Blind Humans Research Articles

The extent of visual deficit and auditory spatial compensation: evidence from self-positioning from auditory cues

Blindfolded sighted, myopic, amblyopic, adventitiously blind and congenitally blind humans performed a self-positioning task during which they were stimulated only by auditory cues. Results showed that visually deprived subjects used auditory cues to position themselves in their environment with a greater accuracy than normal-sighted subjects. In addition, the magnitude of auditory spatial compensation was found to be strongly related to the extent of the visual deficit.

MEMS technologies for epiretinal stimulation of the retina

It has been shown that electrical stimulation of retinal ganglion cells yields visual sensations. Therefore, a retina implant for blind humans suffering from retinitis pigmentosa based on this concept seems to be feasible. In Germany, there are two projects funded by the government working on different approaches namely the subretinal and the epiretinal approaches. This paper describes the epiretinal approach for such a system. The extraocular part of this system records visual images. The images are transformed by a neural net into corresponding signals for stimulation of the retinal ganglion cells. These signals are transmitted to a receiver unit of an intraocular implant, the retina stimulator. Integrated circuitry of this unit decodes the signals and transfers the data to a stimulation circuitry that selects stimulation electrodes placed onto the retina and generates current pulses to the electrodes. By this, action potentials in retinal ganglion cells are evoked, causing a visual sensation. This paper concentrates on the MEMS part of this implant.

Blind subjects process auditory spectral cues more efficiently than sighted individuals.

The goal of the present study was to investigate how monaural sound localization on the horizontal plane in blind humans is affected by manipulating spectral cues. As reported in a previous study (Lessard et al. 1998), blind subjects are able to calibrate their auditory space despite their congenital lack of vision. Moreover, the performance level of half of the blind subjects was superior to that of sighted subjects under monaural listening conditions. Here, we first tested ten blind subjects and five controls in free-field (1) binaural and (2) monaural sound localization tasks. Results showed that, contrary to controls and half the blind subjects, five of the blind listeners were able to localize the sounds with one ear blocked. The blind subjects who showed good monaural localization performances were then re-tested in three additional monaural tasks, but we manipulated their ability to use spectral cues to carry out their discrimination. These subjects thus localized these same sounds: (3) with acoustical paste on the pinna, (4) with high-pass sounds and unobstructed pinna and (5) with low-pass sounds and unobstructed pinna. A significant increase in localization errors was observed when their ability to use spectral cues was altered. We conclude that one of the reasons why some blind subjects show supra-normal performances might be that they more effectively utilize auditory spectral cues.

Cross-modal plasticity and deafferentation

To interact successfully with the environment and to compensate for environmental challenges, the human brain must integrate information originating in different sensory modalities. Such integration occurs in non-primary associative regions of the human brain. Additionally, recent investigations have documented the involvement of the primary visual cortex in processing tactile information in blind humans to a larger extent than in sighted controls. This form of cross-modal plasticity highlights the capacity of the human central nervous system to reorganize after chronic visual deprivation.

A118 手の静的・動的知覚の計測 : 視覚障害者と健常者の比較(A1-4 生体計測)

A118 手の静的・動的知覚の計測 : 視覚障害者と健常者の比較(A1-4 生体計測)

Occipito-parietal cortex activation during visuo-spatial imagery in early blind humans

Using positron emission tomography, regional cerebral blood flow was studied in five early blind and five control volunteers during visuo-spatial imagery. Subjects were instructed to generate a mental representation of verbally provided bidimensional patterns that were placed in a grid and to assess pattern symmetry in relation to a grid axis. This condition was contrasted with a verbal memory task. Cerebral activation in both groups was similar during the visuo-spatial imagery task. It involved the precuneus (BA 7), superior parietal lobule (BA 7), and occipital gyrus (BA 19). These results are in accordance with previous studies conducted in sighted subjects that indicated that the same occipito-parietal areas are involved in visual perception as well as in mental imagery dealing with spatial components. The dorsal pathway seems to be involved in visuo-spatial imagery in early blind subjects, indicating that this pathway undergoes development in the absence of vision.

