A functional neuroimaging study of sound localization: visual cortex activity predicts performance in early-blind individuals

Aftereffects of visuomanual prism adaptation in auditory modality: Review and perspectives

Visuomanual prism adaptation (PA), which consists of pointing to visual targets while wearing prisms that shift the visual field, is one of the oldest experimental paradigms used to investigate sensorimotor plasticity. Since the 2000's, a growing scientific interest emerged for the expansion of PA to cognitive functions in several sensory modalities. The present work focused on the aftereffects of PA within the auditory modality. Recent studies showed changes in mental representation of auditory frequencies and a shift of divided auditory attention following PA. Moreover, one study demonstrated benefits of PA in a patient suffering from tinnitus. According to these results, we tried to shed light on the following question: How could this be possible to modulate audition by inducing sensorimotor plasticity with glasses? Based on the literature, we suggest a bottom-up attentional mechanism involving cerebellar, parietal, and temporal structures to explain crossmodal aftereffects of PA. This review opens promising new avenues of research about aftereffects of PA in audition and its implication in the therapeutic field of auditory troubles.

Longitudinal stability of individual brain plasticity patterns in blindness

The primary visual cortex (V1) in blindness is engaged in a wide spectrum of tasks and sensory modalities, including audition, touch, language, and memory. This widespread involvement raises questions regarding the constancy of its role and whether it might exhibit flexibility in its function over time, connecting to diverse network functions specific to task demands. This would suggest that reorganized V1 assumes a role like multiple-demand system regions. Alternatively, varying patterns of plasticity in blind V1 may be attributed to individual factors, with different blind individuals recruiting V1 preferentially for different functions. In support of this, we recently showed that V1 functional connectivity (FC) varies greatly across blind individuals. But do these represent stable individual patterns of plasticity, or are they driven more by instantaneous changes, like a multiple-demand system now inhabiting V1? Here, we tested whether individual FC patterns from the V1 of blind individuals are stable over time. We show that over two years, FC from the V1 is unique and highly stable in a small sample of repeatedly sampled congenitally blind individuals. Further, using multivoxel pattern analysis, we demonstrate that the unique reorganization patterns of these individuals allow decoding of participant identity. Together with recent evidence for substantial individual differences in V1 connectivity, this indicates that there may be a consistent role for V1 in blindness, which may differ for each individual. Further, it suggests that the variability in visual reorganization in blindness across individuals could be used to seek stable neuromarkers for sight rehabilitation and assistive approaches.

Blind individuals' enhanced ability to sense their own heartbeat is related to the thickness of their occipital cortex

Blindness is associated with heightened sensory abilities, such as improved hearing and tactile acuity. Moreover, recent evidence suggests that blind individuals are better than sighted individuals at perceiving their own heartbeat, suggesting enhanced interoceptive accuracy. Structural changes in the occipital cortex have been hypothesized as the basis of these behavioral enhancements. Indeed, several studies have shown that congenitally blind individuals have increased cortical thickness within occipital areas compared to sighted individuals, but how these structural differences relate to behavioral enhancements is unclear. This study investigated the relationship between cardiac interoceptive accuracy and cortical thickness in 23 congenitally blind individuals and 23 matched sighted controls. Our results show a significant positive correlation between performance in a heartbeat counting task and cortical thickness only in the blind group, indicating a connection between structural changes in occipital areas and blind individuals' enhanced ability to perceive heartbeats.

Enhanced Learning and Memory in Patients with CRB1 Retinopathy

Mutations in the CRB1 gene are associated with a diverse spectrum of retinopathies with phenotypic variability causing severe visual impairment. The CRB1 gene has a role in retinal development and is expressed in the cerebral cortex and hippocampus, but its role in cognition has not been described before. This study compares cognitive function in CRB1 retinopathy individuals with subjects with other retinopathies and the normal population.

METHODS

Neuropsychological tests of cognitive function were used to test individuals with CRB1 and non-CRB1 retinopathies and compare results with a standardised normative dataset.

RESULTS

CRB1 retinopathy subjects significantly outperformed those with non-CRB1 retinopathy in list learning tasks of immediate (p = 0.001) and delayed memory (p = 0.007), tests of semantic verbal fluency (p = 0.017), verbal IQ digit span subtest (p = 0.037), and estimation test of higher execution function (p = 0.020) but not in the remaining tests of cognitive function (p > 0.05). CRB1 retinopathy subjects scored significantly higher than the normal population in all areas of memory testing (p < 0.05) and overall verbal IQ tests (p = 0.0012). Non-CRB1 retinopathy subjects scored significantly higher than the normal population in story recall, verbal fluency, and overall verbal IQ tests (p = 0.0016).

CONCLUSIONS

Subjects with CRB1 retinopathy may have enhanced cognitive function in areas of memory and learning. Further work is required to understand the role of CRB1 in cognition.

Stimulus-responsive and task-dependent activations in occipital regions during pitch perception by early blind listeners

Although it has been established that cross-modal activations occur in the occipital cortex during auditory processing among congenitally and early blind listeners, it remains uncertain whether these activations in various occipital regions reflect sensory analysis of specific sound properties, non-perceptual cognitive operations associated with active tasks, or the interplay between sensory analysis and cognitive operations. This fMRI study aimed to investigate cross-modal responses in occipital regions, specifically V5/MT and V1, during passive and active pitch perception by early blind individuals compared to sighted individuals. The data showed that V5/MT was responsive to pitch during passive perception, and its activations increased with task complexity. By contrast, widespread occipital regions, including V1, were only recruited during two active perception tasks, and their activations were also modulated by task complexity. These fMRI results from blind individuals suggest that while V5/MT activations are both stimulus-responsive and task-modulated, activations in other occipital regions, including V1, are dependent on the task, indicating similarities and differences between various visual areas during auditory processing.

Spatial hearing training in virtual reality with simulated asymmetric hearing loss

Sound localization is essential to perceive the surrounding world and to interact with objects. This ability can be learned across time, and multisensory and motor cues play a crucial role in the learning process. A recent study demonstrated that when training localization skills, reaching to the sound source to determine its position reduced localization errors faster and to a greater extent as compared to just naming sources' positions, despite the fact that in both tasks, participants received the same feedback about the correct position of sound sources in case of wrong response. However, it remains to establish which features have made reaching to sound more effective as compared to naming. In the present study, we introduced a further condition in which the hand is the effector providing the response, but without it reaching toward the space occupied by the target source: the pointing condition. We tested three groups of participants (naming, pointing, and reaching groups) each while performing a sound localization task in normal and altered listening situations (i.e. mild-moderate unilateral hearing loss) simulated through auditory virtual reality technology. The experiment comprised four blocks: during the first and the last block, participants were tested in normal listening condition, while during the second and the third in altered listening condition. We measured their performance, their subjective judgments (e.g. effort), and their head-related behavior (through kinematic tracking). First, people's performance decreased when exposed to asymmetrical mild-moderate hearing impairment, more specifically on the ipsilateral side and for the pointing group. Second, we documented that all groups decreased their localization errors across altered listening blocks, but the extent of this reduction was higher for reaching and pointing as compared to the naming group. Crucially, the reaching group leads to a greater error reduction for the side where the listening alteration was applied. Furthermore, we documented that, across blocks, reaching and pointing groups increased the implementation of head motor behavior during the task (i.e., they increased approaching head movements toward the space of the sound) more than naming. Third, while performance in the unaltered blocks (first and last) was comparable, only the reaching group continued to exhibit a head behavior similar to those developed during the altered blocks (second and third), corroborating the previous observed relationship between the reaching to sounds task and head movements. In conclusion, this study further demonstrated the effectiveness of reaching to sounds as compared to pointing and naming in the learning processes. This effect could be related both to the process of implementing goal-directed motor actions and to the role of reaching actions in fostering the implementation of head-related motor strategies.

