Cortical Plasticity and Olfactory Function in Early Blindness

Developmental plasticity of the structural network of the occipital cortex in congenital blindness

The occipital cortex is the visual processing center in the mammalian brain. An unanswered scientific question pertains to the impact of congenital visual deprivation on the development of various profiles within the occipital network. To address this issue, we recruited 30 congenitally blind participants (8 children and 22 adults) as well as 31 sighted participants (10 children and 21 adults). Our investigation focused on identifying the gray matter regions and white matter connections within the occipital cortex, alongside behavioral measures, that demonstrated different developmental patterns between blind and sighted individuals. We discovered significant developmental changes in the gray matter regions and white matter connections of the occipital cortex among blind individuals from childhood to adulthood, in comparison with sighted individuals. Moreover, some of these structures exhibited cognitive functional reorganization. Specifically, in blind adults, the posterior occipital regions (left calcarine fissure and right middle occipital gyrus) showed reorganization of tactile perception, and the forceps major tracts were reorganized for braille reading. These plastic changes in blind individuals may be attributed to experience-dependent neuronal apoptosis, pruning, and myelination. These findings provide valuable insights into the longitudinal neuroanatomical and cognitive functional plasticity of the occipital network following long-term visual deprivation.

Brain Morphological Modifications in Congenital and Acquired Auditory Deprivation: A Systematic Review and Coordinate-Based Meta-Analysis

Neuroplasticity following deafness has been widely demonstrated in both humans and animals, but the anatomical substrate of these changes is not yet clear in human brain. However, it is of high importance since hearing loss is a growing problem due to aging population. Moreover, knowing these brain changes could help to understand some disappointing results with cochlear implant, and therefore could improve hearing rehabilitation. A systematic review and a coordinate-based meta-analysis were realized about the morphological brain changes highlighted by MRI in severe to profound hearing loss, congenital and acquired before or after language onset. 25 papers were included in our review, concerning more than 400 deaf subjects, most of them presenting prelingual deafness. The most consistent finding is a volumetric decrease in gray matter around bilateral auditory cortex. This change was confirmed by the coordinate-based meta-analysis which shows three converging clusters in this region. The visual areas of deaf children is also significantly impacted, with a decrease of the volume of both gray and white matters. Finally, deafness is responsible of a gray matter increase within the cerebellum, especially at the right side. These results are largely discussed and compared with those from deaf animal models and blind humans, which demonstrate for example a much more consistent gray matter decrease along their respective primary sensory pathway. In human deafness, a lot of other factors than deafness could interact on the brain plasticity. One of the most important is the use of sign language and its age of acquisition, which induce among others changes within the hand motor region and the visual cortex. But other confounding factors exist which have been too little considered in the current literature, such as the etiology of the hearing impairment, the speech-reading ability, the hearing aid use, the frequent associated vestibular dysfunction or neurocognitive impairment. Another important weakness highlighted by this review concern the lack of papers about postlingual deafness, whereas it represents most of the deaf population. Further studies are needed to better understand these issues, and finally try to improve deafness rehabilitation.

Altered Brain Functional Connectivity at Resting-State in Patients With Non-arteritic Anterior Ischemic Optic Neuropathy

Purpose: To investigate the possible changes in functional connectivity (FC) in patients with non-arteritic anterior ischemic optic neuropathy (NAION) using resting-state functional MRI (fMRI).

Methods: Thirty-one NAION patients and 31 healthy controls were recruited and underwent resting-state fMRI scans. Regions of interest (ROIs) were defined as bilateral Brodmann’s area 17 (BA17). FC analysis was performed between the ROIs and the rest of the brain regions, and the between group comparisons of FC were performed. We conducted correlation analysis between the FC changes and the clinical variables in NAION patients.

Results: Compared with healthy controls, patients with NAION showed significantly decreased FC between the left BA17 and the right inferior frontal gyrus, left caudate nucleus. As for the right BA17, patients exhibited significantly increased FC with the left olfactory gyrus and decreased FC with the right superior frontal gyrus (SFG), right insula. Moreover, FC values between the right insula and the right BA17 were positively correlated with the right side of mean sensitivity in the central visual field (r = 0.52, P < 0.01) and negatively correlated with the right side of mean defect in the central visual field (r = −0.55, P < 0.01).

Conclusion: Our study indicated that patients with NAION showed significantly abnormal functional reorganization between the primary visual cortex and several other brain regions not directly related to visual function, which supports that NAION may not only be an ophthalmic disease but also a neuro-ophthalmological disease.

