Minimum requirements for a retinal prosthesis to restore useful vision

Le but de ce projet est de déterminer les caractéristiques minimales pour qu'une prothèse rétinienne permette une vision utile. Pour aborder ce sujet, nous avons simulé une vision prosthétique chez des sujets normaux. Les résultats suggèrent que 400-500 phosphènes, arrangés rétinotopiquement sur une surface rétinienne de 3x2 mm², sont nécessaires pour coder l'information visuelle pour la majorité des tâches. Le champ visuel effectif doit cependant être optimisé (en utilisant par exemple un zoom) pour chaque tâche. Un champ visuel effectif de 2°x1.4° est nécessaire pour permettre une lecture efficace. Un champ visuel effectif d'environ 16°x12° permet une performance efficace pour les tâches de coordination visuomotrice. Enfin, les tâches de mobilité requièrent un champ visuel effectif d'environ 33°x23°. Si ces implants devaient être placés loin de la fovéa, une période d'apprentissage relativement longue sera nécessaire à l'obtention des performances optimales

Sensory neuroprostheses: from physiology to clinical application

Our senses are the main information channels through which we perceive and interact with the world. Consequently, patients suffering from severe sensory disabilities are limited at numerous levels of physical and social functioning. This has motivated the development of a novel therapeutic alternative: sensory neuroprostheses. In order to restore lost function, sensory neuroprostheses attempt to take advantage of the information transfer pathway common to all senses: (1) transduction of the physical stimulus by sensory receptors, (2) transmission of relevant information to the primary sensory areas in the brain by sensory afferents, and (3) analysis and integration of the information to generate perception and action. Neurosensory deficits might occur upon damage of any of the structures involved in the process. However, damage to the peripheral sensory receptor can be often the cause of neurosensory loss. Sensory neuroprostheses attempt to “replace” the malfunctioning or missing peripheral sensory organ by directly delivering basic sensory information to the brain using electrical currents. If the prosthesis is able to deliver enough consistent information, the brain will correctly interpret it and useful rehabilitation can be achieved. I have had the opportunity to contribute to this multidisciplinary field at diverse aspects of the development of retinal, cochlear, and vestibular implants. I have chosen to present the main challenges related to the implementation of these devices as a step-to-step approach in the form a collection of selected articles: (1) sensory information should be efficiently delivered to peripheral afferents (Chapter 2); (2) then the expected physiological response can be evoked and quantified (Chapter 3); (3) the restoration of basic sensory abilities can lead to useful rehabilitation in meaningful every-day activities (Chapter 4); (4) however, sensory pathophysiology can fundamentally limit our ability to transmit the appropriate message to the brain (Chapter 5). Finally, Chapter 6 presents the good clinical outcomes that can be achieved, highlighting the importance of proper technical and rehabilitation follow-up. To conclude, in Chapter 7 the present and future of sensory neuroprostheses will be discussed. This will specifically include current clinical and technical challenges, future prospects, as well as the potential of these devices of improving our fundamental knowledge of sensory physiology and neurosensory deficits

Prospects and Limitations of Spatial Resolution

Our sense of vision permanently captures, transmits and interprets enormous amounts of visual information. The amount of visual information that can be transmitted to the brain by the means of visual prosthesis will be severely limited and thus also limit the rehabilitation prospects of such devices. While several parameters contribute to the information content of visual stimuli, this chapter concentrates essentially on spatial resolution . The fi rst part of the chapter is dedicated to discuss the results of simulation studies of prosthetic vision on normal subjects. These studies aimed to respond to the question of how much visual information should be transmitted to the brain to rehabilitate patients. The amount of visual information, necessary to accomplish daily living tasks (such as reading, eye-hand coordination or whole body mobility) is task-dependent and not only image resolution itself, but also other parameters such as the size of the effective visual fi eld seem to be important. In the second part of the chapter we tried to discuss to which extent the information made available by the stimulation device is lost or degraded before reaching the brain. The experience with actual retinal implants shows us that only part of the information provided by the device fi nds its way to the central nervous system and that this information loss can be highly variable from patient to patient: the spatial resolution provided by the devices corresponds rarely to the spatial resolution perceived by the patients

Milestones in the development of a vestibular implant

PURPOSE OF REVIEW: Bilateral vestibular deficits exist and their prevalence is more important than believed by the medical community. Their severe impact has inspired several teams to develop technical solutions in an attempt to rehabilitate patients. A particularly promising pathway is the vestibular implant. This article describes the main milestones in this field, mainly focusing on work conducted in human patients. RECENT FINDINGS: There have been substantial research efforts, first in animals and more recently in humans, toward the development of vestibular implants. Humans have demonstrated surprising adaptation capabilities to the artificial vestibular signal. Today, the possibility of restoring vestibular reflexes, particularly the vestibulo-ocular reflex, and even achieving useful function in close-to-reality tasks (i.e. improving visual abilities while walking) have been demonstrated in humans. SUMMARY: The vestibular implant opens new perspectives, not only as an effective therapeutic tool, but also pushes us to go beyond current knowledge and well-established clinical concepts

Simultaneous Development of 2 Oral Languages by Child Cochlear Implant Recipients