Task-dependent spatial processing in the visual cortex

To solve spatial tasks, the human brain asks for support from the visual cortices. Nonetheless, representing spatial information is not fixed but depends on the reference frames in which the spatial inputs are involved. The present study investigates how the kind of spatial representations influences the recruitment of visual areas during multisensory spatial tasks. Our study tested participants in an electroencephalography experiment involving two audio-visual (AV) spatial tasks: a spatial bisection, in which participants estimated the relative position in space of an AV stimulus in relation to the position of two other stimuli, and a spatial localization, in which participants localized one AV stimulus in relation to themselves. Results revealed that spatial tasks specifically modulated the occipital event-related potentials (ERPs) after the onset of the stimuli. We observed a greater contralateral early occipital component (50-90 ms) when participants solved the spatial bisection, and a more robust later occipital response (110-160 ms) when they processed the spatial localization. This observation suggests that different spatial representations elicited by multisensory stimuli are sustained by separate neurophysiological mechanisms.

Decoding Reach Direction in Early "Visual" Cortex of Congenitally Blind Individuals

Motor actions, such as reaching or grasping, can be decoded from fMRI activity of early visual cortex (EVC) in sighted humans. This effect can depend on vision or visual imagery, or alternatively, could be driven by mechanisms independent of visual experience. Here, we show that the actions of reaching in different directions can be reliably decoded from fMRI activity of EVC in congenitally blind humans (both sexes). Thus, neither visual experience nor visual imagery is necessary for EVC to represent action-related information. We also demonstrate that, within EVC of blind humans, the accuracy of reach direction decoding is highest in areas typically representing foveal vision and gradually decreases in areas typically representing peripheral vision. We propose that this might indicate the existence of a predictive, hard-wired mechanism of aligning action and visual spaces. This mechanism might send action-related information primarily to the high-resolution foveal visual areas, which are critical for guiding and online correction of motor actions. Finally, we show that, beyond EVC, the decoding of reach direction in blind humans is most accurate in dorsal stream areas known to be critical for visuo-spatial and visuo-motor integration in the sighted. Thus, these areas can develop space and action representations even in the lifelong absence of vision. Overall, our findings in congenitally blind humans match previous research on the action system in the sighted, and suggest that the development of action representations in the human brain might be largely independent of visual experience.SIGNIFICANCE STATEMENT Early visual cortex (EVC) was traditionally thought to process only visual signals from the retina. Recent studies proved this account incomplete, and showed EVC involvement in many activities not directly related to incoming visual information, such as memory, sound, or action processing. Is EVC involved in these activities because of visual imagery? Here, we show robust reach direction representation in EVC of humans born blind. This demonstrates that EVC can represent actions independently of vision and visual imagery. Beyond EVC, we found that reach direction representation in blind humans is strongest in dorsal brain areas, critical for action processing in the sighted. This suggests that the development of action representations in the human brain is largely independent of visual experience.

Longitudinal stability of individual brain plasticity patterns in blindness

The primary visual cortex (V1) in individuals born blind is engaged in a wide spectrum of tasks and sensory modalities, including audition, touch, language, and memory. This widespread involvement raises questions regarding the constancy of its role and whether it might exhibit flexibility in its function over time, connecting to diverse network functions in response to task-specific demands. This would suggest that reorganized V1 takes on a role similar to cognitive multiple-demand system regions. Alternatively, it is possible that the varying patterns of plasticity observed in the blind V1 can be attributed to individual factors, whereby different blind individuals recruit V1 for different functions, highlighting the immense idiosyncrasy of plasticity. In support of this second account, we have recently shown that V1 functional connectivity varies greatly across blind individuals. But do these represent stable individual patterns of plasticity or merely instantaneous changes, for a multiple-demand system now inhabiting V1? Here we tested if individual connectivity patterns from the visual cortex of blind individuals are stable over time. We show that over two years, fMRI functional connectivity from the primary visual cortex is unique and highly stable in a small sample of repeatedly sampled congenitally blind individuals. Further, using multivoxel pattern analysis, we demonstrate that the unique reorganization patterns of these individuals allow decoding of participant identity. Together with recent evidence for substantial individual differences in visual cortex connectivity, this indicates there may be a consistent role for the visual cortex in blindness, which may differ for each individual. Further, it suggests that the variability in visual reorganization in blindness across individuals could be used to seek stable neuromarkers for sight rehabilitation and assistive approaches.

Monaural auditory spatial abilities in early blind individuals

Early blind individuals can localize single sound sources better than sighted participants, even under monaural conditions. Yet, in binaural listening, they struggle with understanding the distances between three different sounds. The latter ability has never been tested under monaural conditions. We investigated the performance of eight early blind and eight blindfolded healthy individuals in monaural and binaural listening during two audio-spatial tasks. In the localization task, a single sound was played in front of participants who needed to localize it properly. In the auditory bisection task, three consecutive sounds were played from different spatial positions, and participants reported which sound the second one was closer to. Only early blind individuals improved their performance in the monaural bisection, while no statistical difference was present for the localization task. We concluded that early blind individuals show superior ability in using spectral cues under monaural conditions.

