Sensory temporal processing in adults with early hearing loss

Investigating the impact of early deafness on learned action-effect contingency for action linked to peripheral sensory effects

Investigating peripheral visual processing in individuals with early auditory deprivation is a critical research area in the field of neuroscience, since it helps understanding the phenomenon of sensory adaptation and brain plasticity after sensory loss. Prior research has already demonstrated that the absence of auditory input, which is crucial to detect events occurring out of the central egocentric visual space, leads to an improved processing of visual and tactile stimuli occurring in peripheral regions of the sensory space. Nevertheless, no prior studies have explored whether such enhanced processing also takes place within the domain of action, particularly when individuals are required to perform actions that produce peripheral sensory outcomes. To test this hypothesis, we recruited 15 hearing (31 ± 3.3 years) and 15 early deaf adults (42 ± 2.6 years) for a neuro-behavioral experiment involving: 1) a behavioral task where participants executed a simple motor action (i.e., a button press) and received a visual feedback either in the center or in a peripheral region of the visual field, and 2) the electrophysiological recording of brain electrical potentials (EEG). We measured and compared neural activity preceding the motor action (the readiness potentials) and visual evoked responses (the N1 and P2 ERP components) and found that deaf individuals did not exhibit more pronounced modulation of neural responses when their motor actions resulted in peripheral visual stimuli compared to their hearing counterparts. Instead they showed a reduced modulation when visual stimuli were presented in the center. Our results suggest a redistribution of attentional resources from center to periphery in deaf individuals during sensorimotor coupling.

Do auditory brainstem implants favor the development of sensory integration and cognitive functions?

BACKGROUND

Information about the development of cognitive skills and the effect of sensory integration in children using auditory brainstem implants (ABIs) is still limited.

OBJECTIVE

This study primarily aims to investigate the relationship between sensory processing skills and attention and memory abilities in children with ABI, and secondarily aims to examine the effects of implant duration on sensory processing and cognitive skills in these children.

METHODS

The study included 25 children between the ages of 6 and 10 years (mean age: 14 girls and 11 boys) with inner ear and/or auditory nerve anomalies using auditory brainstem implants. Visual-Aural Digit Span Test B, Marking Test, Dunn Sensory Profile Questionnaire were applied to all children.

RESULTS

The sensory processing skills of children are statistically significant and positive, and moderately related to their cognitive skills. As the duration of implant use increases, better attention and memory performances have been observed (p < .05).

CONCLUSION

The study demonstrated the positive impact of sensory processing on the development of memory and attention skills in children with ABI. It will contribute to evaluating the effectiveness of attention, memory, and sensory integration skills, and aiding in the development of more effective educational strategies for these children.

Late Blindness and Deafness are Associated with Decreased Tactile Sensitivity, But Early Blindness is Not

Perceptual experience is shaped by a complex interaction between our sensory systems in which each sense conveys information on specific properties of our surroundings. This multisensory processing of complementary information improves the accuracy of our perceptual judgments and leads to more precise and faster reactions. Sensory impairment or loss in one modality leads to information deficiency that can impact other senses in various ways. For early auditory or visual loss, impairment and/or compensatory increase of the sensitivity of other senses are equally well described. Investigating individuals with deafness (N = 73), early (N = 51), late blindness (N = 49) and corresponding controls, we compared tactile sensitivity using the standard monofilament test on two locations, the finger and handback. Results indicate lower tactile sensitivity in people with deafness and late blindness but not in people with early blindness compared to respective controls, irrespective of stimulation location, gender, and age. Results indicate that neither sensory compensation nor simple use-dependency or a hindered development of the tactile sensory system is sufficient to explain changes in somatosensation after the sensory loss but that a complex interaction of effects is present.