Speech processing activates visual cortex in congenitally blind humans.

Neurophysiological recordings and neuroimaging data in blind and deaf animals and humans suggest that perceptual functions may be organized differently after sensory deprivation. It has been argued that neural plasticity contributes to compensatory performance in blind humans, such as faster speech processing. The present study employed functional magnetic resonance imaging (fMRI) to map language-related brain activity in congenitally blind adults. Participants listened to sentences, with either an easy or a more difficult syntactic structure, which were either semantically meaningful or meaningless. Results show that blind adults not only activate classical left-hemispheric perisylvian language areas during speech comprehension, as did a group of sighted adults, but that they additionally display an activation in the homologueous right-hemispheric structures and in extrastriate and striate cortex. Both the perisylvian and occipital activity varied as a function of syntactic difficulty and semantic content. The results demonstrate that the cerebral organization of complex cognitive systems such as the language system is significantly shaped by the input available.

Cross-modal plasticity: where and how?

Animal studies have shown that sensory deprivation in one modality can have striking effects on the development of the remaining modalities. Although recent studies of deaf and blind humans have also provided convincing behavioural, electrophysiological and neuroimaging evidence of increased capabilities and altered organization of spared modalities, there is still much debate about the identity of the brain systems that are changed and the mechanisms that mediate these changes. Plastic changes across brain systems and related behaviours vary as a function of the timing and the nature of changes in experience. This specificity must be understood in the context of differences in the maturation rates and timing of the associated critical periods, differences in patterns of transiently existing connections, and differences in molecular factors across brain systems.

Critical Period for Cross-Modal Plasticity in Blind Humans: A Functional MRI Study

The primary visual cortex (V1) in congenitally blind humans has been shown to be involved in tactile discrimination tasks, indicating that there is a shift in function of this area of cortex, but the age dependency of the reorganization is not fully known. To investigate the reorganized network, we measured the change of regional cerebral blood flow using 3.0 Tesla functional MRI during passive tactile tasks performed by 15 blind and 8 sighted subjects. There was increased activity in the postcentral gyrus to posterior parietal cortex and decreased activity in the secondary somatosensory area in blind compared with sighted subjects during a tactile discrimination task. This suggests that there is a greater demand for shape discrimination processing in blind subjects. Blind subjects, irrespective of the age at onset of blindness, exhibited higher activity in the visual association cortex than did sighted subjects. V1 was activated in blind subjects who lost their sight before 16 years of age, whereas it was suppressed in blind subjects who lost their sight after 16 years of age during a tactile discrimination task. This suggests that the first 16 years of life represent a critical period for a functional shift of V1 from processing visual stimuli to processing tactile stimuli. Because of the age-dependency, V1 is unlikely to be the “entry node” of the cortex for the redirection of tactile signals into visual cortices after blinding. Instead, the visual association cortex may mediate the circuitry by which V1 is activated during tactile stimulation.

Opposing effects of head position on sound localization in blind and sighted human subjects.

Up to now, there is an unsolved contradiction between the view that the development of an auditory spatial representation needs calibration by vision and the psychophysical demonstration of quite precise sound localization in early blind humans. The present study provides a link between these two competing conceptions. Two experiments were conducted with congenitally or early blind subjects and sighted controls. In the first experiment, subjects pointed with their head to actual sound sources located in the azimuthal plane. In the second experiment, lateralization of dichotic sound stimuli, presented via headphones, was investigated with variation of head-to-trunk position. The results showed opposing systematic errors of sound localization or lateralization, depending on head position, made by blind and sighted subjects. These differences suggest that audiomotor feedback replaces vision so as to calibrate auditory space in blind individuals. That is, in contrast to the widespread opinion of compensation of visual loss by a general sharpening of audition, compensatory plasticity in the blind may specifically be related to enhanced processing of proprioceptive and vestibular information with the auditory spatial input.