The Topo-Speech sensory substitution system as a method of conveying spatial information to the blind and vision impaired

Humans, like most animals, integrate sensory input in the brain from different sensory modalities. Yet humans are distinct in their ability to grasp symbolic input, which is interpreted into a cognitive mental representation of the world. This representation merges with external sensory input, providing modality integration of a different sort. This study evaluates the Topo-Speech algorithm in the blind and visually impaired. The system provides spatial information about the external world by applying sensory substitution alongside symbolic representations in a manner that corresponds with the unique way our brains acquire and process information. This is done by conveying spatial information, customarily acquired through vision, through the auditory channel, in a combination of sensory (auditory) features and symbolic language (named/spoken) features. The Topo-Speech sweeps the visual scene or image and represents objects' identity by employing naming in a spoken word and simultaneously conveying the objects' location by mapping the x-axis of the visual scene or image to the time it is announced and the y-axis by mapping the location to the pitch of the voice. This proof of concept study primarily explores the practical applicability of this approach in 22 visually impaired and blind individuals. The findings showed that individuals from both populations could effectively interpret and use the algorithm after a single training session. The blind showed an accuracy of 74.45%, while the visually impaired had an average accuracy of 72.74%. These results are comparable to those of the sighted, as shown in previous research, with all participants above chance level. As such, we demonstrate practically how aspects of spatial information can be transmitted through non-visual channels. To complement the findings, we weigh in on debates concerning models of spatial knowledge (the persistent, cumulative, or convergent models) and the capacity for spatial representation in the blind. We suggest the present study's findings support the convergence model and the scenario that posits the blind are capable of some aspects of spatial representation as depicted by the algorithm comparable to those of the sighted. Finally, we present possible future developments, implementations, and use cases for the system as an aid for the blind and visually impaired.

Reorganization of thalamocortical connections in congenitally blind humans

Cross-modal plasticity in blind individuals has been reported over the past decades showing that nonvisual information is carried and processed by "visual" brain structures. However, despite multiple efforts, the structural underpinnings of cross-modal plasticity in congenitally blind individuals remain unclear. We mapped thalamocortical connectivity and assessed the integrity of white matter of 10 congenitally blind individuals and 10 sighted controls. We hypothesized an aberrant thalamocortical pattern of connectivity taking place in the absence of visual stimuli from birth as a potential mechanism of cross-modal plasticity. In addition to the impaired microstructure of visual white matter bundles, we observed structural connectivity changes between the thalamus and occipital and temporal cortices. Specifically, the thalamic territory dedicated to connections with the occipital cortex was smaller and displayed weaker connectivity in congenitally blind individuals, whereas those connecting with the temporal cortex showed greater volume and increased connectivity. The abnormal pattern of thalamocortical connectivity included the lateral and medial geniculate nuclei and the pulvinar nucleus. For the first time in humans, a remapping of structural thalamocortical connections involving both unimodal and multimodal thalamic nuclei has been demonstrated, shedding light on the possible mechanisms of cross-modal plasticity in humans. The present findings may help understand the functional adaptations commonly observed in congenitally blind individuals.

Diverging patterns of plasticity in the nucleus basalis of Meynert in early- and late-onset blindness

Plasticity in the brain is impacted by an individual's age at the onset of the blindness. However, what drives the varying degrees of plasticity remains largely unclear. One possible explanation attributes the mechanisms for the differing levels of plasticity to the cholinergic signals originating in the nucleus basalis of Meynert. This explanation is based on the fact that the nucleus basalis of Meynert can modulate cortical processes such as plasticity and sensory encoding through its widespread cholinergic projections. Nevertheless, there is no direct evidence indicating that the nucleus basalis of Meynert undergoes plastic changes following blindness. Therefore, using multiparametric magnetic resonance imaging, we examined if the structural and functional properties of the nucleus basalis of Meynert differ between early blind, late blind and sighted individuals. We observed that early and late blind individuals had a preserved volumetric size and cerebrovascular reactivity in the nucleus basalis of Meynert. However, we observed a reduction in the directionality of water diffusion in both early and late blind individuals compared to sighted individuals. Notably, the nucleus basalis of Meynert presented diverging patterns of functional connectivity between early and late blind individuals. This functional connectivity was enhanced at both global and local (visual, language and default-mode networks) levels in the early blind individuals, but there were little-to-no changes in the late blind individuals when compared to sighted controls. Furthermore, the age at onset of blindness predicted both global and local functional connectivity. These results suggest that upon reduced directionality of water diffusion in the nucleus basalis of Meynert, cholinergic influence may be stronger for the early blind compared to the late blind individuals. Our findings are important to unravelling why early blind individuals present stronger and more widespread cross-modal plasticity compared to late blind individuals.

Calibrating Vision: Concepts and Questions

The idea that visual coding and perception are shaped by experience and adjust to changes in the environment or the observer is universally recognized as a cornerstone of visual processing, yet the functions and processes mediating these calibrations remain in many ways poorly understood. In this article we review a number of facets and issues surrounding the general notion of calibration, with a focus on plasticity within the encoding and representational stages of visual processing. These include how many types of calibrations there are - and how we decide; how plasticity for encoding is intertwined with other principles of sensory coding; how it is instantiated at the level of the dynamic networks mediating vision; how it varies with development or between individuals; and the factors that may limit the form or degree of the adjustments. Our goal is to give a small glimpse of an enormous and fundamental dimension of vision, and to point to some of the unresolved questions in our understanding of how and why ongoing calibrations are a pervasive and essential element of vision.

Gyrification in relation to cortical thickness in the congenitally blind

Greater cortical gyrification (GY) is linked with enhanced cognitive abilities and is also negatively related to cortical thickness (CT). Individuals who are congenitally blind (CB) exhibits remarkable functional brain plasticity which enables them to perform certain non-visual and cognitive tasks with supranormal abilities. For instance, extensive training using touch and audition enables CB people to develop impressive skills and there is evidence linking these skills to cross-modal activations of primary visual areas. There is a cascade of anatomical, morphometric and functional-connectivity changes in non-visual structures, volumetric reductions in several components of the visual system, and CT is also increased in CB. No study to date has explored GY changes in this population, and no study has explored how variations in CT are related to GY changes in CB. T1-weighted 3D structural magnetic resonance imaging scans were acquired to examine the effects of congenital visual deprivation in cortical structures in a healthy sample of 11 CB individuals (6 male) and 16 age-matched sighted controls (SC) (10 male). In this report, we show for the first time an increase in GY in several brain areas of CB individuals compared to SC, and a negative relationship between GY and CT in the CB brain in several different cortical areas. We discuss the implications of our findings and the contributions of developmental factors and synaptogenesis to the relationship between CT and GY in CB individuals compared to SC. F.

Neural substrates of spatial processing and navigation in blindness: An activation likelihood estimation meta-analysis

Even though vision is considered the best suited sensory modality to acquire spatial information, blind individuals can form spatial representations to navigate and orient themselves efficiently in space. Consequently, many studies support the amodality hypothesis of spatial representations since sensory modalities other than vision contribute to the formation of spatial representations, independently of visual experience and imagery. However, given the high variability in abilities and deficits observed in blind populations, a clear consensus about the neural representations of space has yet to be established. To this end, we performed a meta-analysis of the literature on the neural correlates of spatial processing and navigation via sensory modalities other than vision, like touch and audition, in individuals with early and late onset blindness. An activation likelihood estimation (ALE) analysis of the neuroimaging literature revealed that early blind individuals and sighted controls activate the same neural networks in the processing of non-visual spatial information and navigation, including the posterior parietal cortex, frontal eye fields, insula, and the hippocampal complex. Furthermore, blind individuals also recruit primary and associative occipital areas involved in visuo-spatial processing via cross-modal plasticity mechanisms. The scarcity of studies involving late blind individuals did not allow us to establish a clear consensus about the neural substrates of spatial representations in this specific population. In conclusion, the results of our analysis on neuroimaging studies involving early blind individuals support the amodality hypothesis of spatial representations.

