Early Blind Subjects Research Articles

The Impact of Spectral and Temporal Degradation on Vocoded Speech Recognition in Early-Blind Individuals.

This study compared the impact of spectral and temporal degradation on vocoded speech recognition between early-blind and sighted subjects. The participants included 25 early-blind subjects (30.32 ± 4.88 years; male:female, 14:11) and 25 age- and sex-matched sighted subjects. Tests included monosyllable recognition in noise at various signal-to-noise ratios (-18 to -4 dB), matrix sentence-in-noise recognition, and vocoded speech recognition with different numbers of channels (4, 8, 16, and 32) and temporal envelope cutoff frequencies (50 vs 500 Hz). Cortical-evoked potentials (N2 and P3b) were measured in response to spectrally and temporally degraded stimuli. The early-blind subjects displayed superior monosyllable and sentence recognition than sighted subjects (all p < 0.01). In the vocoded speech recognition test, a three-way repeated-measure analysis of variance (two groups × four channels × two cutoff frequencies) revealed significant main effects of group, channel, and cutoff frequency (all p < 0.001). Early-blind subjects showed increased sensitivity to spectral degradation for speech recognition, evident in the significant interaction between group and channel (p = 0.007). N2 responses in early-blind subjects exhibited shorter latency and greater amplitude in the 8-channel (p = 0.022 and 0.034, respectively) and shorter latency in the 16-channel (p = 0.049) compared with sighted subjects. In conclusion, early-blind subjects demonstrated speech recognition advantages over sighted subjects, even in the presence of spectral and temporal degradation. Spectral degradation had a greater impact on speech recognition in early-blind subjects, while the effect of temporal degradation was similar in both groups.

Brain structural plasticity in rats subjected to early binocular enucleation characterized by high resolution anatomical magnetic resonance imaging and diffusion tensor imaging

Visual deprivation leads to structural neuroplasticity in the blind subjects, including gray matter (GM) and white matter (WM) atrophy and alterations in structural connectivity. The rat model of binocular enucleation (BE) is a frequently used animal model for studying brain plasticity induced by early blindness. Yet few neuroimaging studies have been performed on this model to investigate whether or not the BE rats have image phenotypes similar to or comparable to, those observed in the early blind subjects. The current study aimed to assess brain structural plasticity in BE rats using anatomical magnetic resonance imaging (MRI) and diffusion tensor imaging (DTI). The results demonstrated that early BE at postnatal day 4 (P4) caused almost complete degeneration of optic nerve (ON) and optic chiasma (OCH), atrophy in a number of visual and non-visual structures, including optic tract (OT), dorsal lateral geniculate nucleus (DLG) and corpus callosum (CC). The BE rats also exhibited impairments of WM microstructural integrity in the OT, and reduction of structural connectivity between the normal-appearing visual cortex (VC) and somatosensory/motor cortices at 4 months of age, likely as manifestations of deafferentation-induced maldevelopment. The structural neuroplasticity in BE rats observable to structural MRI parallels largely with what has been reported in blind subjects, suggesting that longitudinal neuroimaging studies on animal models of sensory deprivation can provide insights into how the brain changes its wiring and function during development/adaption in response to the lack of sensory stimuli.

Enhanced Dichotic Listening and Temporal Sequencing Ability in Early-Blind Individuals.

Several studies have reported the better auditory performance of early-blind subjects over sighted subjects. However, few studies have compared the auditory functions of both hemispheres or evaluated interhemispheric transfer and binaural integration in blind individuals. Therefore, we evaluated whether there are differences in dichotic listening, auditory temporal sequencing ability, or speech perception in noise (all of which have been used to diagnose central auditory processing disorder) between early-blind subjects and sighted subjects. The study included 23 early-blind subjects and 22 age-matched sighted subjects. In the dichotic listening test (three-digit pair), the early-blind subjects achieved higher scores than the sighted subjects in the left ear (p = 0.003, Bonferroni’s corrected α = 0.05/6 = 0.008), but not in the right ear, indicating a right ear advantage in sighted subjects (p < 0.001) but not in early-blind subjects. In the frequency patterning test (five tones), the early-blind subjects performed better (both ears in the humming response, but the left ear only in the labeling response) than the sighted subjects (p < 0.008, Bonferroni’s corrected α = 0.05/6 = 0.008). Monosyllable perception in noise tended to be better in early-blind subjects than in sighted subjects at a signal-to-noise ratio of –8 (p = 0.054), the results at signal-to-noise ratios of –4, 0, +4, and +8 did not differ. Acoustic change complex responses to/ba/in babble noise, recorded with electroencephalography, showed a greater N1 peak amplitude at only FC5 electrode under a signal-to-noise ratio of –8 and –4 dB in the early-blind subjects than in the sighted subjects (p = 0.004 and p = 0.003, respectively, Bonferroni’s corrected α = 0.05/5 = 0.01). The results of this study revealed early-blind subjects exhibited some advantages in dichotic listening, and temporal sequencing ability compared to those shown in sighted subjects. These advantages may be attributable to the enhanced activity of the central auditory nervous system, especially the right hemisphere function, and the transfer of auditory information between the two hemispheres.