Vertical sound localization in blind humans

It is widely held that early-blind people compensate their visual loss by a general sharpening of spatial hearing. The present study reports a possible exception to this view: when the vertical position (elevation) of a sound source had to be localized, four out of six early-blind subjects exhibited systematic deviations in pointing, while two early-blind subjects were as accurate as sighted controls. On the other hand, blind and sighted individuals were able to judge relative positions of different sound locations with similar precision. These results suggest that visual experience may be used to accurately calibrate the relation between the vertical coordinates of auditory space and body, but is not needed to develop sufficiently high resolution of spatial hearing.

Two-dimensional sound-localization behavior of early-blind humans.

To investigate whether the visual system is crucial for adequate calibration of acoustic localization cues, sound-localization performance of early blind humans was compared with that of sighted controls. Because a potential benefit of vision is mainly expected for targets within the two-dimensional (2D) frontal hemifield, localization was tested within this target range, while using sounds of various durations and spectral content. Subjects were instructed to point, in separate experimental sessions, either with their left arm, or with their nose, in the direction of the perceived target position as accurately as possible. The experiments required the use of all available sound-localization cues such as interaural differences in phase and intensity, as well as the complex spectral shape cues provided by the pinnae. In addition, for long-duration stimuli, subjects could have had access to head motion-induced acoustic feedback. Moreover, the two pointing methods allowed us to assess different response strategies for the two groups. In an additional series, subjects were instructed to respond as quickly as possible. The results show that, in general, 2D sound-localization performance of blind subjects was indistinguishable from that of sighted subjects, both for broad-band noise and for pure tones. In the fast head-pointing task, the latency distributions of both groups were equal. These findings suggest that visual feedback is not required to calibrate the available localization cues--including the idiosyncratic and complex spectral shape cues for elevation. Instead, the localization abilities of blind people show that the putative supervising role of vision may be supported, or taken over, by other non-visual feedback systems. The results do not provide support for the hypothesis that blind people can hypercompensate for the loss of vision in the frontal hemifield by developing superior sound-localization abilities. Despite the general correspondence in localization behavior, some specific differences related to pointing strategies as well as to those between blind and sighted subjects were apparent. Most importantly, the reconstructed origin (bias) of arm pointing was located near the shoulder for the blind subjects, whereas it was shifted and located near the cyclopean eye for the sighted subjects. The results indicate that both early blind and sighted humans adequately transform the head-centered acoustic target coordinates into the required reference frame of either motor system, but that the adopted response strategy may be specific to the subject group and pointer method.

Auditory Triggered Mental Imagery of Shape Involves Visual Association Areas in Early Blind Humans

Previous neuroimaging studies identified a large network of cortical areas involved in visual imagery in the human brain, which includes occipitotemporal and visual associative areas. Here we test whether the same processes can be elicited by tactile and auditory experiences in subjects who became blind early in life. Using positron emission tomography, regional cerebral blood flow was assessed in six right-handed early blind and six age-matched control volunteers during three conditions: resting state, passive listening to noise sounds, and mental imagery task (imagery of object shape) triggered by the sound of familiar objects. Activation foci were found in occipitotemporal and visual association areas, particularly in the left fusiform gyrus (Brodmann areas 19-37), during mental imagery of shape by both groups. Since shape imagery by early blind subjects does involve similar visual structures as controls at an adult age, it indicates their developmental crossmodal reorganization to allow perceptual representation in the absence of vision.

Effects of aging on the intrinsic circadian period of totally blind humans.

Age-related changes in the intrinsic circadian period (tau) have been hypothesized to account for sleep symptoms in the elderly such as early morning awakening. The authors sought to determine whether the aging process produced quantifiable differences in the tau of totally blind men who had free-running circadian rhythms. The melatonin onset was used as the indicator of circadian phase. Melatonin rhythms had been characterized about a decade previously when the participants were 38 +/- 6 (SD) years old. Both previous and current assessments of tau were derived from at least 3 serial measurements of the 24-h melatonin profile from which the melatonin onset was determined. All 6 participants exhibited a longer tau in the 2nd assessment (mean increase +/- SD of 0.13 +/- 0.08 h; p < 0.01). Four participants exhibited differences in tau with nonoverlapping 95% confidence intervals. The results do not support the commonly held view that tau shortens during human aging. On the contrary, tau appears to slightly, but significantly, lengthen during at least 1 decade in midlife.

