How does experience modulate auditory spatial processing in individuals with blindness?

Clinical assessment of the TechArm system on visually impaired and blind children during uni- and multi-sensory perception tasks

We developed the TechArm system as a novel technological tool intended for visual rehabilitation settings. The system is designed to provide a quantitative assessment of the stage of development of perceptual and functional skills that are normally vision-dependent, and to be integrated in customized training protocols. Indeed, the system can provide uni- and multisensory stimulation, allowing visually impaired people to train their capability of correctly interpreting non-visual cues from the environment. Importantly, the TechArm is suitable to be used by very young children, when the rehabilitative potential is maximal. In the present work, we validated the TechArm system on a pediatric population of low-vision, blind, and sighted children. In particular, four TechArm units were used to deliver uni- (audio or tactile) or multi-sensory stimulation (audio-tactile) on the participant's arm, and subject was asked to evaluate the number of active units. Results showed no significant difference among groups (normal or impaired vision). Overall, we observed the best performance in tactile condition, while auditory accuracy was around chance level. Also, we found that the audio-tactile condition is better than the audio condition alone, suggesting that multisensory stimulation is beneficial when perceptual accuracy and precision are low. Interestingly, we observed that for low-vision children the accuracy in audio condition improved proportionally to the severity of the visual impairment. Our findings confirmed the TechArm system's effectiveness in assessing perceptual competencies in sighted and visually impaired children, and its potential to be used to develop personalized rehabilitation programs for people with visual and sensory impairments.

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.

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.

The Neurophysiological Basis of the Trial-Wise and Cumulative Ventriloquism Aftereffects

Our senses often receive conflicting multisensory information, which our brain reconciles by adaptive recalibration. A classic example is the ventriloquism aftereffect, which emerges following both cumulative (long-term) and trial-wise exposure to spatially discrepant multisensory stimuli. Despite the importance of such adaptive mechanisms for interacting with environments that change over multiple timescales, it remains debated whether the ventriloquism aftereffects observed following trial-wise and cumulative exposure arise from the same neurophysiological substrate. We address this question by probing electroencephalography recordings from healthy humans (both sexes) for processes predictive of the aftereffect biases following the exposure to spatially offset audiovisual stimuli. Our results support the hypothesis that discrepant multisensory evidence shapes aftereffects on distinct timescales via common neurophysiological processes reflecting sensory inference and memory in parietal-occipital regions, while the cumulative exposure to consistent discrepancies additionally recruits prefrontal processes. During the subsequent unisensory trial, both trial-wise and cumulative exposure bias the encoding of the acoustic information, but do so distinctly. Our results posit a central role of parietal regions in shaping multisensory spatial recalibration, suggest that frontal regions consolidate the behavioral bias for persistent multisensory discrepancies, but also show that the trial-wise and cumulative exposure bias sound position encoding via distinct neurophysiological processes.

SIGNIFICANCE STATEMENT Our brain easily reconciles conflicting multisensory information, such as seeing an actress on screen while hearing her voice over headphones. These adaptive mechanisms exert a persistent influence on the perception of subsequent unisensory stimuli, known as the ventriloquism aftereffect. While this aftereffect emerges following trial-wise or cumulative exposure to multisensory discrepancies, it remained unclear whether both arise from a common neural substrate. We here rephrase this hypothesis using human electroencephalography recordings. Our data suggest that parietal regions involved in multisensory and spatial memory mediate the aftereffect following both trial-wise and cumulative adaptation, but also show that additional and distinct processes are involved in consolidating and implementing the aftereffect following prolonged exposure.