The Outcome of Cochlear Implantations in Deaf-Blind Patients: A Multicenter Observational Study

OBJECTIVE

This multicenter study aimed to evaluate the auditory and speech outcomes of cochlear implantation (CI) in deaf-blind patients compared with deaf-only patients.

STUDY DESIGN

Retrospective cohort study.

SETTING

Multiple cochlear implant centers.

PATIENTS

The current study was conducted on 17 prelingual deaf-blind children and 12 postlingual deaf-blind adults who underwent CI surgery. As a control group, 17 prelingual deaf children and 12 postlingual deaf adults were selected.

INTERVENTION

Cochlear implantation.

MAIN OUTCOME MEASURES

Auditory and linguistic performances in children were assessed using the categories of auditory performance (CAP) and Speech Intelligibility Rating (SIR) scales, respectively. The word recognition score (WRS) was also used to measure speech perception ability in adults. The mean CAP, SIR, and WRS cores were compared between the deaf-only and deaf-blind groups before CI surgery and at "12 months" and "24 months" after device activation. Cohen's d was used for effect size estimation.

RESULTS

We found no significant differences in the mean CAP and SIR scores between the deaf-blind and deaf-only children before the CI surgery. For both groups, SIR and CAP scores improved with increasing time after the device activation. The mean CAP scores in the deaf-only children were either equivalent or slightly higher than those of the deaf-blind children at "12 months post-CI" (3.94 ± 0.74 vs 3.24 ± 1.25; mean difference score, 0.706) and "24 months post-CI" (6.01 ± 0.79 vs 5.47 ± 1.06; mean difference score, 0.529) time intervals, but these differences were not statistically significant. The SIR scores in deaf-only implanted children were, on average, 0.870 scores greater than the deaf-blind children at "12 months post-CI" (2.94 ± 0.55 vs 2.07 ± 1.4; p = 0.01, d = 0.97) and, on average, 1.067 scores greater than deaf-blind children at "24 months post-CI" (4.35 ± 0.49 vs 3.29 ± 1.20; p = 0.002; d = 1.15) time intervals. We also found an improvement in WRS scores from the "preimplantation" to the "12-month post-CI" and "24-month post-CI" time intervals in both groups. Pairwise comparisons indicated that the mean WRS in the deaf-only adults was, on average, 10.61% better than deaf-blind implanted adults at "12 months post-CI" (62.33 ± 9.09% vs 51.71 ± 10.73%, p = 0.034, d = 1.06) and, on average, 15.81% better than deaf-blind adults at "24-months post-CI" (72.67 ± 8.66% vs 56.8 ± 9.78%, p = 0.002, d = 1.61) follow-ups.

CONCLUSION

Cochlear implantation is a beneficial method for the rehabilitation of deaf-blind patients. Both deaf-blind and deaf-only implanted children revealed similar auditory performances. However, speech perception ability in deaf-blind patients was slightly lower than the deaf-only patients in both children and adults.

Auditory perception of ambiguous and non-ambiguous sound in early and late blind children: A functional connectivity study

INTRODUCTION

Auditory perception and associated cognition involve visual and auditory cortical areas for inference of meaningful soundscape.

OBJECTIVE

To investigate auditory perception of ambiguous and non-ambiguous stimulation in auditory and visual cortical networks for categorical discrimination.

METHODOLOGY

Functional mapping was carried out in twenty early (EB), twenty late blind (LB) and fifteen healthy children, during auditory ambiguous and non-ambiguous stimulation task in a 3 T MR scanner to estimate hemodynamic signal alteration and its effect on functional connectivity. The degree of amplitude low-frequency fluctuation (ALFF), correlation analysis and multiple comparison was carried out to map the impact of duration of education and onset of blindness (EB and LB).

RESULTS AND DISCUSSION

Increased functional connectivity (FC) and cross-modal reorganization was observed in auditory, visual and language networks in EB children. FC was increased in contralateral hemisphere in both the blind children (EB and LB) groups and was positively correlated with duration of education performance. Cognitive assessment scores correlated (p < 0.01) with cluster coefficient of FC and BOLD response.

CONCLUSION

FC alterations depend on onset age and audio-haptic training in children associated with increased auditory language and memory perception.

Naturalistic stimuli reveal a sensitive period in cross modal responses of visual cortex: Evidence from adult-onset blindness

How do life experiences impact cortical function? In people who are born blind, the "visual" cortices are recruited during nonvisual tasks, such as Braille reading and sound localization. Do visual cortices have a latent capacity to respond to nonvisual information throughout the lifespan? Alternatively, is there a sensitive period of heightened plasticity that makes visual cortex repurposing especially possible during childhood? To gain insight into these questions, we leveraged meaningful naturalistic auditory stimuli to simultaneously engage a broad range of cognitive domains and quantify cross-modal responses across congenitally blind (n = 22), adult-onset blind (vision loss >18 years-of-age, n = 14) and sighted (n = 22) individuals. During fMRI scanning, participants listened to two types of meaningful naturalistic auditory stimuli: excerpts from movies and a spoken narrative. As controls, participants heard the same narrative with the sentences shuffled and the narrative played backwards (i.e., meaningless sounds). We correlated the voxel-wise timecourses of different participants within condition and group. For all groups, all stimulus conditions induced synchrony in auditory cortex while only the narrative stimuli synchronized responses in higher-cognitive fronto-parietal and temporal regions. As previously reported, inter-subject synchrony in visual cortices was higher in congenitally blind than sighted blindfolded participants and this between-group difference was particularly pronounced for meaningful stimuli (movies and narrative). Critically, visual cortex synchrony was no higher in adult-onset blind than sighted blindfolded participants and did not increase with blindness duration. Sensitive period plasticity enables cross-modal repurposing in visual cortices.

The Role of Visual Experience in Individual Differences of Brain Connectivity

Visual cortex organization is highly consistent across individuals. But to what degree does this consistency depend on life experience, in particular sensory experience? In this study, we asked whether visual cortex reorganization in congenital blindness results in connectivity patterns that are particularly variable across individuals, focusing on resting-state functional connectivity (RSFC) patterns from the primary visual cortex. We show that the absence of shared visual experience results in more variable RSFC patterns across blind individuals than sighted controls. Increased variability is specifically found in areas that show a group difference between the blind and sighted in their RSFC. These findings reveal a relationship between brain plasticity and individual variability; reorganization manifests variably across individuals. We further investigated the different patterns of reorganization in the blind, showing that the connectivity to frontal regions, proposed to have a role in the reorganization of the visual cortex of the blind toward higher cognitive roles, is highly variable. Further, we link some of the variability in visual-to-frontal connectivity to another environmental factor-duration of formal education. Together, these findings show a role of postnatal sensory and socioeconomic experience in imposing consistency on brain organization. By revealing the idiosyncratic nature of neural reorganization, these findings highlight the importance of considering individual differences in fitting sensory aids and restoration approaches for vision loss.SIGNIFICANCE STATEMENT The typical visual system is highly consistent across individuals. What are the origins of this consistency? Comparing the consistency of visual cortex connectivity between people born blind and sighted people, we showed that blindness results in higher variability, suggesting a key impact of postnatal individual experience on brain organization. Further, connectivity patterns that changed following blindness were particularly variable, resulting in diverse patterns of brain reorganization. Individual differences in reorganization were also directly affected by nonvisual experiences in the blind (years of formal education). Together, these findings show a role of sensory and socioeconomic experiences in creating individual differences in brain organization and endorse the use of individual profiles for rehabilitation and restoration of vision loss.