Effects of auditory feedback on gait behavior, gaze patterns and outcome performance in long jumping

In the current study, we conducted two experiments to investigate the impact of concurrent, action-induced auditory feedback on gait patterns, gaze behavior and outcome performance in long jumping. In Experiment 1, we examined the effects of present vs. absent auditory feedback on gait, gaze and performance outcome measures. Results revealed a significant interaction effect between condition (present vs. absent auditory feedback) and phase (acceleration vs. zeroing-in phase) on participants' step lengths indicating that the absence (rather than the presence) of auditory feedback led to facilitatory effects in terms of a more prototypical gait pattern (i.e., shorter steps in the acceleration phase and longer steps in the zeroing-in phase). Similarly, the absent auditory feedback led to a higher gaze stability in terms of less switches between areas of interest (AOIs). However, there was no effect on jumped distance. In Experiment 2, we scrutinized the influence of concurrent vs. delayed auditory feedback on all three performance parameters. In contrast to concurrent feedback, delayed auditory feedback negatively affected all three measures: participants showed (i) dysfunctional deviations from their prototypical gait pattern (i.e., shorter steps across both phases of the run-up), (ii) less stable, maladaptive gaze patterns (i.e., more switches between AOIs) and (iii) poorer jumping performance (i.e., shorter jumped distances). Together, the two experiments provide clear evidence for the impact of concurrent, action-induced auditory feedback on the coordination of complex, rhythmical motor tasks such as the long jump.

Better Olfactory Performance and Larger Olfactory Bulbs in a Mouse Model of Congenital Blindness

It is well established that early blindness results in enhancement of the remaining nonvisual sensory modalities accompanied by functional and anatomical brain plasticity. While auditory and tactile functions have been largely investigated, the results regarding olfactory functions remained less explored and less consistent. In the present study, we investigated olfactory function in blind mice using 3 tests: the buried food test, the olfactory threshold test, and the olfactory performance test. The results indicated better performance of blind mice in the buried food test and odor performance test while there was no difference in the olfactory threshold test. Using histological measurements, we also investigated if there was anatomical plasticity in the olfactory bulbs (OB), the most salient site for olfactory processing. The results indicated a larger volume of the OB driven by larger glomerular and granular layers in blind mice compared with sighted mice. Structural plasticity in the OB may underlie the enhanced olfactory performance in blind mice.

Structural brain plasticity induced by early blindness

It is well established that early blindness results in behavioural adaptations. While the functional effects of visual deprivation have been well researched, anatomical studies are scarce. The aim of this study was to investigate whole brain structural plasticity in a mouse model of congenital blindness. Volumetric analyses were conducted on high-resolution MRI images and histological sections from the same brains. These morphometric measurements were compared between anophthalmic and sighted ZRDBA mice obtained by breeding ZRDCT and DBA mice. Results from MRI analyses using the Multiple Automatically Generated Templates (MAGeT) method showed smaller volume for the primary visual cortex and superior colliculi in anophthalmic compared with sighted mice. Deformation-based morphometry revealed smaller volumes within the dorsal lateral geniculate nuclei and the lateral secondary visual cortex and larger volumes within olfactory areas, piriform cortex, orbital areas and the amygdala, in anophthalmic compared with sighted mice. Histological analyses revealed a larger volume for the amygdala and smaller volume for the superior colliculi, primary visual cortex and medial secondary visual cortex, in anophthalmic compared with sighted mice. The absence of superficial visual layers of the superior colliculus and the thinner cortical layer IV of the primary and secondary visual cortices may explain the smaller volume of these areas, although this was observed in a limited sample. The present study shows large-scale brain plasticity in a mouse model of congenital blindness. In addition, the congruence of MRI and histological findings support the use of MRI to investigate structural brain plasticity in the mouse.

No Olfactory Compensation in Food-related Hazard Detection Among Blind and Deaf Adults: A Psychophysical Approach

The exposure-driven olfactory compensation associated with sensory loss is likely to be observed in assessment of food-related dangers. Therefore, in the current study we tested the hypothesis that olfactory compensation occurs in the case of protection from food-related hazards. We compared thresholds for detection of an unpleasant rotten food odor (fermented fish sauce) in four groups of subjects: blind subjects (n = 100), sighted controls (n = 100), deaf subjects (n = 74) and hearing controls (n = 99). Overall, we observed no significant differences in smell acuity between the blind and deaf groups and their matched control samples. However, the sensory deprived subjects assessed their sensitivity as higher than did control groups. The present study is yet another example of research among large samples of sensory deprived individuals that shows no evidence of olfactory compensation. This result is consistent with a growing number of studies suggesting no sensory compensation in simple, absolute sensitivity tasks.

Neuroplasticity in visual impairments

The visual acuity loss enables the brain to access new pathways in the quest to overcome the visual limitation and this is wellknown as neuroplasticity which have mechanisms to cortical reorganization. In this review, we related the evidences about the neuroplasticity as well as cortical anatomical differences and functional repercussions in visual impairments. We performed a systematic review of PUBMED database, without date or status publication restrictions. The findings demonstrate that the visual impairment produce a compensatory sensorial effect, in which non-visual areas are related to both cross (visual congenital) and multimodal (late blind) neuroplasticity.