Somatosensory processing in deaf and deafblind individuals: How does the brain adapt as a function of sensory and linguistic experience? A critical review

How do deaf and deafblind individuals process touch? This question offers a unique model to understand the prospects and constraints of neural plasticity. Our brain constantly receives and processes signals from the environment and combines them into the most reliable information content. The nervous system adapts its functional and structural organization according to the input, and perceptual processing develops as a function of individual experience. However, there are still many unresolved questions regarding the deciding factors for these changes in deaf and deafblind individuals, and so far, findings are not consistent. To date, most studies have not taken the sensory and linguistic experiences of the included participants into account. As a result, the impact of sensory deprivation vs. language experience on somatosensory processing remains inconclusive. Even less is known about the impact of deafblindness on brain development. The resulting neural adaptations could be even more substantial, but no clear patterns have yet been identified. How do deafblind individuals process sensory input? Studies on deafblindness have mostly focused on single cases or groups of late-blind individuals. Importantly, the language backgrounds of deafblind communities are highly variable and include the usage of tactile languages. So far, this kind of linguistic experience and its consequences have not been considered in studies on basic perceptual functions. Here, we will provide a critical review of the literature, aiming at identifying determinants for neuroplasticity and gaps in our current knowledge of somatosensory processing in deaf and deafblind individuals.

Deprivation of Auditory Experience Influences Numerosity Discrimination, but Not Numerosity Estimation

Number sense is the ability to estimate the number of items, and it is common to many species. Despite the numerous studies dedicated to unveiling how numerosity is processed in the human brain, to date, it is not clear whether the representation of numerosity is supported by a single general mechanism or by multiple mechanisms. Since it is known that deafness entails a selective impairment in the processing of temporal information, we assessed the approximate numerical abilities of deaf individuals to disentangle these two hypotheses. We used a numerosity discrimination task (2AFC) and an estimation task, in both cases using sequential (temporal) or simultaneous (spatial) stimuli. The results showed a selective impairment of the deaf participants compared with the controls (hearing) in the temporal numerosity discrimination task, while no difference was found to discriminate spatial numerosity. Interestingly, the deaf and hearing participants did not differ in spatial or temporal numerosity estimation. Overall, our results suggest that the deficit in temporal processing induced by deafness also impacts perception in other domains such as numerosity, where sensory information is conveyed in a temporal format, which further suggests the existence of separate mechanisms subserving the processing of temporal and spatial numerosity.

Cross-modal integration and plasticity in the superior temporal cortex

In congenitally deaf people, temporal regions typically believed to be primarily auditory enhance their response to nonauditory information. The neural mechanisms and functional principles underlying this phenomenon, as well as its impact on auditory recovery after sensory restoration, yet remain debated. In this chapter, we demonstrate that the cross-modal recruitment of temporal regions by visual inputs in congenitally deaf people follows organizational principles known to be present in the hearing brain. We propose that the functional and structural mechanisms allowing optimal convergence of multisensory information in the temporal cortex of hearing people also provide the neural scaffolding for feeding visual or tactile information into the deafened temporal areas. Innate in their nature, such anatomo-functional links between the auditory and other sensory systems would represent the common substrate of both early multisensory integration and expression of selective cross-modal plasticity in the superior temporal cortex.

Multisensory temporal processing in early deaf

Navigating the world relies on understanding progressive sequences of multisensory events across time. Early deaf (ED) individuals are more precise in visual detection of space and motion than their normal hearing (NH) counterparts. However, whether ED individuals show altered multisensory temporal processing abilities is less clear. According to the connectome model, brain development depends on experience, and therefore the lack of audition may affect how the brain responds to remaining senses and how they are functionally connected. We used a temporal order judgment (TOJ) task to examine multisensory (visuotactile) temporal processing in ED and NH groups. We quantified BOLD responses and functional connectivity (FC) in both groups. ED and NH groups performed similarly for the visuotactile TOJ task. Bilateral posterior superior temporal sulcus (pSTS) BOLD responses during the TOJ task were significantly larger in the ED group than in NH. Using anatomically defined pSTS seeds, our FC analysis revealed stronger somatomotor and weaker visual regional connections in the ED group than in NH during the TOJ task. These results suggest that a lack of auditory input might alter the balance of tactile and visual area FC with pSTS when a multisensory temporal task is involved.