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.

Visual Cortex Alterations in Early and Late Blind Subjects During Tactile Perception

Involvement of visual cortex varies during tactile perception tasks in early blind (EB) and late blind (LB) human subjects. This study explored differences in sensory motor networks associated with tactile task in EB and LB subjects and between children and adolescents. A total of 40 EB subjects, 40 LB subjects, and 30 sighted controls were recruited in two subgroups: children (6-12 years) and adolescents (13-19 years). Data were acquired using a 3T MR scanner. Analyses of blood oxygen level dependent (BOLD), functional connectivity (FC), correlation, and post hoc test for multiple comparisons were carried out. Difference in BOLD activity was observed in EB and LB groups in visual cortex during tactile perception, with increased FC of visual with dorsal attention and sensory motor networks in EB. EB adolescents exhibited increased connectivity with default mode and salience networks when compared with LB. Functional results correlated with duration of training, suggestive of better performance in EB. Alteration in sensory and visual networks in EB and LB correlated with duration of tactile training. Age of onset of blindness has an effect in cross-modal reorganization of visual cortex in EB and multimodal in LB in children and adolescents.

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.

Reduced Dynamic Interactions Within Intrinsic Functional Brain Networks in Early Blind Patients.

Neuroimaging studies in early blind (EB) patients have shown altered connections or brain networks. However, it remains unclear how the causal relationships are disrupted within intrinsic brain networks. In our study, we used spectral dynamic causal modeling (DCM) to estimate the causal interactions using resting-state data in a group of 20 EB patients and 20 healthy controls (HC). Coupling parameters in specific regions were estimated, including the medial prefrontal cortex (mPFC), posterior cingulate cortex (PCC), and inferior parietal lobule (IPC) in the default mode network (DMN); dorsal anterior cingulate cortex (dACC) and bilateral anterior insulae (AI) in the salience network (SN), and bilateral frontal eye fields (FEF) and superior parietal lobes (SPL) within the dorsal attention network (DAN). Statistical analyses found that all endogenous connections and the connections from the mPFC to bilateral IPCs in EB patients were significantly reduced within the DMN, and the effective connectivity from the PCC and lIPC to the mPFC, and from the mPFC to the PCC were enhanced. For the SN, all significant connections in EB patients were significantly decreased, except the intrinsic right AI connections. Within the DAN, more significant effective connections were observed to be reduced between the EB and HC groups, while only the connections from the right SPL to the left SPL and the intrinsic connection in the left SPL were significantly enhanced. Furthermore, discovery of more decreased effective connections in the EB subjects suggested that the disrupted causal interactions between specific regions are responsive to the compensatory brain plasticity in early deprivation.

Commentary: Cortical Plasticity and Olfactory Function in Early Blindness.

Humans beings are visual creatures, which means that they preferentially map the world guided largely by sight (Aglioti and Pazzaglia, 2010). However, multiple senses contribute to the creation of an integrated experience of the real world (Aglioti and Pazzaglia, 2011). Among these, the olfactory sense is undoubtedly the most neglected. One of the reasons for this is that odors do not always prevail at a conscious level (Sobel et al., 1999), even though olfactory receptors are continuously active in order to acquire sensory, motor and affective information (Pazzaglia, 2015). A second reason is the fact that sight occasionally prevails over, or even hides, the information provided by other senses. In a recent stimulating and timely article, Araneda and colleagues presented an important view of the pathophysiological mechanisms of plasticity that occur in the visual brain of early blind patients (Araneda et al., 2016). The authors produced a behavioral and anatomo-physiological perspective on the guiding role of olfactory information when sight is lacking. Behaviorally, early blind subjects perform better than sighted individuals when it comes to odor detection and awareness, indicating that the sense of smell is highly plastic and suggesting the possibility of incremental odor processing by way of learning and experience. Neurally, active olfactory processing occurs within the occipital cortex, suggesting the privileged access of odor to other, seemingly unrelated, sensory systems and neural regions.