Selective sensory deafferentation induces structural and functional brain plasticity

Sensory-motor integration models have been proposed aiming to explain how the brain uses sensory information to guide and check the planning and execution of movements. Sensory neuronopathy (SN) is a peculiar disease characterized by exclusive, severe and widespread sensory loss. It is a valuable condition to investigate how sensory deafferentation impacts brain organization. We thus recruited patients with clinical and electrophysiological criteria for SN to perform structural and functional MRI analyses. We investigated volumetric changes in gray matter (GM) using anatomical images; the microstructure of WM within segmented regions of interest (ROI), via diffusion images; and brain activation related to a finger tapping task. All significant results were related to the long disease duration subgroup of patients. Structural analysis showed hypertrophy of the caudate nucleus, whereas the diffusion study identified reduction of fractional anisotropy values in ROIs located around the thalamus and the striatum. We also found differences regarding finger-tapping activation in the posterior parietal regions and in the medial areas of the cerebellum. Our results stress the role of the caudate nucleus over the other basal ganglia in the sensory-motor integration models, and suggest an inhibitory function of a recently discovered tract between the thalamus and the striatum. Overall, our findings confirm plasticity in the adult brain and open new avenues to design neurorehabilitation strategies.

Visual loss alters multisensory face maps in humans

Topographically organised responses to visual and tactile stimulation are aligned in the ventral intraparietal cortex. The critical biological importance of this region, which is thought to mediate visually guided defensive movements of the head and upper body, suggests that these maps might be hardwired from birth. Here, we investigated whether visual experience is necessary for the creation and positioning of these maps by assessing the representation of tactile stimulation in congenitally and totally blind participants, who had no visual experience, and late and totally blind participants. We used a single-subject approach to the analysis to focus on the potential individual differences in the functional neuroanatomy that might arise from different causes, durations and sensory experiences of visual impairment among participants. The overall results did not show any significant difference between congenitally and late blind participants; however, single-subject trends suggested that visual experience is not necessary to develop topographically organised maps in the intraparietal cortex, whilst losing vision disrupted topographic maps' integrity and organisation. These results discussed in terms of brain plasticity and sensitive periods.

The Effect of Blindness on Long-Term Episodic Memory for Odors and Sounds

We recently showed that compared with sighted, early blind individuals have better episodic memory for environmental sounds, but not odors, after a short retention interval (∼ 8 – 9 min). Few studies have investigated potential effects of blindness on memory across long time frames, such as months or years. Consequently, it was unclear whether compensatory effects may vary as a function of retention interval. In this study, we followed-up participants (N = 57 out of 60) approximately 1 year after the initial testing and retested episodic recognition for environmental sounds and odors, and identification ability. In contrast to our previous findings, the early blind participants (n = 14) performed at a similar level as the late blind (n = 13) and sighted (n = 30) participants for sound recognition. Moreover, the groups had similar recognition performance of odors and identification ability of odors and sounds. These findings suggest that episodic odor memory is unaffected by blindness after both short and long retention intervals. However, the effect of blindness on episodic memory for sounds may vary as a function of retention interval, such that early blind individuals have an advantage over sighted across short but not long time frames. We speculate that the finding of a differential effect of blindness on auditory episodic memory across retention intervals may be related to different memory strategies at initial and follow-up assessments. In conclusion, this study suggests that blindness does not influence auditory or olfactory episodic memory as assessed after a long retention interval.

Olfactory perception and blindness: a systematic review and meta-analysis

Anecdotal reports suggest that blind people might develop supra-normal olfactory abilities. However, scientific evidence shows a mixed pattern of findings. Inconsistent observations are reported for both sensory-driven olfactory tasks (e.g., odor threshold) and higher-order olfactory functions (e.g., odor identification). To quantify the evidence systematically, we conducted a review and meta-analysis. Studies were included if they examined olfactory function (i.e., odor threshold, odor discrimination, free odor identification, or cued odor identification) in blind compared with a sighted control group. Articles were identified through computerized literature search. A total of 18 studies focused on olfactory threshold (n = 1227: 590 blind and 637 sighted individuals), 14 studies targeted discrimination (n = 940: 455 blind and 485 sighted), 14 studies measured cued identification (n = 968: 468 blind and 500 sighted), and 7 studies (n = 443: 224 blind and 219 sighted individuals) assessed free identification. Overall, there were no differences in effect sizes between the blind and sighted individuals after correcting the results for publication bias. We additionally conducted an exploratory analysis targeting the role played by three moderators of interests: participants’ age, the proportion of women versus men in each of the studies included into meta-analysis and onset of blindness (early blind vs. late-blind). However, none of the moderators affected the observed results. To conclude, blindness seems not to affect cued/free odor identification, odor discrimination or odor thresholds.