Reduced flash lag illusion in early deaf individuals

When a brief flash is quickly presented aligned with a moving target, the flash typically appears to lag behind the moving stimulus. This effect is widely known in the literature as a flash-lag illusion (FLI). The flash-lag is an example of a motion-induced position shift. Since auditory deprivation leads to both enhanced visual skills and impaired temporal abilities, both crucial for the perception of the flash-lag effect, here we hypothesized that lack of audition could influence the FLI. 13 early deaf and 18 hearing individuals were tested in a visual FLI paradigm to investigate this hypothesis. As expected, results demonstrated a reduction of the flash-lag effect following early deafness, both in the central and peripheral visual fields. Moreover, only for deaf individuals, there is a positive correlation between the flash-lag effect in the peripheral and central visual field, suggesting that the mechanisms underlying the effect in the center of the visual field expand to the periphery following deafness. Overall, these findings reveal that lack of audition early in life profoundly impacts early visual processing underlying the flash-lag effect.

Quantitative EEG measures in profoundly deaf and normal hearing individuals while performing a vibrotactile temporal discrimination task

Challenges in early oral language acquisition in profoundly deaf individuals have an impact on cognitive neurodevelopment. This has led to the exploration of alternative sound perception methods involving training of vibrotactile discrimination of sounds within the language spectrum. In particular, stimulus duration plays an important role in linguistic categorical perception. We comparatively evaluated vibrotactile temporal discrimination of sound and how specific training can modify the underlying electrical brain activity. Fifteen profoundly deaf (PD) and 15 normal-hearing (NH) subjects performed a vibrotactile oddball task with simultaneous EEG recording, before and after a short training period (5 one-hour sessions; in 2.5-3 weeks). The stimuli consisted of 700 Hz pure-tones with different duration (target: long 500 ms; non-target: short 250 ms). The sound-wave stimuli were delivered by a small device worn on the right index finger. A similar behavioral training effect was observed in both groups showing significant improvement in sound-duration discrimination. However, quantitative EEG measurements reveal distinct neurophysiological patterns characterized by higher and more diffuse delta band magnitudes in the PD group, together with a generalized decrement in absolute power in both groups that might reflect a facilitating process associated to learning. Furthermore, training-related changes were found in the beta-band in NH. Findings suggest PD have different cognitive adaptive mechanisms which are not a mere amplification effect due to greater cortical excitability.

Vestibular status: A missing factor in our understanding of brain reorganization in deaf individuals

The brain of deaf people is definitely not just deaf, and we have to reconsider what we know about the impact of hearing loss on brain development in light of comorbid vestibular impairments.

Cross-modal motion aftereffects transfer between vision and touch in early deaf adults

Previous research on early deafness has primarily focused on the behavioral and neural changes in the intact visual and tactile modalities. However, how early deafness changes the interplay of these two modalities is not well understood. In the current study, we investigated the effect of auditory deprivation on visuo-tactile interaction by measuring the cross-modal motion aftereffect. Consistent with previous findings, motion aftereffect transferred between vision and touch in a bidirectional manner in hearing participants. However, for deaf participants, the cross-modal transfer occurred only in the tactile-to-visual direction but not in the visual-to-tactile direction. This unidirectional cross-modal motion aftereffect found in the deaf participants could not be explained by unisensory motion aftereffect or discrimination threshold. The results suggest a reduced visual influence on tactile motion perception in early deaf individuals.

Improvements and Degradation to Spatial Tactile Acuity Among Blind and Deaf Individuals

Cross-modal reorganization takes place for sensory cortices when there is no more primary input. For instance, the visual cortex in blind individuals which receives no visual input starts responding to auditory and tactile stimuli. Reorganization may improve or degrade processing of other modality inputs, via bottom-up compensational processes and top-down updating. In two experiments, we measured the spatial tactile response in a large sample of early- (N = 49) and late-blind (N = 51) individuals with varying levels of Braille proficiencies, and early-deaf (N = 69) with varying levels of hearing devices against separate hearing and sighted controls. Spatial tactile responses were measured using a standard gradient orientation task on two locations, the finger and tongue. Experiments show limited to no advantage in passive tactile response for blind individuals and degradation for deaf individuals at the finger. However, the use of hearing devices decreased the tactile impairment in early-deaf individuals. Also, no differences in age-related decline in both sensory-impaired groups were shown. Results show less tactile acuity differences between blind and sighted than previously reported, but supports recent reports of tactile impairment among the early-deaf.

