Decoding brain responses to pixelized images in the primary visual cortex: implications for visual cortical prostheses

Visual cortical prostheses have the potential to restore partial vision. Still limited by the low-resolution visual percepts provided by visual cortical prostheses, implant wearers can currently only ＂see＂ pixelized images, and how to obtain the specific brain responses to different pixelized images in the primary visual cortex（the implant area） is still unknown. We conducted a functional magnetic resonance imaging experiment on normal human participants to investigate the brain activation patterns in response to 18 different pixelized images. There were 100 voxels in the brain activation pattern that were selected from the primary visual cortex, and voxel size was 4 mm × 4 mm × 4 mm. Multi-voxel pattern analysis was used to test if these 18 different brain activation patterns were specific. We chose a Linear Support Vector Machine（LSVM） as the classifier in this study. The results showed that the classification accuracies of different brain activation patterns were significantly above chance level, which suggests that the classifier can successfully distinguish the brain activation patterns. Our results suggest that the specific brain activation patterns to different pixelized images can be obtained in the primary visual cortex using a 4 mm × 4 mm × 4 mm voxel size and a 100-voxel pattern.

Microfabrication and Evaluation of a Silicon Microelectrode Based on SOI Wafer

An implantable seven-channel silicon microelectrode was fabricated by MEMS （micro-electro-mechanical system） micromachining techniques for optic-nerve visual prosthesis applications. Theoretical analyses of noise contributed to determining the size of the exposed recording sites of the microprobe. The geometry configuration was optimized for the silicon microprobe to have enough strength and flexibility and to reduce the insertion-induced tissue trauma. Impedance test results showed that the average value of the channels was 2.3MΩ at lkHz when applied with a stimulating voltage with the amplitude of 50mVpp,which is suitable for neural signal recordings. In-vivo animal experiment showed that the recorded neural signal amplitude from the primary visual cortex was 8μV.

Prototype chemical synapse chip for spatially patterned neurotransmitter stimulation of the retina ex vivo

Biomimetic stimulation of the retina with neurotransmitters,the natural agents of communication at chemical synapses,could be more effective than electrical stimulation for treating blindness from photoreceptor degenerative diseases.Recent studies have demonstrated the feasibility of neurotransmitter stimulation by injecting glutamate,a primary retinal neurotransmitter,into the retina at isolated single sites.Here,we demonstrate spatially patterned multisite stimulation of the retina with glutamate,offering the first experimental evidence for applicability of this strategy for translating visual patterns into afferent neural signals.To accomplish pattern stimulation,we fabricated a special microfluidic device comprising an array of independently addressable microports connected to tiny on-chip glutamate reservoirs via microchannels.The device prefilled with glutamate was interfaced with explanted rat retinas placed over a multielectrode array(MEA)with the retinal ganglion cells(RGC)contacting the electrodes and photoreceptor surface contacting the microports.By independently and simultaneously activating a subset of the microports with modulated pressure pulses,small boluses of glutamate were convectively injected at multiple sites in alphabet patterns over the photoreceptor surface.We found that the glutamate-driven RGC responses recorded through the MEA system were robust and spatially laid out in patterns strongly resembling the injection patterns.The stimulations were also highly localized with spatial resolutions comparable to or better than electrical retinal prostheses.Our findings suggest that surface stimulation of the retina with neurotransmitters in pixelated patterns of visual images is feasible and an artificial chemical synapse chip based on this approach could potentially circumvent the limitations of electrical retinal prostheses.

In-Vitro and In-Vivo Electrical Characteristics of a Penetrating Microelectrode Array for Optic Nerve Electrical Stimulation

The Chinese C-Sight team aims to restore vision to blind patients by means of stimulating the optic nerve with a penetrating microelectrode array. A biocompatible, implantable microwire array was developed having four platinum-iridium shafts, each 100μm in diameter. This penetrating microwire array is described in this paper, including its fabrication techniques and its in-vitro electrical characteristics. Every set of four shafts was spaced 0.4mm from center to center, comprising two short shafts that were 0.3mm long and two that were 0.9mm long. This design was intended to stimulate ganglion cell axons at different depths within the optic nerve. In-vitro electrochemical impedance testing results showed that the impedance at 1kHz ranged from 8 to 10kΩ at room temperature. The voltage responses of the arrays to current pulse stimulation indicated a charge-injection capacity of 210μC/cm2. Finally, in-vivo acute animal experiments showed that the amplitude of the electrically evoked potentials (EEPs) measured in primary visual cortex could be as large as 100 μV upon direct stimulation of the optic nerve.