Cortical thickness development of human primary visual cortex related to the age of blindness onset.

Blindness primarily induces structural alteration in the primary visual cortex (V1). Some studies have found that the early blind subjects had a thicker V1 compared to sighted controls, whereas late blind subjects showed no significant differences in the V1. This implies that the age of blindness onset may exert significant effects on the development of cortical thickness of the V1. However, no previous research used a trajectory of the age of blindness onset-related changes to investigate these effects. Here we explored this issue by mapping the cortical thickness trajectory of the V1 against the age of blindness onset using data from 99 blind individuals whose age of blindness onset ranged from birth to 34years. We found that the cortical thickness of the V1 could be fitted well with a quadratic curve in both the left (F=11.59, P=3×10-5) and right hemispheres (F=6.54, P=2×10-3). Specifically, the cortical thickness of the V1 thinned rapidly during childhood and adolescence and did not change significantly thereafter. This trend was not observed in the primary auditory cortex (A1), primary motor cortex (M1), or primary somatosensory cortex (S1). These results provide evidence that an onset of blindness before adulthood significantly affects the cortical thickness of the V1 and suggest a critical period for cortical development of the human V1.

Early Blindness Results in Developmental Plasticity for Auditory Motion Processing within Auditory and Occipital Cortex.

Early blind subjects exhibit superior abilities for processing auditory motion, which are accompanied by enhanced BOLD responses to auditory motion within hMT+ and reduced responses within right planum temporale (rPT). Here, by comparing BOLD responses to auditory motion in hMT+ and rPT within sighted controls, early blind, late blind, and sight-recovery individuals, we were able to separately examine the effects of developmental and adult visual deprivation on cortical plasticity within these two areas. We find that both the enhanced auditory motion responses in hMT+ and the reduced functionality in rPT are driven by the absence of visual experience early in life; neither loss nor recovery of vision later in life had a discernable influence on plasticity within these areas. Cortical plasticity as a result of blindness has generally be presumed to be mediated by competition across modalities within a given cortical region. The reduced functionality within rPT as a result of early visual loss implicates an additional mechanism for cross modal plasticity as a result of early blindness—competition across different cortical areas for functional role.

FMRI correlates of different components of Braille reading by the blind

Brain activations in early-blind subjects during Braille reading indicate considerable cross-modal neuroplasticity in posterior brain areas. Up to now it is, however, not clear how far such neuroplastic reorganization processes reach and whether a specific brain activation pattern corresponds to a specific component of the Braille reading task. Therefore, this fMRI study investigates whether different cortical areas are functionally specialized for different aspects of Braille reading. The comprehensive Braille reading task was contrasted to three control tasks representing subcomponents of Braille reading (passive tactile stimulation, active tactile pattern recognition, Braille imagery). Results in 14 early-blind subjects indicate that only occipital and basal temporo-occipital brain areas show specific fMRI correlates for Braille reading. Central rolandic brain activations correlated with basic somatosensory processing and superior temporal activations were associated with higher level stimulus-independent language processing. In these latter areas no indications for neuroplastic reorganizations specific for Braille reading were found, despite strong activations during the Braille reading task.

Auditory motion processing after early blindness.

Studies showing that occipital cortex responds to auditory and tactile stimuli after early blindness are often interpreted as demonstrating that early blind subjects "see" auditory and tactile stimuli. However, it is not clear whether these occipital responses directly mediate the perception of auditory/tactile stimuli, or simply modulate or augment responses within other sensory areas. We used fMRI pattern classification to categorize the perceived direction of motion for both coherent and ambiguous auditory motion stimuli. In sighted individuals, perceived motion direction was accurately categorized based on neural responses within the planum temporale (PT) and right lateral occipital cortex (LOC). Within early blind individuals, auditory motion decisions for both stimuli were successfully categorized from responses within the human middle temporal complex (hMT+), but not the PT or right LOC. These findings suggest that early blind responses within hMT+ are associated with the perception of auditory motion, and that these responses in hMT+ may usurp some of the functions of nondeprived PT. Thus, our results provide further evidence that blind individuals do indeed "see" auditory motion.