Event-related potentials during auditory language processing in congenitally blind and sighted people

While behavioral studies have documented delayed language acquisition in blind children, other studies have revealed better speech discrimination abilities for blind than sighted adults. Several brain imaging studies have provided evidence for cortical reorganization due to visual deprivation but the cerebral organization of language in blind humans is not known yet. We hypothesized that the increasing specialization of language systems normally observed during development may not take place to the same degree in blind individuals since posterior visual areas do not receive their adequate input. On the other hand, we hypothesized that blind people, due to their greater reliance upon the auditory language signal, may process speech faster than sighted people. To test these assumptions, event-related potentials were recorded while 11 congenitally blind and 11 sighted adults matched in age, gender, handedness and education were engaged in a language task. Participants listened to sentences in order to decide after each sentence if it was meaningful or not. Incongruous sentence-final words elicited an N400 effect in both groups. The N400 effect had a left-lateralized fronto-central scalp distribution in the sighted but a symmetric and broad topography in the blind. Furthermore, the N400 effect started earlier in the blind than in the sighted. Closed class compared to open class sentence middle words elicited a more pronounced late negativity in the blind than in the sighted. These results suggest that blind people process auditory language stimuli faster than sighted people and that some language functions may be reorganized in the blind.

Brain functional reorganization in early blind humans revealed by auditory event-related potentials.

Visually challenged individuals often compensate for their handicap by developing supra-normal abilities in their remaining sensory systems. Here, we examined the scalp distribution of components N1 and P3 of auditory evoked potentials during a sound localization task in four totally blind subjects who had previously shown better performance than sighted subjects. Both N1 and P3 waves peaked at their usual positions while blind and sighted individuals performed the task. However, in blind subjects these two components were also found to be robust over occipital regions while in sighted individuals this pattern was not seen. We conclude that deafferented posterior visual areas in blind individuals are recruited to carry out auditory functions, enabling these individuals to compensate for their lack of vision.

Improved auditory spatial acuity in visually deprived ferrets

We have examined the effects on auditory spatial acuity in the horizontal plane of depriving ferrets of patterned visual cues by binocular eyelid suture in infancy or for a comparable period in adulthood. Minimum audible angles (MAAs) were measured for 500-, 100- and 40-ms broadband noise bursts at the midline and at 45 degrees to one side. A logistic regression analysis revealed no consistent difference between the midline MAAs of normal and infant lid-sutured ferrets. However, the lateral field MAAs of the infant-deprived group were significantly smaller and showed less inter-subject variability than those of normal-sighted ferrets. The animals deprived in adulthood were tested in the lateral field only, firstly 6 months after binocular eyelid suture and again after a further 10 months. For the first test, the MAAs achieved by these animals with 500- and 100-ms noise bursts were significantly smaller than the normal values and no different from those of the infant-deprived group. A significant improvement in performance at the two shortest stimulus durations (100 and 40 ms) was observed when the adult-deprived animals were re-tested. Their second-test MAAs did not differ from those of the infant-deprived group at any of the three stimulus durations used, and both groups achieved significantly better scores than the normal-sighted control animals. These results show that prolonged visual deprivation in both juvenile and adult ferrets can lead to a significant improvement in auditory spatial acuity in the lateral sound field. This is consistent with reports that congenitally blind humans can localize peripheral sounds more accurately than normal controls.