Years of Blindness Lead to "Visualize" Space Through Time

Spatial representation has been widely studied in early blindness, whereas research about late blindness is still limited. We recently demonstrated that the early (50-90 ms) event-related potential (ERP) response observed in sighted people during a spatial bisection task, is altered in early blind people and is influenced by the amount of time spent without vision in late blind individuals. Specifically, in late blind people a shorter period of blindness is associated with strong contralateral activation in occipital cortex and good performance during the spatial task-similar to that of sighted people. In contrast, non-lateralized occipital activation and lower performance characterize late blind individuals who have experienced a longer period of blindness-similar to that of early blind people. However, the same early occipital response activated in sighted individuals by spatial cues has been found to be activated by temporal cues in early blind individuals. Here, we investigate whether a similar temporal attraction can explain the neural and behavioral changes observed after many years of blindness in late blind people. An EEG recording was taken during a spatial bisection task where coherent and conflicting spatio-temporal information was presented. In participants with long blindness duration, the early recruitment of both visual and auditory areas is sensitive to temporal instead of spatial coordinates. These findings highlight some limits of neuroplasticity. Perceptual advantages from cross-sensory calibration during development seem to be subsequently lost following years of visual deprivation. This result has important implications for clinical outcomes following late blindness, highlighting the importance of timing in intervention and rehabilitation programs that activate compensatory strategies soon after sensory loss.

What and where in the auditory systems of sighted and early blind individuals: Evidence from representational similarity analysis

Separated ventral and dorsal streams in auditory system have been proposed to process sound identification and localization respectively. Despite the popularity of the dual-pathway model, it remains controversial how much independence two neural pathways enjoy and whether visual experiences can influence the distinct cortical organizational scheme. In this study, representational similarity analysis (RSA) was used to explore the functional roles of distinct cortical regions that lay within either the ventral or dorsal auditory streams of sighted and early blind (EB) participants. We found functionally segregated auditory networks in both sighted and EB groups where anterior superior temporal gyrus (aSTG) and inferior frontal junction (IFJ) were more related to the sound identification, while posterior superior temporal gyrus (pSTG) and inferior parietal lobe (IPL) preferred the sound localization. The findings indicated visual experiences may not have an influence on this functional dissociation and the cortex of the human brain may be organized as task-specific and modality-independent strategies. Meanwhile, partial overlap of spatial and non-spatial auditory information processing was observed, illustrating the existence of interaction between the two auditory streams. Furthermore, we investigated the effect of visual experiences on the neural bases of auditory perception and observed the cortical reorganization in EB participants in whom middle occipital gyrus was recruited to process auditory information. Our findings examined the distinct cortical networks that abstractly encoded sound identification and localization, and confirmed the existence of interaction from the multivariate perspective. Furthermore, the results suggested visual experience might not impact the functional specialization of auditory regions.

Influence of Visual Deprivation on Auditory Spectral Resolution, Temporal Resolution, and Speech Perception

We evaluated whether blind subjects have advantages in auditory spectral resolution, temporal resolution, and speech perception in noise compared with sighted subjects. We also compared psychoacoustic performance between early blind (EB) subjects and late blind (LB) subjects. Nineteen EB subjects, 16 LB subjects, and 20 sighted individuals were enrolled. All subjects were right-handed with normal and symmetric hearing thresholds and without cognitive impairments. Three psychoacoustic measurements of the subjects’ right ears were performed via an inserted earphone to determine spectral-ripple discrimination (SRD), temporal modulation detection (TMD), and speech recognition threshold (SRT) in noisy conditions. Acoustic change complex (ACC) responses were recorded during passive listening to standard ripple-inverted ripple stimuli. EB subjects exhibited better SRD than did LB (p = 0.020) and sighted (p = 0.003) subjects. TMD was better in EB (p < 0.001) and LB (p = 0.007) subjects compared with sighted subjects. SRD was positively correlated with the duration of blindness (r = 0.386, p = 0.024). Acoustic change complex data for ripple noise change at the Cz and Fz electrodes showed trends toward significant correlations with the behavioral results. In conclusion, compared with sighted subjects, EB subjects showed advantages in terms of auditory spectral and temporal resolution, while LB subjects showed an advantage in temporal resolution exclusively. These findings suggest that it might take longer for auditory spectral resolution to functionally enhance following visual deprivation compared to temporal resolution. Alternatively, a critical period of very young age may be required for auditory spectral resolution to improve following visual deprivation.