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

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

Visual duration bisection in profoundly deaf individuals

Background

Previous research has been designed to study the effect of hearing loss on supra-second duration estimation in the visual channel and position effect of visual abilities among deaf populations. The current study aimed to investigate the sub-second duration perception of different visual fields in profoundly deaf individuals.

Methods

A total of 16 profoundly deaf undergraduates and 16 hearing undergraduates completed a visual duration bisection task in which participants made judgments about whether a series of probe durations that were linearly spaced from 200 ms to 800 ms at 100 ms intervals were more similar to a standard short duration (200 ms) or a standard long duration (800 ms). The probe stimuli were presented in the center, left, or right of the screen. A repeated measure analysis of variance (ANOVA) with a between-participants factor of group and a within-participants factor of position, and a one-sample t-test were conducted.

Results

The Weber ratio (WR) values of deaf participants were significantly higher than those of hearing participants, regardless of the presented positions of the visual stimulus. The bisection point (BP) value of deaf participants was significantly lower than 500 ms (average mean of 200/800 ms) and the BP value of hearing participants did not significantly differ from 500 ms, although the overall difference of BP values between the deaf group and hearing group did not reach significance. For deaf participants, the BP value in the center condition was significantly lower than 500 ms; however, the difference between the BP value in the left condition and 500 ms did not reach significance, indicating that their duration discrimination accuracy in the left visual field was better than that in the center visual field.

Conclusions

Hearing loss impaired visual sub-second duration perception, and deaf individuals showed a left visual field advantage of duration discrimination accuracy during the visual duration bisection task.

Electrophysiological Dynamics of Visual-Tactile Temporal Order Perception in Early Deaf Adults

Studies of compensatory plasticity in early deaf (ED) individuals have mainly focused on unisensory processing, and on spatial rather than temporal coding. However, precise discrimination of the temporal relationship between stimuli is imperative for successful perception of and interaction with the complex, multimodal environment. Although the properties of cross-modal temporal processing have been extensively studied in neurotypical populations, remarkably little is known about how the loss of one sense impacts the integrity of temporal interactions among the remaining senses. To understand how auditory deprivation affects multisensory temporal interactions, ED and age-matched normal hearing (NH) controls performed a visual-tactile temporal order judgment task in which visual and tactile stimuli were separated by varying stimulus onset asynchronies (SOAs) and subjects had to discern the leading stimulus. Participants performed the task while EEG data were recorded. Group averaged event-related potential waveforms were compared between groups in occipital and fronto-central electrodes. Despite similar temporal order sensitivities and performance accuracy, ED had larger visual P100 amplitudes for all SOA levels and larger tactile N140 amplitudes for the shortest asynchronous (± 30 ms) and synchronous SOA levels. The enhanced signal strength reflected in these components from ED adults are discussed in terms of compensatory recruitment of cortical areas for visual-tactile processing. In addition, ED adults had similar tactile P200 amplitudes as NH but longer P200 latencies suggesting reduced efficiency in later processing of tactile information. Overall, these results suggest that greater responses by ED for early processing of visual and tactile signals are likely critical for maintained performance in visual-tactile temporal order discrimination.

Effects of longstanding degraded auditory signal on visuospatial, visuomotor, and visual attention skills in adults with hearing loss

OBJECTIVE

This study examined the consequences of long-term auditory deprivation on visuospatial functions, visuomotor functions, and visual attention skills in adults with early-onset hearing loss.

METHODS

Fifteen adults with bilateral, early-onset (before age 3), severe-to-profound hearing loss who used spoken language participated in this study. Visuospatial (figure ground, form constancy, visual perception) and visuomotor functions (visuomotor integration, visual search) were examined using norm-referenced tests. Visual attention scales were examined using a computerized program, IVA plus continuous performance test.

RESULTS

As a group, participants performed in the average range on visuospatial and visuomotor functions when compared to normative data presented in the test instruments. However, participants demonstrated below average performance on sustained visual attention. Duration of cochlear implant use positively correlated with one of the visual attention scales, namely visual speed.

CONCLUSIONS

The findings of the current study suggest that adults with early-onset hearing loss may demonstrate difficulties in sustaining attention to visual information. Data also suggest that increased experience with cochlear implant may ameliorate deficits associated with visual attention. Future studies should explore challenges experienced by adults with early-onset hearing loss in their daily activities that may result from this deficit and intervention programs that may enhance visual attention skills.