Language and nonverbal auditory processing in the occipital cortex of individuals who are congenitally blind due to anophthalmia

Individuals with congenital blindness due to bilateral anophthalmia offer a unique opportunity to examine cross-modal plasticity in the complete absence of any stimulation of the 'visual' pathway even during development in utero. Our previous work has suggested that this complete sensory deafferentation results in different patterns of reorganisation compared with those seen in other early blind populations. Here, we further test the functional specialisation of occipital cortex in six well-studied cases with anophthalmia. Whole brain functional MRI was obtained while these human participants and a group of sighted controls performed two experiments involving phonological and semantic processing of words (verbal experiment) and spatial and identity processing of piano chords (nonverbal experiment). Both experiments were predicted to show a dorsal-ventral difference in activity based on the specific task performed. All tasks evoked activation in occipital cortex in the individuals with anophthalmia but not in the sighted controls. For the verbal experiment, both dorsal and ventral occipital areas were strongly activated by the phonological and semantic tasks in anophthalmia. For the nonverbal experiment, both the spatial and the identity task robustly activated the dorsal occipital area V3a but showed inconsistent activity elsewhere in the occipital lobe. V1 was most strongly activated by the verbal tasks, showing greater activity on the left for the verbal task relative to the nonverbal one. For individual anophthalmic participants, however, activity in V1 was inconsistent across tasks and hemispheres with many participants showing activity levels in the control range, which was not significantly above baseline. Despite the homogeneous nature of the cause of blindness in the anophthalmic group, there remain differences in patterns of activation among the individuals with this condition. Investigation at the case level might further our understanding of how post-natal experiences shape functional reorganisation in deafferented cortex.

Superior verbal but not nonverbal memory in congenital blindness

Previous studies suggest that people who are congenitally blind outperform sighted people on some memory tasks. Whether blindness-associated memory advantages are specific to verbal materials or are also observed with nonverbal sounds has not been determined. Congenitally blind individuals (n = 20) and age and education matched blindfolded sighted controls (n = 22) performed a series of auditory memory tasks. These included: verbal forward and backward letter spans, a complex letter span with intervening equations, as well as two matched recognition tasks: one with verbal stimuli (i.e., letters) and one with nonverbal complex meaningless sounds. Replicating previously observed findings, blind participants outperformed sighted people on forward and backward letter span tasks. Blind participants also recalled more letters on the complex letter span task despite the interference of intervening equations. Critically, the same blind participants showed larger advantages on the verbal as compared to the nonverbal recognition task. These results suggest that blindness selectively enhances memory for verbal material. Possible explanations for blindness-related verbal memory advantages include blindness-induced memory practice and 'visual' cortex recruitment for verbal processing.

A natural history of vertebrate vision loss: Insight from mammalian vision for human visual function

Research on the origin of vision and vision loss in naturally "blind" animal species can reveal the tasks that vision fulfills and the brain's role in visual experience. Models that incorporate evolutionary history, natural variation in visual ability, and experimental manipulations can help disentangle visual ability at a superficial level from behaviors linked to vision but not solely reliant upon it, and could assist the translation of ophthalmological research in animal models to human treatments. To unravel the similarities between blind individuals and blind species, we review concepts of 'blindness' and its behavioral correlates across a range of species. We explore the ancestral emergence of vision in vertebrates, and the loss of vision in blind species with reference to an evolution-based classification scheme. We applied phylogenetic comparative methods to a mammalian tree to explore the evolution of visual acuity using ancestral state estimations. Future research into the natural history of vision loss could help elucidate the function of vision and inspire innovations in how to address vision loss in humans.

Reorganization of large-scale brain networks in deaf signing adults: The role of auditory cortex in functional reorganization following deafness

If the brain is deprived of input from one or more senses during development, functional and structural reorganization of the deprived regions takes place. However, little is known about how sensory deprivation affects large-scale brain networks. In the present study, we use data-driven independent component analysis (ICA) to characterize large-scale brain networks in 15 deaf early signers and 24 hearing non-signers based on resting-state functional MRI data. We found differences between the groups in independent components representing the left lateralized control network, the default network, the ventral somatomotor network, and the attention network. In addition, we showed stronger functional connectivity for deaf compared to hearing individuals from the middle and superior temporal cortices to the cingulate cortex, insular cortex, cuneus and precuneus, supramarginal gyrus, supplementary motor area, and cerebellum crus 1, and stronger connectivity for hearing non-signers to hippocampus, middle and superior frontal gyri, pre- and postcentral gyri, and cerebellum crus 8. These results show that deafness induces large-scale network reorganization, with the middle/superior temporal cortex as a central node of plasticity. Cross-modal reorganization may be associated with behavioral adaptations to the environment, including superior ability in some visual functions such as visual working memory and visual attention, in deaf signers.

Limits of Cross-modal Plasticity? Short-term Visual Deprivation Does Not Enhance Cardiac Interoception, Thermosensation, or Tactile Spatial Acuity

In the present study, we investigated the effect of short-term visual deprivation on discriminative touch, cardiac interoception, and thermosensation by asking 64 healthy volunteers to perform four behavioral tasks. The experimental group contained 32 subjects who were blindfolded and kept in complete darkness for 110minutes, while the control group consisted of 32 volunteers who were not blindfolded but were otherwise kept under identical experimental conditions. Both groups performed the required tasks three times: before and directly after deprivation (or control) and after an additional washout period of 40minutes, in which all participants were exposed to normal light conditions. Our results showed that short-term visual deprivation had no effect on any of the senses tested. This finding suggests that short-term visual deprivation does not modulate basic bodily senses and extends this principle beyond tactile processing to the interoceptive modalities of cardiac and thermal sensations.

