Auditory deprivation affects biases of visuospatial attention as measured by line bisection

In this study, we investigated whether early deafness affects the typical pattern of hemispheric lateralization [i.e., right hemisphere (RH) dominance] in the control of spatial attention. To this aim, deaf signers, deaf non-signers, hearing signers, and hearing non-signers were required to bisect a series of centrally presented visual lines. The directional bisection bias was found to be significantly different between hearing and deaf participants, irrespective of sign language use. Hearing participants (both signers and non-signers) showed a consistent leftward bias, reflecting RH dominance. Conversely, we observed no evidence of a clear directional bias in deaf signers or non-signers (deaf participants overall showing a non-significant tendency to deviate rightward), suggesting that deafness may be associated to a more bilateral hemispheric engagement in visuospatial tasks.     

Routes to short-term memory indexing: Lessons from deaf native users of American Sign Language

Models of working memory (WM) have been instrumental in understanding foundational cognitive processes and sources of individual differences. However, current models cannot conclusively explain the consistent group differences between deaf signers and hearing speakers on a number of short-term memory (STM) tasks. Here we take the perspective that these results are not due to a temporal order-processing deficit in deaf individuals, but rather reflect different biases in how different types of memory cues are used to do a given task. We further argue that the main driving force behind the shifts in relative biasing is a consequence of language modality (sign vs. speech) and the processing they afford, and not deafness, per se.     

A critical period for right hemisphere recruitment in American Sign Language processing.

Signed languages such as American Sign Language (ASL) are natural languages that are formally similar to spoken languages, and thus present an opportunity to examine the effects of language structure and modality on the neural organization for language. Native learners of spoken languages show predominantly left-lateralized patterns of neural activation for language processing, whereas native learners of ASL show extensive right hemisphere (RH) and LH activation. We demonstrate that the RH angular gyrus is active during ASL processing only in native signers (hearing, ASL-English bilinguals) but not in those who acquired ASL after puberty (hearing, native English speakers). This is the first demonstration of a 'sensitive' or 'critical' period for language in an RH structure. This has implications for language acquisition and for understanding age-related changes in neuroplasticity more generally.     

Superior spatial touch: improved haptic orientation processing in deaf individuals

The present study investigated haptic spatial orientation processing in deaf signers, hearing sign language interpreters, and hearing controls. Blindfolded participants had to set two bars parallel in the horizontal plane, with either a 2-s or a 10-s delay between inspection of the reference bar and the setting of the test bar. The deaf group outperformed the other two groups which did not differ from each other. Together these results indicate that deaf individuals can better identify the allocentric spatial coordinates of haptically inspected orientations. These results are discussed in terms of the possible neurocognitive consequences of auditory deprivation.     

Selective attentional processing and the right hemisphere: effects of aging and alcoholism.

Thirty-seven nonalcoholic individuals (22 women, 15 men), ages 26-76, and 36 abstinent alcoholic individuals (11 women, 25 men), ages 31-74, participated in a cued-detection task that assessed right hemisphere (RH) functioning associated with aging and alcoholism. Young controls were less reliant on cues following RH activation, which is consistent with the view that the RH has an advantage because it has the ability to attend to a broader spatial array than does the left hemisphere (LH). This RH advantage was not obtained in older controls or alcoholic participants. The pattern of results for the older nonalcoholic participants indicated that they neither benefited from valid cues following LH activation nor exhibited enhanced processing on invalid cue trials following RH activation. The results for the alcoholic participants were consistent with RH functional decline, but did not support the view that alcoholism and aging have synergistic effects.     

Hemiface contributions to hemispheric dominance in visual speech perception

Lateralized displays are used widely to investigate hemispheric asymmetry in language perception. However, few studies have used lateralized displays to investigate hemispheric asymmetry in visual speech perception, and those that have yielded mixed results. This issue was investigated in the current study by presenting visual speech to either the left hemisphere (LH) or the right hemisphere (RH) using the face as recorded (normal), a mirror image of the normal face (reversed), and chimeric displays constructed by duplicating and reversing just one hemiface (left or right) to form symmetrical images (left-duplicated, right-duplicated). The projection of displays to each hemisphere was controlled precisely by an automated eye-tracking technique. Visual speech perception showed the same, clear LH advantage for normal and reversed displays, a greater LH advantage for right-duplicated displays, and no hemispheric difference for left-duplicated displays. Of particular note is that perception of LH displays was affected greatly by the presence of right-hemiface information, whereas perception of RH displays was unaffected by changes in hemiface content. Thus, when investigated under precise viewing conditions, the indications are not only that the dominant processes of visual speech perception are located in the LH but that these processes are uniquely sensitive to right-hemiface information.     