The Cross-Modal Effects of Sensory Deprivation on Spatial and Temporal Processes in Vision and Audition: A Systematic Review on Behavioral and Neuroimaging Research since 2000

One of the most significant effects of neural plasticity manifests in the case of sensory deprivation when cortical areas that were originally specialized for the functions of the deprived sense take over the processing of another modality. Vision and audition represent two important senses needed to navigate through space and time. Therefore, the current systematic review discusses the cross-modal behavioral and neural consequences of deafness and blindness by focusing on spatial and temporal processing abilities, respectively. In addition, movement processing is evaluated as compiling both spatial and temporal information. We examine whether the sense that is not primarily affected changes in its own properties or in the properties of the deprived modality (i.e., temporal processing as the main specialization of audition and spatial processing as the main specialization of vision). References to the metamodal organization, supramodal functioning, and the revised neural recycling theory are made to address global brain organization and plasticity principles. Generally, according to the reviewed studies, behavioral performance is enhanced in those aspects for which both the deprived and the overtaking senses provide adequate processing resources. Furthermore, the behavioral enhancements observed in the overtaking sense (i.e., vision in the case of deafness and audition in the case of blindness) are clearly limited by the processing resources of the overtaking modality. Thus, the brain regions that were previously recruited during the behavioral performance of the deprived sense now support a similar behavioral performance for the overtaking sense. This finding suggests a more input-unspecific and processing principle-based organization of the brain. Finally, we highlight the importance of controlling for and stating factors that might impact neural plasticity and the need for further research into visual temporal processing in deaf subjects.

Spatial Cues Influence Time Estimations in Deaf Individuals

Recent studies have reported a strong interaction between spatial and temporal representation when visual experience is missing: blind people use temporal representation of events to represent spatial metrics. Given the superiority of audition on time perception, we hypothesized that when audition is not available complex temporal representations could be impaired, and spatial representation of events could be used to build temporal metrics. To test this hypothesis, deaf and hearing subjects were tested with a visual temporal task where conflicting and not conflicting spatiotemporal information was delivered. As predicted, we observed a strong deficit of deaf participants when only temporal cues were useful and space was uninformative with respect to time. However, the deficit disappeared when coherent spatiotemporal cues were presented and increased for conflicting spatiotemporal stimuli. These results highlight that spatial cues influence time estimations in deaf participants, suggesting that deaf individuals use spatial information to infer temporal environmental coordinates.

Multisensory Interference in Early Deaf Adults

Multisensory interactions in deaf cognition are largely unexplored. Unisensory studies suggest that behavioral/neural changes may be more prominent for visual compared to tactile processing in early deaf adults. Here we test whether such an asymmetry results in increased saliency of vision over touch during visuo-tactile interactions. About 23 early deaf and 25 hearing adults performed two consecutive visuo-tactile spatial interference tasks. Participants responded either to the elevation of the tactile target while ignoring a concurrent visual distractor at central or peripheral locations (respond to touch/ignore vision), or they performed the opposite task (respond to vision/ignore touch). Multisensory spatial interference emerged in both tasks for both groups. Crucially, deaf participants showed increased interference compared to hearing adults when they attempted to respond to tactile targets and ignore visual distractors, with enhanced difficulties with ipsilateral visual distractors. Analyses on task-order revealed that in deaf adults, interference of visual distractors on tactile targets was much stronger when this task followed the task in which vision was behaviorally relevant (respond to vision/ignore touch). These novel results suggest that behavioral/neural changes related to early deafness determine enhanced visual dominance during visuo-tactile multisensory conflict.

