A model of ganglion axon pathways accounts for percepts elicited by retinal implants.

Degenerative retinal diseases such as retinitis pigmentosa and macular degeneration cause irreversible vision loss in more than 10 million people worldwide. Retinal prostheses, now implanted in over 250 patients worldwide, electrically stimulate surviving cells in order to evoke neuronal responses that are interpreted by the brain as visual percepts ('phosphenes'). However, instead of seeing focal spots of light, current implant users perceive highly distorted phosphenes that vary in shape both across subjects and electrodes. We characterized these distortions by asking users of the Argus retinal prosthesis system (Second Sight Medical Products Inc.) to draw electrically elicited percepts on a touchscreen. Using ophthalmic fundus imaging and computational modeling, we show that elicited percepts can be accurately predicted by the topographic organization of optic nerve fiber bundles in each subject's retina, successfully replicating visual percepts ranging from 'blobs' to oriented 'streaks' and 'wedges' depending on the retinal location of the stimulating electrode. This provides the first evidence that activation of passing axon fibers accounts for the rich repertoire of phosphene shape commonly reported in psychophysical experiments, which can severely distort the quality of the generated visual experience. Overall our findings argue for more detailed modeling of biological detail across neural engineering applications

Simulation of thalamic prosthetic vision: reading accuracy, speed, and acuity in sighted humans

The psychophysics of reading with artificial sight has received increasing attention as visual prostheses are becoming a real possibility to restore useful function to the blind through the coarse, pseudo-pixelized vision they generate. Studies to date have focused on simulating retinal and cortical prostheses; here we extend that work to report on thalamic designs. This study examined the reading performance of normally sighted human subjects using a simulation of three thalamic visual prostheses that varied in phosphene count, to help understand the level of functional ability afforded by thalamic designs in a task of daily living. Reading accuracy, reading speed, and reading acuity of 20 subjects were measured as a function of letter size, using a task based on the MNREAD chart. Results showed that fluid reading was feasible with appropriate combinations of letter size and phosphene count, and performance degraded smoothly as font size was decreased, with an approximate doubling of phosphene count resulting in an increase of 0.2 logMAR in acuity. Results here were consistent with previous results from our laboratory. Results were also consistent with those from the literature, despite using naive subjects who were not trained on the simulator, in contrast to other reports

Visual Acuity of Simulated Thalamic Visual Prostheses in Normally Sighted Humans

Simulation in normally sighted individuals is a crucial tool to evaluate the performance of potential visual prosthesis designs prior to human implantation of a device. Here, we investigated the effects of electrode count on visual acuity, learning rate and response time in 16 normally sighted subjects using a simulated thalamic visual prosthesis, providing the first performance reports for thalamic designs. A new letter recognition paradigm using a multiple-optotype two-alternative forced choice task was adapted from the Snellen eye chart, and specifically devised to be readily communicated to both human and non-human primate subjects. Validation of the method against a standard Snellen acuity test in 21 human subjects showed no significant differences between the two tests. The novel task was then used to address three questions about simulations of the center-weighted phosphene patterns typical of thalamic designs: What are the expected Snellen acuities for devices with varying numbers of contacts, do subjects display rapid adaptation to the new visual modality, and can response time in the task provide clues to the mechanisms of perception in low-resolution artificial vision? Population performance (hit rate) was significantly above chance when viewing Snellen 20/200 optotypes (Log MAR 1.0) with 370 phosphenes in the central 10 degrees of vision, ranging to Snellen 20/800 (Log MAR 1.6) with 25 central phosphenes. Furthermore, subjects demonstrated learning within the 1–2 hours of task experience indicating the potential for an effective rehabilitation and possibly better visual performance after a longer period of training. Response time differences suggest that direct letter perception occurred when hit rate was above 75%, whereas a slower strategy like feature-based pattern matching was used in conditions of lower relative resolution. As pattern matching can substantially boost effective acuity, these results suggest post-implant therapy should specifically address feature detection skills

Photogenetic Retinal Prosthesis

The last few decades have witnessed an immense effort to develop working retinal implants for patients suffering from retinal degeneration diseases such as retinitis pigmentosa. However, it is becoming apparent that this approach is unable to restore levels of vision that will be sufficient to offer significant improvement in the quality of life of patients. Herein, a new type of retinal prosthesis that is based on genetic expression of microbial light sensitive ion channel, Chanelrhodopsin-2 (ChR2), and a remote light stimulation is examined. First, the dynamics of the ChR2 stimulation is characterized and it is shown that (1) the temporal resolution of ChR2-evoked spiking is limited by a continuous drop in its depolarization efficiency that is due to (a) frequency-independent desensitization process and (b) slow photocurrent shutting, which leads to a frequency-dependent post-spike depolarization and (2) the ChR2 response to light can be accurately reproduced by a four-state model consisting of two interconnected branches of open and close states. Then, a stimulation prototype is developed and its functionality is demonstrated in-vitro. The prototype uses a new micro-emissive matrix which enables generating of two-dimensional stimulation patterns with enhanced resolution compared to the conventional retinal implants. Finally, based on the micro-emitters matrix, a new technique for sub-cellular and network-level neuroscience experimentations is shown. The capacity to excite sub-cellular compartments is demonstrated and an example utility to fast map variability in dendrites conductance is shown. The outcomes of this thesis present an outline and a first proof-of-concept for a future photogenetic retinal prosthesis. In addition, they provide the emerging optogenetic technology with a detailed analysis of its temporal resolution and a tool to expand its spatial resolution, which can have immediate high impact applications in modulating the activity of sub-cellular compartments, mapping neuronal networks and studying synchrony and plasticity effects

Aerospace Medicine and Biology: A continuing bibliography with indexes, supplement 199

This bibliography lists 82 reports, articles, and other documents introduced into the NASA scientific and technical information system in October 1979

The Speed, Precision and Accuracy of Human Multisensory Perception following Changes to the Visual Sense

Human adults can combine information from multiple senses to improve their perceptual judgments. Visual and multisensory experience plays an important role in the development of multisensory integration, however it is unclear to what extent changes in vision impact multisensory processing later in life. In particular, it is not known whether adults account for changes to the relative reliability of their senses, following sensory loss, treatment or training. Using psychophysical methods, this thesis studied the multisensory processing of individuals experiencing changes to the visual sense. Chapters 2 and 3 assessed whether patients implanted with a retinal prosthesis (having been blinded by a retinal degenerative disease) could use this new visual signal with non-visual information to improve their speed or precision on multisensory tasks. Due to large differences between the reliabilities of the visual and non-visual cues, patients were not always able to benefit from the new visual signal. Chapter 4 assessed whether patients with degenerative visual loss adjust the weight given to visual and non-visual cues during audio-visual localization as their relative reliabilities change. Although some patients adjusted their reliance on vision across the visual field in line with predictions based on cue relative reliability, others - patients with visual loss limited to their central visual field only - did not. Chapter 5 assessed whether training with either more reliable or less reliable visual feedback could enable normally sighted adults to overcome an auditory localization bias. Findings suggest that visual information, irrespective of reliability, can be used to overcome at least some non-visual biases. In summary, this thesis documents multisensory changes following changes to the visual sense. The results improve our understanding of adult multisensory plasticity and have implications for successful treatments and rehabilitation following sensory loss