Functional MRI of Sensory Substitution in the Blind

Visual cortex functionality in the blind has been shown to shift away from sensory networks toward task-positive networks that are involved in top-down modulation. However, how such modulation is shaped by experience and reflected behaviorally remains unclear. This study evaluates the visual cortex activity and functional connectivity among congenitally blind, acquired blind, and sighted subjects using blood-oxygenation-level-dependent functional MRI during sensory substitution tasks and at rest. We found that primary visual cortex activity due to active interpretation not only depends on the blindness duration, but also negatively associates with behavioral reaction time. In addition, alterations in visual and task-positive functional connectivity progress over the duration of blindness. In summary, this work suggests that functional plasticity in the primary visual cortex can be reshaped in the blind over time, even in the adult stage. Furthermore, the degree of top-down activity in the primary visual cortex may reflect the speed of performance during sensory substitution.

Properties of cross-modal occipital responses in early blindness: An ALE meta-analysis

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ALE meta-analysis reveals distributed brain networks for object and spatial functions in individuals with early blindness.

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ALE contrast analysis reveals specific activations in the left cuneus and lingual gyrus for language function, suggesting a reverse hierarchical organization of the visual cortex for early blind individuals.

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The findings contribute to visual rehabilitation in blind individuals by revealing the function-dependent and sensory-independent networks during nonvisual processing.

Cross-modal occipital responses appear to be essential for nonvisual processing in individuals with early blindness. However, it is not clear whether the recruitment of occipital regions depends on functional domain or sensory modality. The current study utilized a coordinate-based meta-analysis to identify the distinct brain regions involved in the functional domains of object, spatial/motion, and language processing and the common brain regions involved in both auditory and tactile modalities in individuals with early blindness. Following the PRISMA guidelines, a total of 55 studies were included in the meta-analysis. The specific analyses revealed the brain regions that are consistently recruited for each function, such as the dorsal fronto-parietal network for spatial function and ventral occipito-temporal network for object function. This is consistent with the literature, suggesting that the two visual streams are preserved in early blind individuals. The contrast analyses found specific activations in the left cuneus and lingual gyrus for language function. This finding is novel and suggests a reverse hierarchical organization of the visual cortex for early blind individuals. The conjunction analyses found common activations in the right middle temporal gyrus, right precuneus and a left parieto-occipital region. Clinically, this work contributes to visual rehabilitation in early blind individuals by revealing the function-dependent and sensory-independent networks during nonvisual processing.

Effects of Visual Working Memory on Individual Differences in Echolocation Performance in Sighted Participants

Echolocation performance differs widely among individuals. This study examined a possible factor that may explain this variation, namely, visual working memory, which is a subcomponent of spatial working memory. Sighted participants performed an object-detection task consisting of initial testing on 2 separate days (up to 8 days apart) with follow-up testing on a third day (up to 1 month after the second day of testing) while manipulating the target distance from 20 to 50 cm. Participants performed two types of visual spatial working memory tasks, one of which required them to memorize color-location combinations and the other, an imaginary pathway. The participants' performance on the object-detection task generally improved in the first 2 days, but there were substantial individual differences in detection ability. A positive correlation was observed between performance on these tasks and visual working memory capacity, except on the second day, after detection ability had improved. These findings suggest that factors contributing to echolocation skill are related to nonauditory factors in a sighted group.

Impact of years of blindness on neural circuits underlying auditory spatial representation

Early visual deprivation impacts negatively on spatial bisection abilities. Recently, an early (50-90 ms) ERP response, selective for sound position in space, has been observed in the visual cortex of sighted individuals during the spatial but not the temporal bisection task. Here, we clarify the role of vision on spatial bisection abilities and neural correlates by studying late blind individuals. Results highlight that a shorter period of blindness is linked to a stronger contralateral activation in the visual cortex and a better performance during the spatial bisection task. Contrarily, not lateralized visual activation and lower performance are observed in individuals with a longer period of blindness. To conclude, the amount of time spent without vision may gradually impact on neural circuits underlying the construction of spatial representations in late blind participants. These findings suggest a key relationship between visual deprivation and auditory spatial abilities in humans.