Cross-Modal Reorganization of Horizontal Connectivity in Auditory Cortex without Altering Thalamocortical Projections

The development of the different, highly specialized regions of the mammalian cerebral cortex depends in part on neural activity, either intrinsic spontaneous activity or externally driven sensory activity. To determine whether patterned sensory activity instructs the development of intrinsic cortical circuitry, we have experimentally altered the modality of sensory inputs to cerebral cortex. Neonatal diversion of retinal axons to the auditory thalamus (cross-modal rewiring) results in a primary auditory cortex (AI) that resembles visual cortex in its response properties and topography (Roe et al., 1990, 1992). To test the hypothesis that the visual response properties are created by a visually driven reorganization of auditory cortical circuitry, we investigated the effect of early visual experience on the development of intrinsic, horizontal connections within AI. Horizontal connections are likely to play an important role in the construction of visual response properties in AI as they do in visual cortex. Here we show that early visual inputs to auditory thalamus can reorganize horizontal connections in AI, causing both an increase in their extent and a change in pattern, so that projections are not restricted to the isofrequency axis, but extend in a more isotropic pattern around the injection site. Thus, changing afferent modality, without altering the source of the thalamocortical axons, can profoundly alter cortical circuitry. Similar changes may underlie cortical compensatory processes in deaf or blind humans and may also have played a role in the parcellation of neocortex during mammalian evolution.

Tuned in when out of sight

It has been reported previously that blind humans show improved auditory discrimination, as if to compensate for the sensory lack in the visual modality (e.g. 1 Lessard N. et al. Early-blind human subjects localize sound sources better than sighted subjects. Nature. 1998; 395: 278-280 Crossref PubMed Scopus (478) Google Scholar ). However, it is unclear how this sensory enhancement relates to altered cortical activation patterns found in these blind individuals. A recent study by Röder et al. makes an important contribution to the investigation of this phenomenon by comparing behavioural and electrophysiological measures of auditory spatial tuning in congenitally blind individuals and normally sighted but blindfolded control subjects 2 Röder B. et al. Improved auditory spatial tuning in blind humans. Nature. 1999; 400: 162-166 Crossref PubMed Scopus (510) Google Scholar . In an attentional paradigm, brief noise bursts were presented in random sequence with equal probability from one of four central and four peripheral speakers. Subjects were instructed to attend either to a central or to a peripheral loudspeaker and to detect infrequent higher-pitched sounds when they came from the target speaker, but to ignore sounds from all other speakers. The results showed that the blind subjects had better localization abilities than those of the control subjects, but only when they attended to peripheral auditory space. ERP recordings measured at the same time revealed sharper tuning curves for the component of the ERP related to early spatial attention in the blind subjects, and differences in the scalp topography of the ERP responses strongly suggest that there is a reorganization of the neural substrates for early auditory processing in the congenitally blind. This study adds support to the notion that when the brain is faced with the loss of one sensory modality, it reacts with compensatory changes in the remaining modalities.

Cross-modal reorganization of callosal connectivity without altering thalamocortical projections.

Mammalian cerebral cortex is composed of a multitude of different areas that are each specialized for a unique purpose. It is unclear whether the activity pattern and modality of sensory inputs to cortex play an important role in the development of cortical regionalization. The modality of sensory inputs to cerebral cortex can be altered experimentally. Neonatal diversion of retinal axons to the auditory thalamus (cross-modal rewiring) results in a primary auditory cortex (AI) that resembles the primary visual cortex in its visual response properties and topography. Functional reorganization could occur because the visual inputs use existing circuitry in AI, or because the early visual inputs promote changes in AI's circuitry that make it capable of constructing visual receptive field properties. The present study begins to distinguish between these possibilities by exploring whether the callosal connectivity of AI is altered by early visual experience. Here we show that early visual inputs to auditory thalamus can reorganize callosal connections in auditory cortex, causing both a reduction in their extent and a reorganization of the pattern. This result is distinctly different from that in deafened animals, which have widespread callosal connections, as in early postnatal development. Thus, profound changes in cortical circuitry can result simply from a change in the modality of afferent input. Similar changes may underlie cortical compensatory processes in deaf and blind humans.