What is the function of auditory cortex when it develops in the absence of acoustic input?

Unraveling the impact of congenital deafness on individual brain organization

Research on brain plasticity, particularly in the context of deafness, consistently emphasizes the reorganization of the auditory cortex. But to what extent do all individuals with deafness show the same level of reorganization? To address this question, we examined the individual differences in functional connectivity (FC) from the deprived auditory cortex. Our findings demonstrate remarkable differentiation between individuals deriving from the absence of shared auditory experiences, resulting in heightened FC variability among deaf individuals, compared to more consistent FC in the hearing group. Notably, connectivity to language regions becomes more diverse across individuals with deafness. This does not stem from delayed language acquisition; it is found in deaf native signers, who are exposed to natural language since birth. However, comparing FC diversity between deaf native signers and deaf delayed signers, who were deprived of language in early development, we show that language experience also impacts individual differences, although to a more moderate extent. Overall, our research points out the intricate interplay between brain plasticity and individual differences, shedding light on the diverse ways reorganization manifests among individuals. It joins findings of increased connectivity diversity in blindness and highlights the importance of considering individual differences in personalized rehabilitation for sensory loss.

Neural adaptations in short-term learning of sign language revealed by fMRI and DTI

While vocal articulation is a unique feature of spoken languages, signed languages use facial expressions and hand movements for communication. Despite this substantial difference, neuroimaging studies show that spoken and sign language rely on similar areas of the brain in the frontal and parietal regions. However, little is known about the specific roles of these areas and how early they get involved. In the present study, we investigate the impact of short-term training-related changes in learners of British sign language (BSL). Pre- and post-training functional magnetic resonance imaging (fMRI) and diffusion tensor imaging scans were taken from twenty-six right-handed healthy volunteers. During the training course, participants were taught to discriminate and sign basic sentences using BSL for three consecutive days (1 h per day). fMRI results show increasing brain activity in the right cerebellum and cerebral brain areas including bilateral middle temporal gyrus, left angular gyrus, left middle and inferior frontal gyrus. Moreover, functional connectivity increased significantly after training between these areas. Microstructural findings show significant mean diffusivity and radial diffusivity reductions in the left angular gyrus, which are significantly correlated with behavioural improvement. These results reveal a high degree of similarity in the neural activity underlying signed and spoken languages. The rapid microstructural changes, identify the left angular gyrus as a structure that rapidly adapts to newly learnt visual-semantic associations.

Development of Mandarin Chinese Vowel Perception in Young Children With Normal Hearing and Cochlear Implants

Purpose Depicting the development pattern of vowel perception for children with normal hearing (NH) and cochlear implants (CIs) would be useful for clinicians and school teachers to monitor children's auditory rehabilitation. The study was to investigate the development of Mandarin Chinese vowel perception for Mandarin Chinese native-speaking children with the ages of 4-6 years. Method Vowel identification of children with NH and CIs were tested. All children with CIs received CIs before the age of 4 years. In a picture identification task with Mandarin Chinese speech stimuli, listeners identified the target consonant-vowel word among two to four contrastive words that differed only in vowels. Each target word represented a concrete object and was spoken by a young female native Mandarin Chinese talker. The target words included 16 monophthongs, 22 diphthongs, and nine triphthongs. Results Children with NH showed significantly better identification of monophthongs and diphthongs than children with CIs at the age of 6 years, whereas the two groups had comparable performance at age of 4 and 5 years. Children with NH significantly outperformed children with CIs for triphthong identification across all three age groups. For children with NH, a rapid development of perception of all three types of vowels occurred between age 4 and 5 years with a rapid development only for monophthong perception between age 5 and 6 years. For children with CIs, a rapid development of both diphthong and triphthong perception occurred between 4 and 5 years old, but not monophthong, with no significant development between 5 and 6 years old for all three types of vowels. Overall, Mandarin-speaking children with NH achieved their ceiling performance in vowel perception before or at the age of 6 years, whereas children with CIs may need more time to reach the typical level of their peers with NH. Conclusions The development of Mandarin vowel perception for Mandarin-native children differed between preschool-age children with NH and CIs, likely due to the deficits of spectral processing for children with CIs. The results would be a supplement to the development of speech recognition in Mandarin-native children with NH and CIs.

Evidence for an effector-independent action system from people born without hands

What are the principles of brain organization? In the motor domain, separate pathways were found for reaching and grasping actions performed by the hand. To what extent is this organization specific to the hand or based on abstract action types, regardless of which body part performs them? We tested people born without hands who perform actions with their feet. Activity in frontoparietal association motor areas showed preference for an action type (reaching or grasping), regardless of whether it was performed by the foot in people born without hands or by the hand in typically-developed controls. These findings provide evidence that some association areas are organized based on abstract functions of action types, independent of specific sensorimotor experience and parameters of specific body parts.

Many parts of the visuomotor system guide daily hand actions, like reaching for and grasping objects. Do these regions depend exclusively on the hand as a specific body part whose movement they guide, or are they organized for the reaching task per se, for any body part used as an effector? To address this question, we conducted a neuroimaging study with people born without upper limbs—individuals with dysplasia—who use the feet to act, as they and typically developed controls performed reaching and grasping actions with their dominant effector. Individuals with dysplasia have no prior experience acting with hands, allowing us to control for hand motor imagery when acting with another effector (i.e., foot). Primary sensorimotor cortices showed selectivity for the hand in controls and foot in individuals with dysplasia. Importantly, we found a preference based on action type (reaching/grasping) regardless of the effector used in the association sensorimotor cortex, in the left intraparietal sulcus and dorsal premotor cortex, as well as in the basal ganglia and anterior cerebellum. These areas also showed differential response patterns between action types for both groups. Intermediate areas along a posterior–anterior gradient in the left dorsal premotor cortex gradually transitioned from selectivity based on the body part to selectivity based on the action type. These findings indicate that some visuomotor association areas are organized based on abstract action functions independent of specific sensorimotor parameters, paralleling sensory feature-independence in visual and auditory cortices in people born blind and deaf. Together, they suggest association cortices across action and perception may support specific computations, abstracted from low-level sensorimotor elements.