Mental Imagery Follows Similar Cortical Reorganization as Perception: Intra-Modal and Cross-Modal Plasticity in Congenitally Blind

Cortical plasticity in congenitally blind individuals leads to cross-modal activation of the visual cortex and may lead to superior perceptual processing in the intact sensory domains. Although mental imagery is often defined as a quasi-perceptual experience, it is unknown whether it follows similar cortical reorganization as perception in blind individuals. In this study, we show that auditory versus tactile perception evokes similar intra-modal discriminative patterns in congenitally blind compared with sighted participants. These results indicate that cortical plasticity following visual deprivation does not influence broad intra-modal organization of auditory and tactile perception as measured by our task. Furthermore, not only the blind, but also the sighted participants showed cross-modal discriminative patterns for perception modality in the visual cortex. During mental imagery, both groups showed similar decoding accuracies for imagery modality in the intra-modal primary sensory cortices. However, no cross-modal discriminative information for imagery modality was found in early visual cortex of blind participants, in contrast to the sighted participants. We did find evidence of cross-modal activation of higher visual areas in blind participants, including the representation of specific-imagined auditory features in visual area V4.

Aging Effect on Audiovisual Integrative Processing in Spatial Discrimination Task

Multisensory integration is an essential process that people employ daily, from conversing in social gatherings to navigating the nearby environment. The aim of this study was to investigate the impact of aging on modulating multisensory integrative processes using event-related potential (ERP), and the validity of the study was improved by including “noise” in the contrast conditions. Older and younger participants were involved in perceiving visual and/or auditory stimuli that contained spatial information. The participants responded by indicating the spatial direction (far vs. near and left vs. right) conveyed in the stimuli using different wrist movements. electroencephalograms (EEGs) were captured in each task trial, along with the accuracy and reaction time of the participants’ motor responses. Older participants showed a greater extent of behavioral improvements in the multisensory (as opposed to unisensory) condition compared to their younger counterparts. Older participants were found to have fronto-centrally distributed super-additive P2, which was not the case for the younger participants. The P2 amplitude difference between the multisensory condition and the sum of the unisensory conditions was found to correlate significantly with performance on spatial discrimination. The results indicated that the age-related effect modulated the integrative process in the perceptual and feedback stages, particularly the evaluation of auditory stimuli. Audiovisual (AV) integration may also serve a functional role during spatial-discrimination processes to compensate for the compromised attention function caused by aging.

Structural neuroplasticity of the superior temporal plane in early and late blindness

Blindness is associated with well-documented changes to the morphometry and function of the occipital cortex. By comparison, its impact on the perisylvian regions in the superior temporal plane (STP) is poorly understood, with many studies reporting null findings on this issue. Here we re-approach this question using a morphometric analysis that relied on fine-scale, manual annotation of 13 sub-regions within the STP and that quantified both univariate and multivariate differences in morphometry. We applied these analyses to both cortical thickness (CT) and surface area (SA) data from congenitally and late blind, as compared to two matched sighted control groups. The univariate analyses indicated that for CT, no region differentiated blind from sighted, and for SA, two regions showed lower values for congenitally blind. Moreover, the multivariate analyses identified more robust signatures of plasticity in blindness. Specifically, pairwise regional correlations of CT values between contralateral regions were significantly higher for both blind groups as compared to sighted controls. A similar pattern for SA data was found for congenitally blind alone. Our findings indicate that blindness strongly impacts STP, resulting in a more coordinated pattern of interhemispheric morphometric development. We discuss implications for theories of language plasticity and models of neuroplasticity in the blind.