Enhanced cortical representation of auditory frequency as a result of early blindness

A fundamental ability found to be enhanced by early blindness is frequency discrimination; however, the neural basis of this enhancement is unclear. Responses to a variety of auditory stimuli have been found throughout occipital cortex, but clear evidence for frequency tuning has not been reported. In auditory cortex, attenuated responses to pure tones have been found in early blind and anophthalmic individuals (Stevens and Weaver, 2009; Watkins et al., 2013). Here, we measured responses to pure tones in 5 early-blind and 5 age-matched sighted subjects to assess group differences in the organization of auditory frequency representations within occipital and auditory cortex. Methods: Stimuli consisted of pure tones ranging from 88 to 8000 Hz, which were presented in partially randomized order while subjects performed a 1-back frequency matching task. Data were analyzed using an adaptation of the population receptive field technique (Dumoulin and Wandell, 2008). We modeled the aggregate receptive field underlying each voxel's response as a one-dimensional Gaussian function of frequency, with a center and standard deviation that reflect preferred frequency and tuning bandwidth, respectively. Results: Within auditory cortex, we could reliably measure individual tonotopic maps in both subject groups, and we saw no evidence for an attenuation of responses. Relative to controls, early-blind subjects showed an enhanced representation of low to middle frequencies (<4 kHz) and narrower tuning bandwidths. In occipital cortex, blind but not sighted subjects showed evidence of auditory frequency tuning. Frequency tuned voxels in occipital cortex favored the mid-frequency range (1-8 kHz) and had narrower bandwidths then those found in auditory cortex. Conclusions: Early blindness results in an increased cortical representation of behaviorally relevant frequencies within both auditory and occipital cortex. Future work will examine how responses in these cortical areas relate to frequency discrimination performance in individual subjects. Meeting abstract presented at VSS 2014

Improved beat asynchrony detection in early blind individuals.

Although early blind (EB) individuals are thought to have a better musical sense than sighted subjects, no study has investigated the musical rhythm and beat processing abilities in EB individuals. Using an adaptive 'up and down' procedure, we measured the beat asynchrony detection threshold and the duration discrimination threshold, in the auditory and vibrotactile modalities in both EB and sighted control (SC) subjects matched for age, gender, and musical experience. We observed that EB subjects were better than SC in the beat asynchrony detection task; that is, they showed lower thresholds than SC, both in the auditory and in the vibrotactile modalities. In addition, EB subjects had a lower threshold than SC for duration discrimination in the vibrotactile modality only. These improved beat asynchrony detection abilities may contribute to the known excellent musical abilities often observed in many blind subjects.

Visual callosal topography in the absence of retinal input

Using probabilistic diffusion tractography, we examined the retinotopic organization of splenial callosal connections within early blind, anophthalmic, and control subjects. Early blind subjects experienced prenatal retinal “waves” of spontaneous activity similar to those of sighted subjects, and only lack postnatal visual experience. In anophthalmia, the eye is either absent or arrested at an early prenatal stage, depriving these subjects of both pre- and postnatal visual input. Therefore, comparing these two groups provides a way of separating the influence of pre- and postnatal retinal input on the organization of visual connections across hemispheres. We found that retinotopic mapping within the splenium was not measurably disrupted in early blind or anophthalmic subjects compared to visually normal controls. No significant differences in splenial volume were observed across groups. No significant differences in diffusivity were found between early blind subjects and sighted controls, though some differences in diffusivity were noted between anophthalmic subjects and controls. These results suggest that neither prenatal retinal activity nor postnatal visual experience plays a role in the large-scale topographic organization of visual callosal connections within the splenium.

Perceiving Space and Optical Cues via a Visuo-Tactile Sensory Substitution System: A Methodological Approach for Training of Blind Subjects for Navigation

A methodological approach to perceptual learning was used to allow both early blind subjects (experimental group) and blindfolded sighted subjects (control group) to experience optical information and spatial phenomena, on the basis of visuo-tactile information transmitted by a 64-taxel pneumatic sensory substitution device. The learning process allowed the subjects to develop abilities in spatial localisation, shape recognition (with generalisation to different points of view), and monocular depth cue interpretation. During the training phase, early blind people initially experienced more difficulties than blindfolded sighted subjects (having previous perceptual experience of perspective) with interpreting and using monocular depth cues. The amelioration of the performance for all blind subjects during training sessions and the quite similar level of performance reached by two groups in the final navigation tasks suggested that early blind people were able to develop and apply cognitive understanding of depth cues. Both groups showed generalisation of the learning from the initial phases to cue identification in the maze, and subjectively experienced shapes facing them. Subjects' performance depended not only on their perceptual experience but also on their previous spatial competencies.