Use of Functional Near-Infrared Spectroscopy to Predict and Measure Cochlear Implant Outcomes: A Scoping Review

Outcomes following cochlear implantation vary widely for both adults and children, and behavioral tests are currently relied upon to assess this. However, these behavioral tests rely on subjective judgements that can be unreliable, particularly for infants and young children. The addition of an objective test of outcome following cochlear implantation is therefore desirable. The aim of this scoping review was to comprehensively catalogue the evidence for the potential of functional near infrared spectroscopy (fNIRS) to be used as a tool to objectively predict and measure cochlear implant outcomes. A scoping review of the literature was conducted following the PRISMA extension for scoping review framework. Searches were conducted in the MEDLINE, EMBASE, PubMed, CINAHL, SCOPUS, and Web of Science electronic databases, with a hand search conducted in Google Scholar. Key terms relating to near infrared spectroscopy and cochlear implants were used to identify relevant publications. Eight records met the criteria for inclusion. Seven records reported on adult populations, with five records only including post-lingually deaf individuals and two including both pre- and post-lingually deaf individuals. Studies were either longitudinal or cross-sectional, and all studies compared fNIRS measurements with receptive speech outcomes. This review identified and collated key work in this field. The homogeneity of the populations studied so far identifies key gaps for future research, including the use of fNIRS in infants. By mapping the literature on this important topic, this review contributes knowledge towards the improvement of outcomes following cochlear implantation.

Partial visual loss disrupts the relationship between judged room size and sound source distance

Visual spatial information plays an important role in calibrating auditory space. Blindness results in deficits in a number of auditory abilities, which have been explained in terms of the hypothesis that visual information is needed to calibrate audition. When judging the size of a novel room when only auditory cues are available, normally sighted participants may use the location of the farthest sound source to infer the nearest possible distance of the far wall. However, for people with partial visual loss (distinct from blindness in that some vision is present), such a strategy may not be reliable if vision is needed to calibrate auditory cues for distance. In the current study, participants were presented with sounds at different distances (ranging from 1.2 to 13.8 m) in a simulated reverberant (T₆₀ = 700 ms) or anechoic room. Farthest distance judgments and room size judgments (volume and area) were obtained from blindfolded participants (18 normally sighted, 38 partially sighted) for speech, music, and noise stimuli. With sighted participants, the judged room volume and farthest sound source distance estimates were positively correlated (p < 0.05) for all conditions. Participants with visual losses showed no significant correlations for any of the conditions tested. A similar pattern of results was observed for the correlations between farthest distance and room floor area estimates. Results demonstrate that partial visual loss disrupts the relationship between judged room size and sound source distance that is shown by sighted participants.

Expanding a Large Inclusive Study of Human Listening Rates

As conversational agents and digital assistants become increasingly pervasive, understanding their synthetic speech becomes increasingly important. Simultaneously, speech synthesis is becoming more sophisticated and manipulable, providing the opportunity to optimize speech rate to save users time. However, little is known about people’s abilities to understand fast speech. In this work, we provide an extension of the first large-scale study on human listening rates, enlarging the prior study run with 453 participants to 1,409 participants and adding new analyses on this larger group. Run on LabintheWild, it used volunteer participants, was screen reader accessible, and measured listening rate by accuracy at answering questions spoken by a screen reader at various rates. Our results show that people who are visually impaired, who often rely on audio cues and access text aurally, generally have higher listening rates than sighted people. The findings also suggest a need to expand the range of rates available on personal devices. These results demonstrate the potential for users to learn to listen to faster rates, expanding the possibilities for human-conversational agent interaction.

Auditory speed processing in sighted and blind individuals

Multisensory experience is crucial for developing a coherent perception of the world. In this context, vision and audition are essential tools to scaffold spatial and temporal representations, respectively. Since speed encompasses both space and time, investigating this dimension in blindness allows deepening the relationship between sensory modalities and the two representation domains. In the present study, we hypothesized that visual deprivation influences the use of spatial and temporal cues underlying acoustic speed perception. To this end, ten early blind and ten blindfolded sighted participants performed a speed discrimination task in which spatial, temporal, or both cues were available to infer moving sounds' velocity. The results indicated that both sighted and early blind participants preferentially relied on temporal cues to determine stimuli speed, by following an assumption that identified as faster those sounds with a shorter duration. However, in some cases, this temporal assumption produces a misperception of the stimulus speed that negatively affected participants' performance. Interestingly, early blind participants were more influenced by this misleading temporal assumption than sighted controls, resulting in a stronger impairment in the speed discrimination performance. These findings demonstrate that the absence of visual experience in early life increases the auditory system's preference for the time domain and, consequentially, affects the perception of speed through audition.

Structural and white matter changes associated with duration of Braille education in early and late blind children

In early (EB) and late blind (LB) children, vision deprivation produces cross-modal plasticity in the visual cortex. The progression of structural- and tract-based spatial statistics changes in the visual cortex in EB and LB, as well as their impact on global cognition, have yet to be investigated. The purpose of this study was to determine the cortical thickness (CT), gyrification index (GI), and white matter (WM) integrity in EB and LB children, as well as their association to the duration of blindness and education. Structural and diffusion tensor imaging data were acquired in a 3T magnetic resonance imaging in EB and LB children (n = 40 each) and 30 sighted controls (SCs) and processed using CAT12 toolbox and FSL software. Two sample t-test was used for group analyses with P < 0.05 (false discovery rate-corrected). Increased CT in visual, sensory-motor, and auditory areas, and GI in bilateral visual cortex was observed in EB children. In LB children, the right visual cortex, anterior-cingulate, sensorimotor, and auditory areas showed increased GI. Structural- and tract-based spatial statistics changes were observed in anterior visual pathway, thalamo-cortical, and corticospinal tracts, and were correlated with education onset and global cognition in EB children. Reduced impairment in WM, increased CT and GI and its correlation with global cognitive functions in visually impaired children suggests cross-modal plasticity due to adaptive compensatory mechanism (as compared to SCs). Reduced CT and increased FA in thalamo-cortical areas in EB suggest synaptic pruning and alteration in WM integrity. In the visual cortical pathway, higher education and the development of blindness modify the morphology of brain areas and influence the probabilistic tractography in EB rather than LB.

Auditory and olfactory findings in patients with USH2A-related retinal degeneration-Findings at baseline from the rate of progression in USH2A-related retinal degeneration natural history study (RUSH2A)

Sensorineural hearing loss (SNHL) is characteristic of Usher syndrome type 2 (USH2), but less is known about SNHL in nonsyndromic autosomal recessive retinitis pigmentosa (ARRP) and olfaction in USH2A-associated retinal degeneration. The Rate of Progression of USH2A-related Retinal Degeneration (RUSH2A) is a natural history study that enrolled 127 participants, 80 with USH2 and 47 with ARRP. Hearing was measured by pure-tone thresholds and word recognition scores, and olfaction by the University of Pennsylvania Smell Identification Test (UPSIT). SNHL was moderate in 72% of USH2 participants and severe or profound in 25%, while 9% of ARRP participants had moderate adult-onset SNHL. Pure-tone thresholds worsened with age in ARRP but not in USH2 participants. The degree of SNHL was not associated with other participant characteristics in either USH2 or ARRP. Median pure-tone thresholds in ARRP participants were significantly higher than the normative population (p < 0.001). Among 14 USH2 participants reporting newborn hearing screening results, 7 reported passing. Among RUSH2A participants, 7% had mild microsmia and 5% had moderate or severe microsmia. Their mean (±SD) UPSIT score was 35 (±3), similar to healthy controls (34 [±3]; p = 0.39). Olfaction differed by country (p = 0.02), but was not significantly associated with clinical diagnosis, age, gender, race/ethnicity, smoking status, visual measures, or hearing. Hearing loss in USH2A-related USH2 did not progress with age. ARRP patients had higher pure-tone thresholds than normal. Newborn hearing screening did not identify all USH2A-related hearing loss. Olfaction was not significantly worse than normal in participants with USH2A-related retinal degeneration.