Visual habituation in deaf and hearing infants

Early cognitive development relies on the sensory experiences that infants acquire as they explore their environment. Atypical experience in one sensory modality from birth may result in fundamental differences in general cognitive abilities. The primary aim of the current study was to compare visual habituation in infants with profound hearing loss, prior to receiving cochlear implants (CIs), and age-matched peers with typical hearing. Two complementary measures of cognitive function and attention maintenance were assessed: the length of time to habituate to a visual stimulus, and look-away rate during habituation. Findings revealed that deaf infants were slower to habituate to a visual stimulus and demonstrated a lower look-away rate than hearing infants. For deaf infants, habituation measures correlated with language outcomes on standardized assessments before cochlear implantation. These findings are consistent with prior evidence suggesting that habituation and look-away rates reflect efficiency of information processing and may suggest that deaf infants take longer to process visual stimuli relative to the hearing infants. Taken together, these findings are consistent with the hypothesis that hearing loss early in infancy influences aspects of general cognitive functioning.

Plasticity based on compensatory effector use in the association but not primary sensorimotor cortex of people born without hands

What forces direct brain organization and its plasticity? When brain regions are deprived of their input, which regions reorganize based on compensation for the disability and experience, and which regions show topographically constrained plasticity? People born without hands activate their primary sensorimotor hand region while moving body parts used to compensate for this disability (e.g., their feet). This was taken to suggest a neural organization based on functions, such as performing manual-like dexterous actions, rather than on body parts, in primary sensorimotor cortex. We tested the selectivity for the compensatory body parts in the primary and association sensorimotor cortex of people born without hands (dysplasic individuals). Despite clear compensatory foot use, the primary sensorimotor hand area in the dysplasic subjects showed preference for adjacent body parts that are not compensatorily used as effectors. This suggests that function-based organization, proposed for congenital blindness and deafness, does not apply to the primary sensorimotor cortex deprivation in dysplasia. These findings stress the roles of neuroanatomical constraints like topographical proximity and connectivity in determining the functional development of primary cortex even in extreme, congenital deprivation. In contrast, increased and selective foot movement preference was found in dysplasics' association cortex in the inferior parietal lobule. This suggests that the typical motor selectivity of this region for manual actions may correspond to high-level action representations that are effector-invariant. These findings reveal limitations to compensatory plasticity and experience in modifying brain organization of early topographical cortex compared with association cortices driven by function-based organization.

Language and Sensory Neural Plasticity in the Superior Temporal Cortex of the Deaf

Visual stimuli are known to activate the auditory cortex of deaf people, presenting evidence of cross-modal plasticity. However, the mechanisms underlying such plasticity are poorly understood. In this functional MRI study, we presented two types of visual stimuli, language stimuli (words, sign language, and lip-reading) and a general stimulus (checkerboard) to investigate neural reorganization in the superior temporal cortex (STC) of deaf subjects and hearing controls. We found that only in the deaf subjects, all visual stimuli activated the STC. The cross-modal activation induced by the checkerboard was mainly due to a sensory component via a feed-forward pathway from the thalamus and primary visual cortex, positively correlated with duration of deafness, indicating a consequence of pure sensory deprivation. In contrast, the STC activity evoked by language stimuli was functionally connected to both the visual cortex and the frontotemporal areas, which were highly correlated with the learning of sign language, suggesting a strong language component via a possible feedback modulation. While the sensory component exhibited specificity to features of a visual stimulus (e.g., selective to the form of words, bodies, or faces) and the language (semantic) component appeared to recruit a common frontotemporal neural network, the two components converged to the STC and caused plasticity with different multivoxel activity patterns. In summary, the present study showed plausible neural pathways for auditory reorganization and correlations of activations of the reorganized cortical areas with developmental factors and provided unique evidence towards the understanding of neural circuits involved in cross-modal plasticity.

Visual cortical activity before and after cochlear implantation: A follow up ERP prospective study in deaf children

ERPs were recorded in response to presentation of static colored patterned stimuli in 25 children (19 to 80months of age at cochlear implantation, CI) with very early prelingual profound deafness (PreLD), 21 postlingual profoundly deaf children (PostLD) (34 to 180months of age at CI) and gender- and age-matched control hearing children. Recording sessions were performed before CI, then 6 and 24months after CI. Results showed that prelingual and, at a lesser degree, postlingual auditory deprivation altered cortical visual neural activity associated to colored shapes from both P1 and N1 cortical processing stages. The P1 and N1 amplitude modifications vanished about 24months after CI in both PreLD and PostLD deaf children. In PreLD the visual processing pattern becomes similar to the typical one essentially by an amplitude decrease of P1 on the left hemisphere together with an amplitude increase of the N1 on the right hemisphere. Finally, in PreLD, increased LH advantage over the RH in N1 amplitude on the cerebellar-occipito-parietal region before CI showed a significant inverse relationship with speech perception outcomes 3years after CI. Investigating early visual processing development and its neural substrates in deaf children would help to understand the variability of CI outcome, because their cortical visual organization diverged from the one of typically developing hearing children, and cannot be predicted from what is observed in deaf adults.

Brain Plasticity: When the Feet and Mouth Replace the Hand

Cortex plasticity after hand amputation is considered harmful, causing phantom limb pain. A new study shows that cortical overtake can occur instead in a compensatory manner in people born with one hand, for multiple body parts used to overcome disability.