Spatial localization in patients with unilateral posterior left or right hemisphere lesions

In this study, perceptual and visuomotor spatial localization were examined in patients with unilateral right (RH) or left (LH) hemisphere lesions and in a group of control subjects. Perceptual localization was measured with a position-discrimination task; in the visuomotor localization task, subjects had to point to a visual target. Both tasks were investigated in conditions with or without background visibility and with central and peripheral targets. In the visuomotor task, hand visibility was also manipulated. In both tasks, targets were presented in the left and right visual hemifield. The perceptual task revealed impairments for both LH and RH patients in the contra-lateral visual hemifield. RH patients also revealed slightly larger impairments in conditions without a visual background. In the visuomotor task, the LH patients were not impaired, whereas the RH patients were impaired in conditions without hand visibility and/or background visibility. Hence, our data strengthen the idea that spatial localization is not a unitary function and that perceptual and visuomotor localization can be selectively impaired. We suggest that one of the important factors distinguishing between localization impairments in RH and LH patients might be absolute versus relative localization.     

Response speed advantage for vision does not extend to touch in early deaf adults

Early deaf adults typically respond faster than hearing controls when performing a speeded simple detection on visual targets. Whether this response time advantage can generalise to another intact modality (touch) or it is instead specific to visual processing remained unexplored. We tested eight early deaf adults and twelve hearing controls in a simple detection task, with visual or tactile targets delivered on the arms and occupying the same locations in external space. Catch trials were included in the experimental paradigm. Results revealed a response time advantage in deaf adults compared to hearing controls, selectively for visual targets. This advantage did not extend to touch. The number of anticipation errors was negligible and comparable in both groups. The present findings strengthen the notion that response time advantage in deaf adults emerges as a consequence of changes specific to visual processing. They also exclude the involvement of sensory-unspecific cognitive mechanisms in this improvement (e.g. increased impulsivity in initiation of response, longer-lasting sustained attention or higher motivation to perform the task). Finally, they provide initial evidence that the intact sensory modalities can reorganise independently from each other following early auditory deprivation.     

ERP evidence for hemispheric asymmetries in abstract but not exemplar‐specific repetition priming

Implicit memory retrieval is thought to be exemplar‐specific in the right hemisphere (RH) but abstract in the left hemisphere (LH). Yet, conflicting behavioral priming results illustrate that the level at which asymmetries take effect is difficult to pinpoint. In the present divided visual field experiment, we tried to address this issue by analyzing ERPs in addition to behavioral measures. Participants made a natural/artificial decision on lateralized visual objects that were either new, identical repetitions, or different exemplars of studied items. Hemispheric asymmetries did not emerge in either behavioral or late positive complex (LPC) priming effects, but did affect the process of implicit memory retrieval proper as indexed by an early frontal negativity (N350/(F)N400). Whereas exemplar‐specific N350/(F)N400 priming effects emerged irrespective of presentation side, abstract implicit memory retrieval of different exemplars was contingent on right visual field presentation and the ensuing initial stimulus processing by the LH.     

Word position affects stimulus recognition: Evidence for early ERP short-term plastic modulation

The present study was aimed at investigating the short-term plastic changes that follow word learning at a neurophysiological level. The main hypothesis was that word position (left or right visual field, LVF/RH or RVF/LH) in the initial learning phase would leave a trace that affected, in the subsequent recognition phase, the Recognition Potential (i.e., the first negative component distinguishing words from other stimuli) elicited 220–240 ms after centrally presented stimuli. Forty-eight students were administered, in the learning phase, 125 words for 4 s, randomly presented half in the left and half in the right visual field. In the recognition phase, participants were split into two equal groups, one was assigned to the Word task, the other to the Picture task (in which half of the 125 pictures were new, and half matched prior studied words). During the Word task, old RVF/LH words elicited significantly greater negativity in left posterior sites with respect to old LVF/RH words, which in turn showed the same pattern of activation evoked by new words. Therefore, correspondence between stimulus spatial position and hemisphere specialized in automatic word recognition created a robust prime for subsequent recognition. During the Picture task, pictures matching old RVF/LH words showed no differences compared with new pictures, but evoked significantly greater negativity than pictures matching old LVF/RH words. Thus, the priming effect vanished when the task required a switch from visual analysis to stored linguistic information, whereas the lack of correspondence between stimulus position and network specialized in automatic word recognition (i.e., when words were presented to the LVF/RH) revealed the implicit costs for recognition. Results support the view that short-term plastic changes occurring in a linguistic learning task interact with both stimulus position and modality (written word vs. picture representation).     