Objective: To study the development French as a mainstream language by children with a cochlear implant who belong to non-French speaking families. Study Design: Matched pairs comparison of postoperative hearing perception and speech development data of monolingual and bilingual children with cochlear implants. Setting: University medical center. Patients: Fourteen congenital profoundly deaf children. Seven were exclusively French speaking and seven bilingual (French-Portuguese, Arab, Turkish, or Serbo-Croatian) children. Interventions: Cochlear implantation before the age of 52years. Main Outcome Measures: Subjects were evaluated using standard hearing perception and oral language development tests. Results: Both monolingual and bilingual groups obtained excellent hearing perception results with the cochlear implant. Monolingual children showed oral language development results equivalent to those of normal hearing children of the same age, except for the morpho-syntax test where they were slightly below average. Bilingual children scored below average in all oral language development tests. Conclusion: Despite the excellent hearing perception obtained with cochlear implants, the acquisition of a second language at home seems to slow down the development of the French mainstream language. Comparison of our results with those of previous studies indicates that bilingual children require intensive and correct input in both languages. Parents’ involvement in rehabilitation efforts also appears as an important factor for successful oral language development

Implantation cochléaire – better safe than sorry

The benefit of cochlear implants in the mtreatent of profound bilateral sensorineural hearing loss is undeniable. In Switzerland, bilateral cochlear implantation is the treatment of choice in bilateral deafness in children. Due to the anatomical relations of the cochlea and the vestibule, vestibular function can be affected during implantation. Bilateral vestibular areflexia can significantly impair the quality of life and a child's development. Therefore, even if the theoretical risk of bilateral vestibular arefilxia after cochlear implantation is only about 1%, it must be taken into account. Vestibular function assessment must be part of the pre-implantation workup. For bilateral implantations, we recommend a sequential procedure. The second implantation will take place only if the vestibular function is preserved in the other side

Maladie de Menière, hydrops endolymphatique et IRM

La maladie de Menière est caractérisée par des épisodes récidivants de vertige associés à des fluctuations de l’audition, des acouphènes et une sensation de plénitude dans l’oreille atteinte. Même s’il ne permet pas d’expliquer toutes les manifestations de la maladie, l’élargissement de l’espace endolymphatique – ou hydrops endolymphatique (HE) – en est une caractéristique histologique classique. Les progrès de l’imagerie par résonance magnétique permettent maintenant la visualisation in vivo des espaces liquidiens de l’oreille interne et l’appréciation d’un éventuel HE. Cet article traite de ces avancées et leurs potentielles implications diagnostiques dans le contexte d’une maladie de Menière et plus généralement dans les affections cochléovestibulaires

Simulation of artificial vision: IV. Visual information required to achieve simple pointing and manipulation tasks

Retinal prostheses attempt to restore some amount of vision to totally blind patients. Vision evoked this way will be however severely constrained because of several factors (e.g., size of the implanted device, number of stimulating contacts, etc.). We used simulations of artificial vision to study how such restrictions of the amount of visual information provided would affect performance on simple pointing and manipulation tasks. Five normal subjects participated in the study. Two tasks were used: pointing on random targets (LEDs task) and arranging wooden chips according to a given model (CHIPs task). Both tasks had to be completed while the amount of visual information was limited by reducing the resolution (number of pixels) and modifying the size of the effective field of view. All images were projected on a 10 degrees x 7 degrees viewing area, stabilised at a given position on the retina. In central vision, the time required to accomplish the tasks remained systematically slower than with normal vision. Accuracy was close to normal at high image resolutions and decreased at 500 pixels or below, depending on the field of view used. Subjects adapted quite rapidly (in less than 15 sessions) to performing both tasks in eccentric vision (15 degrees in the lower visual field), achieving after adaptation performances close to those observed in central vision. These results demonstrate that, if vision is restricted to a small visual area stabilised on the retina (as would be the case in a retinal prosthesis), the perception of several hundreds of retinotopically arranged phosphenes is still needed to restore accurate but slow performance on pointing and manipulation tasks. Considering that present prototypes afford less than 100 stimulation contacts and that our simulations represent the most favourable visual input conditions that the user might experience, further development is required to achieve optimal rehabilitation prospects

Development of a viewing strategy during adaptation to an artificial central scotoma

Although many individuals with a central scotoma develop eccentric fixation most often beneath or left of the scotoma, little is known about how they come to develop a particular viewing strategy. We investigated this by asking eight subjects with normal vision to read isolated letters, words and text passages while an artificial scotoma covered a central portion of the visual field. We quantified viewing strategy and analysed changes in their viewing behaviour over 8-10 sessions within a two-week period. Subjects read while either a horizontal (n=4) or vertical bar scotoma (n=4), 10 degrees wide, covered the entire horizontal or vertical meridian of the stimulus field. For the horizontal scotoma group: (1) there was an increasing preference to use the inferior visual field for isolated letters/words and text passages, which was essentially complete within the test period; (2) the superior visual field was preferred when reading letters/words initially presented in upper visual space and the inferior visual field when reading letters/words initially presented in lower visual space; (3) in general, variation in viewing strategy according to stimulus position diminished over the sessions for all stimuli. For the vertical scotoma group: (1) two subjects used the left and right visual fields in approximately equal proportion to view isolated letters/words, one subject showed a weak preference to use the left visual field and one subject developed a strong preference for using the right visual field; (2) the text passages could be read with combined use of left and right visual fields in a specific manner; (3) the left visual field was preferred to view stimuli initially presented in left visual space while the right visual field was preferred for words initially presented in right visual space. This effect diminished across sessions. Overall, these findings indicate that (1) a specific viewing strategy can be developed through as little as 5 hours of reading experience without guided training; (2) two distinctly separate retinal areas can be used in an integrated manner during reading; (4) stimulus position in visual space can influence viewing strategy; (5) in general, reading encourages a preference for the inferior over the superior visual field, but not the left over right visual field. Letter/word/text recognition and reading speeds increased progressively across sessions, even after scotoma lateralisation appeared stabilised suggesting that multiple mechanism are involved in adaptive changes