Spatial modulation of motor-sensory recalibration in early deaf individuals

Audition dominates other senses in temporal processing, and in the absence of auditory cues, temporal perception can be compromised. Moreover, after auditory deprivation, visual attention is selectively enhanced for peripheral visual stimuli. In this study, we assessed whether early hearing loss affects motor-sensory recalibration, the ability to adjust the timing of an action and its sensory effect based on the recent experience. Early deaf participants and hearing controls were asked to discriminate the temporal order between a motor action (a keypress) and a visual stimulus (a white circle) before and after adaptation to a delay between the two events. To examine the effects of spatial modulation, we presented visual stimuli in both central and peripheral visual fields. Results showed overall higher temporal JNDs (Just Noticeable Difference) for deaf participants as compared to hearing controls suggesting that the auditory information is important for the calibration of motor-sensory timing. Adaptation to a motor-sensory delay induced distinctive effect in the two groups of participants, with hearing controls showing a recalibration effect for central stimuli only whereas deaf individuals for peripheral visual stimuli only. Our results suggest that auditory deprivation affects motor-sensory recalibration and that the mechanism underlying motor-sensory recalibration is susceptible to spatial modulation.

Tactile short-term memory in sensory-deprived individuals

To verify whether loosing a sense or two has consequences on a spared sensory modality, namely touch, and whether these consequences depend on practice or are biologically determined, we investigated 13 deafblind participants, 16 deaf participants, 15 blind participants, and 13 matched normally sighted and hearing controls on a tactile short-term memory task, using checkerboard matrices of increasing length in which half of the squares were made up of a rough texture and half of a smooth one. Time of execution of a fixed matrix, number of correctly reproduced matrices, largest matrix correctly reproduced and tactile span were recorded. The three groups of sensory-deprived individuals did not differ in any measure, while blind and deaf participants outscored controls in all parameters except time of execution; the difference approached significance for deafblind people compared to controls only in one measure, namely correctly reproduced matrices. In blind and deafblind participants, performance negatively correlated with age of Braille acquisition, the older being the subject when acquiring Braille, the lower the performance, suggesting that practice plays a role. However, the fact that deaf participants, who did not share tactile experience, performed similarly to blind participants and significantly better than controls highlights that practice cannot be the only contribution to better tactile memory.

An exploratory study of visual sequential processing in children with cochlear implants

OBJECTIVE

The objective of the study was to compare visual sequential processing in school-age children with cochlear implants (CIs) and their normal-hearing (NH) peers. Visual sequential processing was examined using both behavioral and an event-related potential (ERP) measures.

METHODS

Eighteen children with CIs and nineteen children who had hearing within normal limits (NH) participated in the behavioral study. Subtests from the Test of Visual Perceptual Skills and the Sensory Integration and Praxis Test were administered to all children. ERP measures were collected from five children with CI and five age-matched peers. Peak latencies (N200 and P300) and reaction times for visual sequential processing were compared in these two groups.

RESULTS

The findings of the study revealed significant group differences in visual sequential memory and visuo-motor sequencing tasks suggesting that children with severe-profound hearing loss may have difficulties in visual sequential tasks. The study also revealed longer P300 latencies and longer reaction times for a visual sequential matching task in children with CI when compared to their NH peers suggesting slower or delayed processing of visual sequential stimuli.

CONCLUSIONS

This exploratory study involving behavioral and ERP measures showed that as a group, children with prelingual, severe-profound hearing loss who use CIs have difficulties with visual sequential processing. These findings may have implications for rehabilitation for children with hearing loss in the light of recent evidence that accurate and efficient processing of sequentially presented visual stimuli is important for language and reading outcomes.

Revisiting the adaptive and maladaptive effects of crossmodal plasticity

One of the most striking demonstrations of experience-dependent plasticity comes from studies of sensory-deprived individuals (e.g., blind or deaf), showing that brain regions deprived of their natural inputs change their sensory tuning to support the processing of inputs coming from the spared senses. These mechanisms of crossmodal plasticity have been traditionally conceptualized as having a double-edged sword effect on behavior. On one side, crossmodal plasticity is conceived as adaptive for the development of enhanced behavioral skills in the remaining senses of early-deaf or blind individuals. On the other side, crossmodal plasticity raises crucial challenges for sensory restoration and is typically conceived as maladaptive since its presence may prevent optimal recovery in sensory-re-afferented individuals. In the present review we stress that this dichotomic vision is oversimplified and we emphasize that the notions of the unavoidable adaptive/maladaptive effects of crossmodal reorganization for sensory compensation/restoration may actually be misleading. For this purpose we critically review the findings from the blind and deaf literatures, highlighting the complementary nature of these two fields of research. The integrated framework we propose here has the potential to impact on the way rehabilitation programs for sensory recovery are carried out, with the promising prospect of eventually improving their final outcomes.