Visual temporal order judgment in profoundly deaf individuals

We investigated temporal processing in profoundly deaf individuals by testing their ability to make temporal order judgments (TOJs) for pairs of visual stimuli presented at central or peripheral visual eccentricities. Ten profoundly deaf participants judged which of the two visual stimuli appearing on opposite sides of central fixation was delivered first. Stimuli were presented either symmetrically, at central or peripheral locations, or asymmetrically (i.e. one central and the other peripheral) at varying stimulus onset asynchronies (SOAs) using the method of constant stimuli. Two groups of hearing controls were also tested in this task: 10 hearing controls auditory-deprived during testing and 12 hearing controls who were not subjected to any deprivation procedure. Temporal order thresholds (i.e. just noticeable differences) and points of subjective simultaneity for the two visual stimuli did not differ between groups. However, faster discrimination responses were systematically observed in the deaf than in either group of hearing controls, especially when the first of the two stimuli appeared at peripheral locations. Contrary to some previous findings, our results show that a life-long auditory deprivation does not alter temporal processing abilities in the millisecond range. In fact, we show that deaf participants obtain similar temporal thresholds to hearing controls, while also responding much faster. This enhanced reactivity is documented here for the first time in the context of a temporal processing task, and we suggest it may constitute a critical aspect of the functional changes occurring as a consequence of profound deafness.     

Exploring cerebral asymmetries for the verb generation task

This study investigated potential right hemisphere involvement in the verb generation task. Six divided visual field experiments explored cerebral asymmetries for word retrieval in the verb generation task as well as in rhyme generation and immediate and delayed word pronunciation. The typical right visual field/left hemisphere (RVF/LH) advantage was observed for pronunciation and rhyme generation. For verb generation, the RVF/LH advantage was obtained only when stimulus items had a single prepotent response and not when there were multiple response alternatives. A semantic priming experiment suggested that activation for less common, related verbs was maintained for a longer time course within the right than within the left hemisphere. The authors suggest that the right hemisphere may play a role in continued activation of semantically related response alternatives in word generation and discuss methodological implications of their findings.     

Fitness Among Individuals with Early Childhood Deafness: Studies in Alumni Families from Gallaudet University

The genetic fitness of an individual is influenced by their phenotype, genotype and family and social structure of the population in which they live. It is likely that the fitness of deaf individuals was quite low in the Western European population during the Middle Ages. The establishment of residential schools for deaf individuals nearly 400 years ago resulted in relaxed genetic selection against deaf individuals which contributed to the improved fitness of deaf individuals in recent times. As part of a study of deaf probands from Gallaudet University, we collected pedigree data, including the mating type and the number and hearing status of the children of 686 deaf adults and 602 of their hearing siblings. Most of these individuals had an onset of severe to profound hearing loss by early childhood. Marital rates of deaf adults were similar to their hearing siblings (0.83 vs. 0.85). Among married individuals, the fertility of deaf individuals is lower than their hearing siblings (2.06 vs. 2.26, p = 0.005). The fitness of deaf individuals was reduced (p = 0.002). Analysis of fertility rates after stratification by mating type reveals that matings between two deaf individuals produced more children (2.11) than matings of a deaf and hearing individual (1.85), suggesting that fertility among deaf individuals is influenced by multiple factors.     

Night-time right hemisphere superiority and daytime left hemisphere superiority: a repatterning of laterality across wake-sleep-wake states

During the sleep onset period (SOP), there is a state-specific relative advantage of the right hemisphere (RH). This superiority could either be due to the ability of the RH when operating at levels of reduced arousal, as suggested by its superiority in sustaining vigilance, or it could depend on a more pronounced homeostatic deactivation of the left hemisphere (LH). To evaluate whether variations of lateralization from wake to sleep are consistent with one of these two hypotheses, we considered a finger tapping task (FTT) bimanually performed during two wakeful conditions, during the SOP, upon experimental awakenings from stage 2 and REM sleep in the first and in the second part of the night, and upon morning awakening. The RH advantage in sustaining vigilance would be supported if the relative RH advantage was present throughout the sleep period. Finding that the repatterning of hemispheric asymmetry is present and/or prominent in the first part of the night would support the homeostatic hypothesis, i.e. when the homeostatic process is more pronounced. Results from 16 subjects revealed a significant superiority of the LH during the two wakeful conditions. At sleep onset, a clear reversal of dominance was observed and it was followed by a steady RH superiority upon both REM and NREM sleep awakenings, and upon the morning sleep-wake transition. Therefore, performance in a FTT reveals a repatterning of laterality across wake-sleep-wake states. These results are interpreted as consistent with the hypothesis concerning an advantage of the RH in sustaining vigilance.     

Short-term memory span: insights from sign language

Short-term memory (STM), or the ability to hold information in mind for a few seconds, is thought to be limited in its capacity to about 7 +/- 2 items. Notably, the average STM capacity when using American Sign Language (ASL) rather than English is only 5 +/- 1 items. Here we show that, contrary to previous interpretations, this difference cannot be attributed to phonological factors, item duration or reduced memory abilities in deaf people. We also show that, despite this difference in STM span, hearing speakers and deaf ASL users have comparable working memory resources during language use, indicating similar abilities to maintain and manipulate linguistic information. The shorter STM span in ASL users therefore confirms the view that the spoken span of 7 +/- 2 is an exception, probably owing to the reliance of speakers on auditory-based rather than visually based representations in linguistic STM, and calls for adjustments in the norms used with deaf individuals.     