Seeing Beyond Your Nose? The Effects of Lifelong Olfactory Sensory Deprivation on Cerebral Audio-visual Integration

Lifelong auditory and visual sensory deprivation have been demonstrated to alter both perceptual acuity and the neural processing of remaining senses. Recently, it was demonstrated that individuals with anosmia, i.e. complete olfactory sensory deprivation, displayed enhanced multisensory integration performance. Whether this ability is due to a reorganization of olfactory processing regions to focus on cross-modal multisensory information or whether it is due to enhanced processing within multisensory integration regions is not known. To dissociate these two outcomes, we investigated the neural processing of dynamic audio-visual stimuli in individuals with congenital anosmia and matched controls (both groups, n = 33) using functional magnetic resonance imaging. Specifically, we assessed whether the previously demonstrated multisensory enhancement is related to cross-modal processing of multisensory stimuli in olfactory associated regions, the piriform and olfactory orbitofrontal cortices, or enhanced multisensory processing in established multisensory integration regions, the superior temporal and intraparietal sulci. No significant group differences were found in the a priori hypothesized regions using region of interest analyses. However, exploratory whole-brain analysis suggested higher activation related to multisensory integration within the posterior superior temporal sulcus, in close proximity to the multisensory region of interest, in individuals with congenital anosmia. No group differences were demonstrated in olfactory associated regions. Although results were outside our hypothesized regions, combined, they tentatively suggest that enhanced processing of audio-visual stimuli in individuals with congenital anosmia may be mediated by multisensory, and not primary sensory, cerebral regions.

Beat Detection Recruits the Visual Cortex in Early Blind Subjects

Using functional magnetic resonance imaging, here we monitored the brain activity in 12 early blind subjects and 12 blindfolded control subjects, matched for age, gender and musical experience, during a beat detection task. Subjects were required to discriminate regular ("beat") from irregular ("no beat") rhythmic sequences composed of sounds or vibrotactile stimulations. In both sensory modalities, the brain activity differences between the two groups involved heteromodal brain regions including parietal and frontal cortical areas and occipital brain areas, that were recruited in the early blind group only. Accordingly, early blindness induced brain plasticity changes in the cerebral pathways involved in rhythm perception, with a participation of the visually deprived occipital brain areas whatever the sensory modality for input. We conclude that the visually deprived cortex switches its input modality from vision to audition and vibrotactile sense to perform this temporal processing task, supporting the concept of a metamodal, multisensory organization of this cortex.

Data-Driven Classification of Spectral Profiles Reveals Brain Region-Specific Plasticity in Blindness

Congenital blindness has been shown to result in behavioral adaptation and neuronal reorganization, but the underlying neuronal mechanisms are largely unknown. Brain rhythms are characteristic for anatomically defined brain regions and provide a putative mechanistic link to cognitive processes. In a novel approach, using magnetoencephalography resting state data of congenitally blind and sighted humans, deprivation-related changes in spectral profiles were mapped to the cortex using clustering and classification procedures. Altered spectral profiles in visual areas suggest changes in visual alpha-gamma band inhibitory-excitatory circuits. Remarkably, spectral profiles were also altered in auditory and right frontal areas showing increased power in theta-to-beta frequency bands in blind compared with sighted individuals, possibly related to adaptive auditory and higher cognitive processing. Moreover, occipital alpha correlated with microstructural white matter properties extending bilaterally across posterior parts of the brain. We provide evidence that visual deprivation selectively modulates spectral profiles, possibly reflecting structural and functional adaptation.

Spatial navigation with horizontally spatialized sounds in early and late blind individuals

Blind individuals often report difficulties to navigate and to detect objects placed outside their peri-personal space. Although classical sensory substitution devices could be helpful in this respect, these devices often give a complex signal which requires intensive training to analyze. New devices that provide a less complex output signal are therefore needed. Here, we evaluate a smartphone-based sensory substitution device that offers navigation guidance based on strictly spatial cues in the form of horizontally spatialized sounds. The system uses multiple sensors to either detect obstacles at a distance directly in front of the user or to create a 3D map of the environment (detection and avoidance mode, respectively), and informs the user with auditory feedback. We tested 12 early blind, 11 late blind and 24 blindfolded-sighted participants for their ability to detect obstacles and to navigate in an obstacle course. The three groups did not differ in the number of objects detected and avoided. However, early blind and late blind participants were faster than their sighted counterparts to navigate through the obstacle course. These results are consistent with previous research on sensory substitution showing that vision can be replaced by other senses to improve performance in a wide variety of tasks in blind individuals. This study offers new evidence that sensory substitution devices based on horizontally spatialized sounds can be used as a navigation tool with a minimal amount of training.

Enhanced Odorant Localization Abilities in Congenitally Blind but not in Late-Blind Individuals

Although often considered a nondominant sense for spatial perception, chemosensory perception can be used to localize the source of an event and potentially help us navigate through our environment. Would blind people who lack the dominant spatial sense-vision-develop enhanced spatial chemosensation or suffer from the lack of visual calibration on spatial chemosensory perception? To investigate this question, we tested odorant localization abilities across nostrils in blind people compared to sighted controls and if the time of vision loss onset modulates those abilities. We observed that congenitally blind individuals (10 subjects) outperformed sighted (20 subjects) and late-blind subjects (10 subjects) in a birhinal localization task using mixed olfactory-trigeminal stimuli. This advantage in congenitally blind people was selective to olfactory localization but not observed for odorant detection or identification. We, therefore, showed that congenital blindness but not blindness acquired late in life is linked to enhanced localization of chemosensory stimuli across nostrils, most probably of the trigeminal component. In addition to previous studies highlighting enhanced localization abilities in auditory and tactile modalities, our current results extend such enhanced abilities to chemosensory localization.