Audiotactile interaction can change over time in cochlear implant users

Recent results suggest that audiotactile interactions are disturbed in cochlear implant (CI) users. However, further exploration regarding the factors responsible for such abnormal sensory processing is still required. Considering the temporal nature of a previously used multisensory task, it remains unclear whether any aberrant results were caused by the specificity of the interaction studied or rather if it reflects an overall abnormal interaction. Moreover, although duration of experience with a CI has often been linked with the recovery of auditory functions, its impact on multisensory performance remains uncertain. In the present study, we used the parchment-skin illusion, a robust illustration of sound-biased perception of touch based on changes in auditory frequencies, to investigate the specificities of audiotactile interactions in CI users. Whereas individuals with relatively little experience with the CI performed similarly to the control group, experienced CI users showed a significantly greater illusory percept. The overall results suggest that despite being able to ignore auditory distractors in a temporal audiotactile task, CI users develop to become greatly influenced by auditory input in a spectral audiotactile task. When considered with the existing body of research, these results confirm that normal sensory interaction processing can be compromised in CI users.

Temporary deafness can impair multisensory integration: a study of cochlear-implant users

Previous investigations suggest that temporary deafness can have a dramatic impact on audiovisual speech processing. The aim of this study was to test whether temporary deafness disturbs other multisensory processes in adults. A nonspeech task involving an audiotactile illusion was administered to a group of normally hearing individuals and a group of individuals who had been temporarily auditorily deprived. Members of this latter group had their auditory detection thresholds restored to normal levels through the use of a cochlear implant. Control conditions revealed that auditory and tactile discrimination capabilities were identical in the two groups. However, whereas normally hearing individuals integrated auditory and tactile information, so that they experienced the audiotactile illusion, individuals who had been temporarily deprived did not. Given the basic nature of the task, failure to integrate multisensory information could not be explained by the use of the cochlear implant. Thus, the results suggest that normally anticipated audiotactile interactions are disturbed following temporary deafness.

Reduced visual discrimination in cochlear implant users

The aim of the study was to investigate low-level visual function in cochlear implant users. Spatial frequency discrimination was assessed in 16 adults with normal hearing and 18 adults with profound deafness who had a cochlear implant. Thresholds were measured with sinusoidal gratings using a two-alternative temporal forced-choice procedure combined with an adaptive staircase. Cochlear implant users had significantly poorer spatial frequency discrimination compared with normal hearing participants. Therefore, auditory privation leads to substantial changes in this particular visual function and these changes remain even after the restoration of hearing with a cochlear implant.

Multisensory processing in children with cochlear implants

OBJECTIVE

The objective of the study was to investigate the functions of the vestibular, tactile, visual, and proprioceptive modalities in children with prelinguistic hearing loss. Specifically, the study sought to examine evidence for the compensation hypothesis (auditory deprivation leads to enhancements in functions of other sensory systems) versus the deficiency hypothesis (functions of the spared senses are compromised as a result of auditory deprivation).

METHODS

Twelve children between the ages of 5 years, 1 month and 8 years, 11 months with bilateral severe-profound hearing loss participated in the study. All children used bilateral cochlear implants. Subtests of two norm-referenced instruments, the Sensory Integration and Praxis Test and the Beery-Buktenica Developmental Test of Visual-Motor Integration, were administered to all of the children. Mean subtest scores for children with hearing loss were compared with the mean scores for the normative group.

RESULTS

As expected, a majority of children demonstrated vestibular dysfunction. Assessment of other modalities revealed significantly below average performance on two tasks by children with hearing loss when compared to the normative group. Both tasks involved temporal processing of tactile or proprioceptive signals. Conversely, children with hearing loss showed significantly better performance compared to the normative group for a spatial task that involved localization of a tactile stimulus. No group differences were found for any of the visual tasks.

CONCLUSIONS

Results supported both the compensation and the deficiency hypotheses. The findings have implications for both assessment and intervention of young children with hearing loss. Further investigations are necessary to replicate these findings with a more comprehensive set of measures on a larger cohort of children with prelinguistic, severe-profound hearing loss.