Prior Visual Experience Modulates Learning of Sound Localization Among Blind Individuals

Cross-modal learning requires the use of information from different sensory modalities. This study investigated how the prior visual experience of late blind individuals could modulate neural processes associated with learning of sound localization. Learning was realized by standardized training on sound localization processing, and experience was investigated by comparing brain activations elicited from a sound localization task in individuals with (late blind, LB) and without (early blind, EB) prior visual experience. After the training, EB showed decreased activation in the precuneus, which was functionally connected to a limbic-multisensory network. In contrast, LB showed the increased activation of the precuneus. A subgroup of LB participants who demonstrated higher visuospatial working memory capabilities (LB-HVM) exhibited an enhanced precuneus-lingual gyrus network. This differential connectivity suggests that visuospatial working memory due to the prior visual experience gained via LB-HVM enhanced learning of sound localization. Active visuospatial navigation processes could have occurred in LB-HVM compared to the retrieval of previously bound information from long-term memory for EB. The precuneus appears to play a crucial role in learning of sound localization, disregarding prior visual experience. Prior visual experience, however, could enhance cross-modal learning by extending binding to the integration of unprocessed information, mediated by the cognitive functions that these experiences develop.

Auditory distance perception in humans: a review of cues, development, neuronal bases, and effects of sensory loss

Auditory distance perception plays a major role in spatial awareness, enabling location of objects and avoidance of obstacles in the environment. However, it remains under-researched relative to studies of the directional aspect of sound localization. This review focuses on the following four aspects of auditory distance perception: cue processing, development, consequences of visual and auditory loss, and neurological bases. The several auditory distance cues vary in their effective ranges in peripersonal and extrapersonal space. The primary cues are sound level, reverberation, and frequency. Nonperceptual factors, including the importance of the auditory event to the listener, also can affect perceived distance. Basic internal representations of auditory distance emerge at approximately 6 months of age in humans. Although visual information plays an important role in calibrating auditory space, sensorimotor contingencies can be used for calibration when vision is unavailable. Blind individuals often manifest supranormal abilities to judge relative distance but show a deficit in absolute distance judgments. Following hearing loss, the use of auditory level as a distance cue remains robust, while the reverberation cue becomes less effective. Previous studies have not found evidence that hearing-aid processing affects perceived auditory distance. Studies investigating the brain areas involved in processing different acoustic distance cues are described. Finally, suggestions are given for further research on auditory distance perception, including broader investigation of how background noise and multiple sound sources affect perceived auditory distance for those with sensory loss.

Successful tactile based visual sensory substitution use functions independently of visual pathway integrity

PURPOSE

Neuronal reorganization after blindness is of critical interest because it has implications for the rational prescription of artificial vision devices. The purpose of this study was to distinguish the microstructural differences between perinatally blind (PB), acquired blind (AB), and normally sighted controls (SCs) and relate these differences to performance on functional tasks using a sensory substitution device (BrainPort).

METHODS

We enrolled 52 subjects (PB n = 11; AB n = 35; SC n = 6). All subjects spent 15 h undergoing BrainPort device training. Outcomes of light perception, motion, direction, temporal resolution, grating, and acuity were tested at baseline and after training. Twenty-six of the subjects were scanned with a three Tesla MRI scanner for diffusion tensor imaging (DTI), and with a positron emission tomography (PET) scanner for mapping regional brain glucose consumption during sensory substitution function. Non-parametric models were used to analyze fractional anisotropy (FA; a DTI measure of microstructural integrity) of the brain via region-of-interest (ROI) analysis and tract-based spatial statistics (TBSS).

RESULTS

At baseline, all subjects performed all tasks at chance level. After training, light perception, time resolution, location and grating acuity tasks improved significantly for all subject groups. ROI and TBSS analyses of FA maps show areas of statistically significant differences (p ≤ 0.025) in the bilateral optic radiations and some visual association connections between all three groups. No relationship was found between FA and functional performance with the BrainPort.

DISCUSSION

All subjects showed performance improvements using the BrainPort irrespective of nature and duration of blindness. Definite brain areas with significant microstructural integrity changes exist among PB, AB, and NC, and these variations are most pronounced in the visual pathways. However, the use of sensory substitution devices is feasible irrespective of microstructural integrity of the primary visual pathways between the eye and the brain. Therefore, tongue based devices devices may be usable for a broad array of non-sighted patients.