Early visual experience and the recognition of basic facial expressions: involvement of the middle temporal and inferior frontal gyri during haptic identification by the early blind.

Face perception is critical for social communication. Given its fundamental importance in the course of evolution, the innate neural mechanisms can anticipate the computations necessary for representing faces. However, the effect of visual deprivation on the formation of neural mechanisms that underlie face perception is largely unknown. We previously showed that sighted individuals can recognize basic facial expressions by haptics surprisingly well. Moreover, the inferior frontal gyrus (IFG) and posterior superior temporal sulcus (pSTS) in the sighted subjects are involved in haptic and visual recognition of facial expressions. Here, we conducted both psychophysical and functional magnetic-resonance imaging (fMRI) experiments to determine the nature of the neural representation that subserves the recognition of basic facial expressions in early blind individuals. In a psychophysical experiment, both early blind and sighted subjects haptically identified basic facial expressions at levels well above chance. In the subsequent fMRI experiment, both groups haptically identified facial expressions and shoe types (control). The sighted subjects then completed the same task visually. Within brain regions activated by the visual and haptic identification of facial expressions (relative to that of shoes) in the sighted group, corresponding haptic identification in the early blind activated regions in the inferior frontal and middle temporal gyri. These results suggest that the neural system that underlies the recognition of basic facial expressions develops supramodally even in the absence of early visual experience.

Age of Onset of Blindness Affects Brain Anatomical Networks Constructed Using Diffusion Tensor Tractography

Studying blindness with various onset ages may elucidate the ways that unimodal sensory deprivation at different periods of development shape the human brain. In order to determine the effect of the onset age on brain anatomical networks, we extended a previous study of 17 early blind (EB) subjects with an additional 97 subjects with various onset ages. We constructed binary anatomical networks of these subjects and sighted controls (SC) using diffusion tensor tractography and calculated the topological properties of the network. Based on onset age, the subjects were divided into congenitally blind (CB), EB, adolescent-blind (AB), and late-blind (LB) subgroups. The LB subjects demonstrated a greater connectivity density and a higher global efficiency, similar to the SC. The CB and EB subgroups showed large group differences from the other groups in their topological networks, specifically, a reduced connectivity density and a decreased global efficiency compared with the SC, especially in the frontal and occipital cortices. Additionally, significant correlations were found between age of onset and the topological properties of the anatomical network in the blind. Our results suggest that visual experience during an early period of development is critical for establishing an intact efficient anatomical network in the human brain.

The spatial distribution of auditory attention in early blindness

Early blind people compensate for their lack of vision by developing superior abilities in the remaining senses such as audition (Collignon et al., 2006; Gougoux et al., 2004; Wan et al., 2010). Previous studies reported supra-normal abilities in auditory spatial attention, particularly for the localization of peripheral stimuli in comparison with frontal stimuli (Lessard et al., 1998; Röder et al., 1999). However, it is unknown whether this specific supra-normal ability extends to the non-spatial attention domain. Here we compared the performance of early blind subjects and sighted controls, who were blindfolded, during an auditory non-spatial attention task: target detection among distractors according to tone frequency. We paid a special attention to the potential effect of the sound source location, comparing the accuracy and speed in target detection in the peripheral and frontal space. Blind subjects displayed shorter reaction times than sighted controls for both peripheral and frontal stimuli. Moreover, in the two groups of subjects, we observed an interaction effect between the target location and the distractors location: the target was detected faster when its location was different from the location of the distractors. However, this effect was attenuated in early blind subjects and even cancelled in the condition with frontal targets and peripheral distractors. We conclude that early blind people compensate for the lack of vision by enhancing their ability to process auditory information but also by changing the spatial distribution of their auditory attention resources.

Comparison of amplitude and latency of cognitive potential (P3)with high and low frequency stimuli in early and late onset blind subjects

The aim of this study was to study P3 with high and low frequency stimulus in blind subjects to study neuroplasticity. The P3 was recorded in 70 dBnHL. Comparison of results between two groups showed that there were no statistically significant differences between amplitudes and latencies of P3 with high frequency stimuli. However, in low frequency the difference between amplitudes was significant. Neuroplasticity seemed to occur in visual cortex in both groups. The activation patterns of the occipital cortex varied between these two groups and this could be demonstrated with low frequency stimuli better than with high frequency stimuli.