Brain structural changes in blindness: a systematic review and an anatomical likelihood estimation (ALE) meta-analysis.

In recent decades, numerous structural brain imaging studies investigated purported morphometric changes in early (EB) and late onset blindness (LB). The results of these studies have not yielded very consistent results, neither with respect to the type, nor to the anatomical locations of the brain morphometric alterations. To better characterize the effects of blindness on brain morphometry, we performed a systematic review and an Anatomical-Likelihood-Estimation (ALE) coordinate-based-meta-analysis of 65 eligible studies on brain structural changes in EB and LB, including 890 EB, 466 LB and 1257 sighted controls. Results revealed atrophic changes throughout the whole extent of the retino-geniculo-striate system in both EB and LB, whereas changes in areas beyond the occipital lobe occurred in EB only. We discuss the nature of some of the contradictory findings with respect to the used brain imaging methodologies and characteristics of the blind populations such as the onset, duration and cause of blindness. Future studies should aim for much larger sample sizes, eventually by merging data from different brain imaging centers using the same imaging sequences, opt for multimodal structural brain imaging, and go beyond a purely structural approach by combining functional with structural connectivity network analyses.

Neural substrates of spatial processing and navigation in blindness: An activation likelihood estimation meta-analysis.

Even though vision is considered the best suited sensory modality to acquire spatial information, blind individuals can form spatial representations to navigate and orient themselves efficiently in space. Consequently, many studies support the amodality hypothesis of spatial representations since sensory modalities other than vision contribute to the formation of spatial representations, independently of visual experience and imagery. However, given the high variability in abilities and deficits observed in blind populations, a clear consensus about the neural representations of space has yet to be established. To this end, we performed a meta-analysis of the literature on the neural correlates of spatial processing and navigation via sensory modalities other than vision, like touch and audition, in individuals with early and late onset blindness. An activation likelihood estimation (ALE) analysis of the neuroimaging literature revealed that early blind individuals and sighted controls activate the same neural networks in the processing of non-visual spatial information and navigation, including the posterior parietal cortex, frontal eye fields, insula, and the hippocampal complex. Furthermore, blind individuals also recruit primary and associative occipital areas involved in visuo-spatial processing via cross-modal plasticity mechanisms. The scarcity of studies involving late blind individuals did not allow us to establish a clear consensus about the neural substrates of spatial representations in this specific population. In conclusion, the results of our analysis on neuroimaging studies involving early blind individuals support the amodality hypothesis of spatial representations.

A quantitative analysis of the retinofugal projections in congenital and late-onset blindness.

Vision loss early in life has dramatic consequences on the organization of the visual system and hence on structural plasticity of its remnant components. Most of the studies on the anatomical changes in the brain following visual deprivation have focused on the re-organization of the visual cortex and its afferent and efferent projections. In this study, we performed a quantitative analysis of the volume and size of the optic chiasm, optic nerve, optic tract and the lateral geniculate nucleus (LGN), the retino recipient thalamic nucleus. Analysis was carried out on structural T1-weighted MRIs from 22 congenitally blind (CB), 14 late blind (LB) and 29 age -and sex-matched sighted control (SC) subjects. We manually segmented the optic nerve, optic chiasm and optic tract, while LGN volumes were extracted using in-house software. We also measured voxel intensity of optic nerve, optic chiasm and optic tract. Mean volumes of the optic nerve, optic tract and optic chiasm were reduced by 50 to 60% in both CB and LB participants. No significant differences were found between the congenitally and late-onset blind participants for any of the measures. Our data further revealed reduced white matter voxel intensities in optic nerve, optic chiasm and optic tract in blind compared to sighted participants, suggesting decreased myelin content in the atrophied white matter. The LGN was reduced by 50% and 44% in CB and LB, respectively. In LB, optic nerve volume correlated negatively with the blindness duration index; no such correlation was found for optic chiasm, optic tract and LGN. The observation that despite the absence of visual input about half of the subcortical retinofugal projections are structurally preserved raises the question of their functional role. One possibility is that the surviving fibers play a role in the maintenance of circadian rhythms in the blind through the intrinsically photosensitive melanopsin-containing retinal ganglion cells.

Blindness and the Reliability of Downwards Sensors to Avoid Obstacles: A Study with the EyeCane.

Vision loss has dramatic repercussions on the quality of life of affected people, particularly with respect to their orientation and mobility. Many devices are available to help blind people to navigate in their environment. The EyeCane is a recently developed electronic travel aid (ETA) that is inexpensive and easy to use, allowing for the detection of obstacles lying ahead within a 2 m range. The goal of this study was to investigate the potential of the EyeCane as a primary aid for spatial navigation. Three groups of participants were recruited: early blind, late blind, and sighted. They were first trained with the EyeCane and then tested in a life-size obstacle course with four obstacles types: cube, door, post, and step. Subjects were requested to cross the corridor while detecting, identifying, and avoiding the obstacles. Each participant had to perform 12 runs with 12 different obstacles configurations. All participants were able to learn quickly to use the EyeCane and successfully complete all trials. Amongst the various obstacles, the step appeared to prove the hardest to detect and resulted in more collisions. Although the EyeCane was effective for detecting obstacles lying ahead, its downward sensor did not reliably detect those on the ground, rendering downward obstacles more hazardous for navigation.

Brain-Machine Interfaces to Assist the Blind.

The loss or absence of vision is probably one of the most incapacitating events that can befall a human being. The importance of vision for humans is also reflected in brain anatomy as approximately one third of the human brain is devoted to vision. It is therefore unsurprising that throughout history many attempts have been undertaken to develop devices aiming at substituting for a missing visual capacity. In this review, we present two concepts that have been prevalent over the last two decades. The first concept is sensory substitution, which refers to the use of another sensory modality to perform a task that is normally primarily sub-served by the lost sense. The second concept is cross-modal plasticity, which occurs when loss of input in one sensory modality leads to reorganization in brain representation of other sensory modalities. Both phenomena are training-dependent. We also briefly describe the history of blindness from ancient times to modernity, and then proceed to address the means that have been used to help blind individuals, with an emphasis on modern technologies, invasive (various type of surgical implants) and non-invasive devices. With the advent of brain imaging, it has become possible to peer into the neural substrates of sensory substitution and highlight the magnitude of the plastic processes that lead to a rewired brain. Finally, we will address the important question of the value and practicality of the available technologies and future directions.

Spatial navigation with horizontally spatialized sounds in early and late blind individuals.

Blind individuals often report difficulties to navigate and to detect objects placed outside their peri-personal space. Although classical sensory substitution devices could be helpful in this respect, these devices often give a complex signal which requires intensive training to analyze. New devices that provide a less complex output signal are therefore needed. Here, we evaluate a smartphone-based sensory substitution device that offers navigation guidance based on strictly spatial cues in the form of horizontally spatialized sounds. The system uses multiple sensors to either detect obstacles at a distance directly in front of the user or to create a 3D map of the environment (detection and avoidance mode, respectively), and informs the user with auditory feedback. We tested 12 early blind, 11 late blind and 24 blindfolded-sighted participants for their ability to detect obstacles and to navigate in an obstacle course. The three groups did not differ in the number of objects detected and avoided. However, early blind and late blind participants were faster than their sighted counterparts to navigate through the obstacle course. These results are consistent with previous research on sensory substitution showing that vision can be replaced by other senses to improve performance in a wide variety of tasks in blind individuals. This study offers new evidence that sensory substitution devices based on horizontally spatialized sounds can be used as a navigation tool with a minimal amount of training.