Brain (re)organization following visual loss

The study of the neural consequences of sensory loss provides a unique window into the brain's functional and organizational principles. Although the blind visual cortex has been implicated in the cross-modal processing of nonvisual inputs for quite some time, recent research has shown that certain cortical organizational principles are preserved even in the case of complete sensory loss. Furthermore, a growing body of work has shown that markers of neuroplasticity extend to neuroanatomical metrics that include cortical thickness and myelinization. Although our understanding of the mechanisms that underlie sensory deprivation-driven cross-modal plasticity is improving, several critical questions remain unanswered. The specific pathways that underlie the rerouting of nonvisual information, for instance, have not been fully elucidated. The fact that important cross-modal recruitment occurs following transient deprivation in sighted individuals suggests that significant rewiring following blindness may not be required. Furthermore, there are marked individual differences regarding the magnitude and functional relevance of the cross-modal reorganization. It is also not clear to what extent precise environmental factors may play a role in establishing the degree of reorganization across individuals, as opposed to factors that might specifically relate to the cause or the nature of the visual loss. In sum, although many unresolved questions remain, sensory deprivation continues to be an excellent model for studying the plastic nature of the brain. This article is categorized under: Psychology > Brain Function and Dysfunction Psychology > Perception and Psychophysics Neuroscience > Plasticity.

Developmental evolution and developmental plasticity of the olfactory epithelium and olfactory skills in Mexican cavefish

The fish Astyanax mexicanus comes in two forms: the normal surface-dwelling (SF) and the blind depigmented cave-adapted (CF) morphs. Among many phenotypic differences, cavefish show enhanced olfactory sensitivity to detect amino-acid odors and they possess large olfactory sensory organs. Here, we questioned the relationship between the size of the olfactory organ and olfactory capacities. Comparing olfactory detection abilities of CF, SF and F1 hybrids with various olfactory epithelium (OE) sizes in behavioral tests, we concluded that OE size is not the only factor involved. Other possibilities were envisaged. First, olfactory behavior was tested in SF raised in the dark or after embryonic lens ablation, which leads to eye degeneration and mimics the CF condition. Both absence of visual function and absence of visual organs improved the SF olfactory detection capacities, without affecting the size of their OE. This suggested that developmental plasticity occurs between the visual and the olfactory modalities, and can be recruited in SF after visual deprivation. Second, the development of the olfactory epithelium was compared in SF and CF in their first month of life. Proliferation, cell death, neuronal lifespan, and olfactory progenitor cell cycling properties were identical in the two morphs. By contrast, the proportions of the three main olfactory sensory neurons subtypes (ciliated, microvillous and crypt) in their OE differed. OMP-positive ciliated neurons were more represented in SF, TRPC2-positive microvillous neurons were proportionately more abundant in CF, and S100-positive crypt cells were found in equal densities in the two morphs. Thus, general proliferative properties of olfactory progenitors are identical but neurogenic properties differ and lead to variations in the neuronal composition of the OE in SF and CF. Together, these experiments suggest that there are at least two components in the evolution of cavefish olfactory skills: (1) one part of eye-dependent developmental phenotypic plasticity, which does not depend on the size of the olfactory organ, and (2) one part of developmental evolution of the OE, which may stem from embryonic specification of olfactory neurons progenitor pools.

Adaptive and maladaptive neural compensatory consequences of sensory deprivation-From a phantom percept perspective

It is suggested that the brain undergoes plastic changes in order to adapt to changing environmental needs. Sensory deprivation results in decreased input to the brain leading to adaptive or maladaptive changes. Although several theories hypothesize the mechanism of these adaptive and maladaptive changes, the course of action taken by the brain heavily depends on the age of incidence of damage. The growing body of literature on the topic proposes that maladaptive changes in the brain are instrumental in creating phantom percepts, defined as the perception of a sensory experience in the absence of a physical stimulus. The current article reviews the mechanisms of adaptive and maladaptive plasticity in the brain in congenital, early, and late-onset sensory deprivation in conjunction with the phantom percepts in the different sensory domains. We propose that the mechanisms of adaptive and maladaptive plasticity fall under a universal construct of updating hierarchical Bayesian prediction errors. This theory of the Bayesian brain hypothesizes that the brain constantly compares its internal milieu with changing environmental cues and either adjusts its predictions or discards the change, depending on the novelty or salience of the external stimulus. We propose that adaptive plasticity reflects both successful bottom-up compensation and top-down updating of the model while maladaptive plasticity reflects failure in one or both mechanisms, resulting in a constant prediction-error. Finally, we hypothesize that phantom percepts are generated by the brain as a solution to this prediction error and are thus a manifestation of unsuccessful adaptation to sensory deprivation.