impact on the type and magnitude of the WM changes. In USCP with periventricular and right-hemispheric lesions, the diffusion imaging parameters correlated with the pa@ents' visuospa@al assessment. Dedr1y and colleagues studied three unique pa@ents with unilateral vocal fold paralysis. The pa@ents were followed for one year with mul@parametric voice assessments and longitudinal fMRI during a sustained phona@on task and rsfMRI. One pa@ent received an augmenta@on injec@on in the paralyzed vocal fold. This pa@ent showed a bilateral ac@va@on of the voice-related nuclei in the brainstem during sustained phona@on. In addi@on, rsFC between the voice motor/sensory brainstem nuclei and other voice-related ROIs correlated with mean airflow measures in this pa@ent. This observa@on supports the hypothesis that promo@ng propriocep@ve feedback, by temporarily rehabilita@ng glocc closure, can enhance the neural recovery process.Finally, the study by Vaessen and co-workers addressed the ques@on whether there is an abstract representa@on of emo@ons in the brain that is shared across s@mulus types (face, body, voice) and sensory origin (visual, auditory). Thereto, the authors studied fMRI responses to ecological types of emo@on expressions of different types and modali@es. Using mul@variate sta@s@cal analyses, the authors showed that there is a specific brain organiza@on for affec@ve signals which depends on s@mulus category and modality. These findings are consistent with the no@on that emo@on expressions conveyed by different s@mulus types have different func@onal roles in triggering rapid adap@ve behavior.We hope that the papers presented in this special issue of Fron@ers in Neuroscience will contribute to a beZer understanding of the mechanisms of cross-modal plas@city following different forms of sensory loss and of sensory subs@tu@on and other restora@ve therapies that may lead to restora@on of the lost func@ons.

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