A hierarchical analysis of block design errors in children with early focal brain damage

This study investigated the differential effects of very early damage to the left hemisphere (LH) or right hemisphere (RH) on visuospatial processing. Twenty-two children who had suffered either LH or RH strokes in the pre- or perinatal period were included in the study. The Block Design subtest of the Wechsler Intelligence Scale for Children-Revised (Wechsler, 1974) was used. Each missed item was coded as either a global error (e.g., broken configuration), local error (e.g., incorrect details), or time fail error (i.e., not completed within the allotted time). Results showed that the LH lesion and RH lesion groups had similar full scale IQs, verbal IQs, and performance IQs and were within the average to low average range. Block Design scaled scores were also within the average to low average range and did not significantly differ between the 2 lesion groups. Error analysis revealed, however, that the RH focal lesion group produced a significantly higher percentage of global errors than did the LH lesion group, whereas the LH lesion group produced a significantly higher percentage of local errors than did the RH lesion group. The groups did not differ on their percentage of time fail errors. These results are consistent with previous findings that suggest that the RH is involved in more global aspects of visual processing, whereas the LH mediates the more detailed, local aspects of visual information. The fact that these differences in processing are present after such early focal damage implies that hemispheric specialization for visuospatial processing occurs very early in brain development.     

Involvement of classical anterior and posterior language areas in sign language production, as investigated by 4 T functional magnetic resonance imaging

To investigate the cerebral organization for language production across the particular channels supporting linguistic behavior, a functional magnetic resonance (fMRI) study was conducted in deaf native users of American Sign Language (ASL) and age-matched hearing controls. Seven native ASL speakers and 15 vocal English speaking subjects covertly performed an object naming task inside the 4 T scanner using their native languages ASL or English, respectively. In subjects of both groups, classical language areas were found to be activated, including posterior Broca's area, the anterior insula, premotor cortex, and the posterior parts of the superior temporal cortex. Activations showed a predominance of the left hemisphere for both groups. In the deaf group, however, there was markedly larger involvement of the cerebellum, the inferior frontal gyrus, and the posterior insula and more robust activation in occipito-temporal and superior parietal cortices. In summary, it could be demonstrated by fMRI that native language production using ASL is associated with activation of classical language areas, although the neural organization for language processing is not identical in the two language modalities ASL and English language.     

Source Localisation of Visual Evoked Potentials in Congenitally Deaf Individuals

Previous studies have suggested that individuals deprived of auditory input can compensate with specific superior abilities in the remaining sensory modalities. To better understand the neural basis of deafness-induced changes, the present study used electroencephalography to examine visual functions and cross-modal reorganization of the auditory cortex in deaf individuals. Congenitally deaf participants and hearing controls were presented with reversing chequerboard stimuli that were systematically modulated in luminance ratio. The two groups of participants showed similar modulation of visual evoked potential (VEP) amplitudes (N85, P110) and latencies (P110) as a function of luminance ratio. Analysis of VEPs revealed faster neural processing in deaf participants compared with hearing controls at early stages of cortical visual processing (N85). Deaf participants also showed higher amplitudes (P110) than hearing participants. In contrast to our expectations, the results from VEP source analysis revealed no clear evidence for cross-modal reorganization in the auditory cortex of deaf participants. However, deaf participants tended to show higher activation in posterior parietal cortex (PPC). Moreover, modulation of PPC responses as a function of luminance was also stronger in deaf than in hearing participants. Taken together, these findings are an indication of more efficient neural processing of visual information in the deaf, which may relate to functional changes, in particular in multisensory parietal cortex, as a consequence of early auditory deprivation.     

Hemispheric differences for identification of words and nonwords in Urdu-English bilinguals

Hemispheric asymmetry was examined for Urdu-English bilinguals identifying printed Urdu words and nonwords, separated Urdu letter strings, digits, and English nonwords. In all cases, fewer errors occurred when stimuli were presented to the right visual field/left hemisphere (RVF/LH) than to the left visual field/right hemisphere (LVF/RH). Qualitative error patterns suggested that separated Urdu letter strings were processed more serially than Urdu letter strings joined to form words or pronounceable nonwords and more serially on RVF/LH than on LVF/RH trials. This qualitative laterality effect is similar to that found for Hebrew and Arabic but opposite that found for English and suggests that the qualitative manner of processing printed verbal material is influenced by language-specific factors such as scanning direction, orthographic-to-phonological mapping rules, and morphology.