Hearing Shapes Our Perception of Time: Temporal Discrimination of Tactile Stimuli in Deaf People

Confronted with the loss of one type of sensory input, we compensate using information conveyed by other senses. However, losing one type of sensory information at specific developmental times may lead to deficits across all sensory modalities. We addressed the effect of auditory deprivation on the development of tactile abilities, taking into account changes occurring at the behavioral and cortical level. Congenitally deaf and hearing individuals performed two tactile tasks, the first requiring the discrimination of the temporal duration of touches and the second requiring the discrimination of their spatial length. Compared with hearing individuals, deaf individuals were impaired only in tactile temporal processing. To explore the neural substrate of this difference, we ran a TMS experiment. In deaf individuals, the auditory association cortex was involved in temporal and spatial tactile processing, with the same chronometry as the primary somatosensory cortex. In hearing participants, the involvement of auditory association cortex occurred at a later stage and selectively for temporal discrimination. The different chronometry in the recruitment of the auditory cortex in deaf individuals correlated with the tactile temporal impairment. Thus, early hearing experience seems to be crucial to develop an efficient temporal processing across modalities, suggesting that plasticity does not necessarily result in behavioral compensation.

Measures of tactual detection and temporal order resolution in congenitally deaf and normal-hearing adults

To guide the development of tactile speech aids, tactual detection and temporal order discrimination by congenitally deaf and normal-hearing adults have been examined. Tactual detection thresholds for sinusoidal vibrations between 2 and 300 Hz were measured at the left thumb and index finger using an adaptive paradigm. Temporal onset- and offset-order discrimination were tested using stimuli of 50 Hz at the thumb and 250 Hz at the index finger, delivered asynchronously and varied independently in amplitude and duration. Mean detection thresholds for the deaf and normal-hearing groups did not differ significantly at any frequency tested. Temporal onset-order discrimination thresholds varied widely, particularly among congenitally deaf individuals, but no statistically significant difference was found between group means. Both experimental groups exhibited a broad range of discrimination thresholds for temporal offset-order, and mean thresholds did not differ significantly. On the whole, tactual offset-order thresholds were substantially higher than onset-order thresholds. Differences in the relative levels of paired stimuli systematically affected sensitivity to both onset- and offset-orders in most subjects. Differences in the relative durations of paired stimuli had little effect on onset-order discrimination, but had a robust effect on offset-order discrimination thresholds, which was consistent across all subjects.

Temporal resolution in regions of normal hearing and speech perception in noise for adults with sloping high-frequency hearing loss

OBJECTIVES

(1) To determine whether high-frequency sensorineural hearing loss (HF SNHL) is accompanied by deterioration in temporal resolution in the low-frequency region where hearing sensitivity is within normal range. (2) To evaluate whether such temporal processing deficits contribute to speech perception difficulty in noise.

DESIGN

A between-group design was employed, using subjects either with or without high-frequency hearing loss and matched by age. Temporal resolution was evaluated in amplitude modulation (AM) detection and gap detection tasks. To restrict evaluation to the low-frequency regions where the auditory sensitivity was virtually normal, low-pass noise carriers (for AM detection) and gap markers (for gap detection) were used. The impact of temporal processing deficits on speech perception was evaluated using hearing in noise tests (HINT) with varied time compression rates of the speech materials.

RESULTS

Adults with high-frequency hearing loss showed poorer performance than the age-matched normal-hearing subjects on both the AM and gap detection tasks, even though the stimuli were restricted to regions of observed normal sensitivity. With increasing time compression, listeners with HF SNHL required a larger signal to noise ratio to maintain accuracy in speech perception in adaptive HINT and exhibited a bigger decrease in score for HINTs at a fixed signal to noise ratio. Multiple regression/correlation analyses show significant correlation across the scores of AM/gap detection tasks and HINTs.

CONCLUSIONS

Temporal resolution in the low-frequency region with near-normal sensitivity seems to be deteriorated in subjects with HF SNHL. They were more sensitive to increases in speech rate, suggesting that poorer temporal processing may be related to speech perception deficits in noise.