Auditory localization should be considered as a sign of minimally conscious state based on multimodal findings

Auditory localization (i.e. turning the head and/or the eyes towards an auditory stimulus) is often part of the clinical evaluation of patients recovering from coma. The objective of this study is to determine whether auditory localization could be considered as a new sign of minimally conscious state, using a multimodal approach. The presence of auditory localization and the clinical outcome at 2 years of follow-up were evaluated in 186 patients with severe brain injury, including 64 with unresponsive wakefulness syndrome, 28 in minimally conscious state minus, 71 in minimally conscious state plus and 23 who emerged from the minimally conscious state. Brain metabolism, functional connectivity and graph theory measures were investigated by means of ¹⁸F-fluorodeoxyglucose positron emission tomography, functional MRI and high-density electroencephalography in two subgroups of unresponsive patients, with and without auditory localization. These two subgroups were also compared to a subgroup of patients in minimally conscious state minus. Auditory localization was observed in 13% of unresponsive patients, 46% of patients in minimally conscious state minus, 62% of patients in minimally conscious state plus and 78% of patients who emerged from the minimally conscious state. The probability to observe an auditory localization increased along with the level of consciousness, and the presence of auditory localization could predict the level of consciousness. Patients with auditory localization had higher survival rates (at 2-year follow-up) than those without localization. Differences in brain function were found between unresponsive patients with and without auditory localization. Higher connectivity in unresponsive patients with auditory localization was measured between the fronto-parietal network and secondary visual areas, and in the alpha band electroencephalography network. Moreover, patients in minimally conscious state minus significantly differed from unresponsive patients without auditory localization in terms of brain metabolism and alpha network centrality, whereas no difference was found with unresponsive patients who presented auditory localization. Our multimodal findings suggest differences in brain function between unresponsive patients with and without auditory localization, which support our hypothesis that auditory localization should be considered as a new sign of minimally conscious state. Unresponsive patients showing auditory localization should therefore no longer be considered unresponsive but minimally conscious. This would have crucial consequences on these patients’ lives as it would directly impact the therapeutic orientation or end-of-life decisions usually taken based on the diagnosis.

Impact of visual and auditory deprivation on speech perception and production in adults

Speech perception relies on auditory and visual cues and there are strong links between speech perception and production. We aimed to evaluate the role of auditory and visual modalities on speech perception and production in adults with impaired hearing or sight versus those with normal hearing and sight. We examined speech perception and production of three isolated vowels (/i/, /y/, /u/), which were selected based on their different auditory and visual perceptual saliencies, in 12 deaf adults who used one or two cochlear implants (CIs), 14 congenitally blind adults, and 16 adults with normal sight and hearing. The results showed that the deaf adults who used a CI had worse vowel identification and discrimination perception and they also produced vowels that were less typical or precise than other participants. They had different tongue positions in speech production, which possibly partly explains the poorer quality of their spoken vowels. Blind individuals had larger lip openings and smaller lip protrusions for the rounded vowel and unrounded vowels, compared to the other participants, but they still produced vowels that were similar to those produced by the adults with normal sight and hearing. In summary, the deaf adults, even though they used CIs, had greater difficulty in producing accurate vowel targets than the blind adults, whereas the blind adults were still able to produce accurate vowel targets, even though they used different articulatory strategies.

The Effect of Blindness on Spatial Asymmetries

The human cerebral cortex is asymmetrically organized with hemispheric lateralization pervading nearly all neural systems of the brain. Whether the lack of normal visual development affects hemispheric specialization subserving the deployment of visuospatial attention asymmetries is controversial. In principle, indeed, the lack of early visual experience may affect the lateralization of spatial functions, and the blind may rely on a different sensory input compared to the sighted. In this review article, we thus present a current state-of-the-art synthesis of empirical evidence concerning the effects of visual deprivation on the lateralization of various spatial processes (i.e., including line bisection, mirror symmetry, and localization tasks). Overall, the evidence reviewed indicates that spatial processes are supported by a right hemispheric network in the blind, hence, analogously to the sighted. Such a right-hemisphere dominance, however, seems more accentuated in the blind as compared to the sighted as indexed by the greater leftward bias shown in different spatial tasks. This is possibly the result of the more pronounced involvement of the right parietal cortex during spatial tasks in blind individuals compared to the sighted, as well as of the additional recruitment of the right occipital cortex, which would reflect the cross-modal plastic phenomena that largely characterize the blind brain.

Early visual cortex response for sound in expert blind echolocators, but not in early blind non-echolocators

Echolocation is a perceptual and navigational skill that can be acquired by some individuals. Regarding blind people, this skill can help them "see" the environment around them via a new form of auditory information based on echoes. Expert human echolocators benefit from using this technique not only in controlled environments but also in their everyday lives. In the current study, we investigate the effect of echolocation on blind people's auditory spatial abilities at the cortical level. In an auditory spatial bisection task, we tested people who are early blinds and early blind expert echolocators, along with sighted people. Our results showed that there is similar early activation (50-90 ms) in the posterior area of the scalp for both early blind expert echolocators and sighted participants, but not in the early blind group. This activation was related to sound stimulation, and it is contralateral to the position of the sound in space. These findings indicate that echolocation is a good substitute for the visual modality that enables the development of auditory spatial representations when vision is not available.

General Enhancement of Spatial Hearing in Congenitally Blind People

Vision is thought to support the development of spatial abilities in the other senses. If this is true, how does spatial hearing develop in people lacking visual experience? We comprehensively addressed this question by investigating auditory-localization abilities in 17 congenitally blind and 17 sighted individuals using a psychophysical minimum-audible-angle task that lacked sensorimotor confounds. Participants were asked to compare the relative position of two sound sources located in central and peripheral, horizontal and vertical, or frontal and rear spaces. We observed unequivocal enhancement of spatial-hearing abilities in congenitally blind people, irrespective of the field of space that was assessed. Our results conclusively demonstrate that visual experience is not a prerequisite for developing optimal spatial-hearing abilities and that, in striking contrast, the lack of vision leads to a general enhancement of auditory-spatial skills.

Normal Olfactory Functional Connectivity Despite Lifelong Absence of Olfactory Experiences

Congenital blindness is associated with atypical morphology and functional connectivity within and from visual cortical regions; changes that are hypothesized to originate from a lifelong absence of visual input and could be regarded as a general (re) organization principle of sensory cortices. Challenging this is the fact that individuals with congenital anosmia (lifelong olfactory sensory loss) display little to no morphological changes in the primary olfactory cortex. To determine whether olfactory input from birth is essential to establish and maintain normal functional connectivity in olfactory processing regions, akin to the visual system, we assessed differences in functional connectivity within the olfactory cortex between individuals with congenital anosmia (n = 33) and matched controls (n = 33). Specifically, we assessed differences in connectivity between core olfactory processing regions as well as differences in regional homogeneity and homotopic connectivity within the primary olfactory cortex. In contrast to congenital blindness, none of the analyses indicated atypical connectivity in individuals with congenital anosmia. In fact, post-hoc Bayesian analysis provided support for an absence of group differences. These results suggest that a lifelong absence of olfactory experience has a limited impact on the functional connectivity in the olfactory cortex, a finding that indicates a clear difference between sensory modalities in how sensory cortical regions develop.