Cortical Implants

Cortical implants in V1 cortex
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Visual perception in humans is a complex neuronal sensory pathway developed through the evolution of optic lenses and photoreceptors. Those specialized neurons convert photons into electrical signals that are transported to the visual cortex, where information is interpreted and processed.[1] Damage to any parts of the optical pathway results in visual impairments. Blindness is a condition suffered by more than 40 million people in the world, and it creates an obstacle to leading a normal life. Hence, a great deal of research has been done in the field of vision restoration. Retinal implants and bionic eyes are pioneering the field in terms of neural prosthetics. However, newer animal models have been recently developed and proposed to replace sensory input by bypassing the pre-cortical visual pathway to provide a direct stimulus to the cortex.[2]

1 Visual pathways

Vision is perceived starting from the retina. Information about the visual world is perceived and encoded as electrical signals by photoreceptors, and transported via optic nerves and optic tracts into the thalamus, specifically the lateral geniculate nucleus. Projections are then sent out from the LGN to the primary visual cortex by way of the geniculostriate pathway. The optic radiation that forms Meyer's loop is a collection of axons originating from neurons in the LGN relay station of the thalamus which carries visual information to the V1 cortex along the calcarine fissure in the occipital lobe.

1.1 Decoding the primary visual cortex (V1)

Functional MRI is currently the best non-invasive tool for measuring human brain activity at below-centimeter resolution1. This spatial resolution is ideal for detecting and studying entire cortical maps, such as those in primary visual cortex (V1), but it is still a long way from measuring the responses of individual neurons. Electrophysiological recording and optical imaging in animals have shown that V1 neurons preferring similar orientations form columns about 500 m across. Measuring the response from these homogenous clusters would be an important step toward increasing the spatial resolution of functional imaging. It is possible to estimate the orientation of a stimulus from the pattern of fMRI responses it produces in V1. This enables us for the first time to study how this fundamental form of visual information is represented in human cortex.

1.2 Cortical and neuronal excitation sites

Blindness results from an interruption in the normal flow of signals at one or more sites along the visual pathway. To provide a profoundly blind individual with functional vision, a prosthesis would therefore have to excite the neurons of the pathway at some point after that is, "downstream" of the damage site. The technique is feasible because conditions that damage a site in the visual pathway normally spare the downstream elements. Blindness is usually of retinal origin; the higher visual centers in totally blind individuals are often completely functional (with the exception, usually, of congenital blindness). For this and a number of other compelling reasons, most vision prosthesis research programs have focused on the visual cortex as the site for stimulation. The cortex is well protected by the cranium. It is nevertheless easily accessible through surgery, and the brain seems to be a good, mechanically stable platform on which to mount an electrode array.

Recently, however, work also has been focused on the retina as a site for stimulation of the visual pathways. If the blindness is localized to the photoreceptors (as it is in a number of conditions), the retinal approach has the advantage of including the LGN in the signal path and making use of its processing capability.

However, there are large potential drawbacks to using the retina as the point of entry to the nervous system. First, the technological problems of producing a suitable electrode array are significant. So, too, is the surgical difficulty of placing it against the retina in such a way that it will remain in place permanently- without, for example, being dislodged by encapsulating tissues.
Moreover, the array will rest not against the ganglion cell bodies but against the optic nerve fibers projecting from them. Electrical stimulation will therefore excite the optic nerve fibers as they cross the retina, not the cell bodies. As a result, the electrodes are likely to evoke random rather than contiguous arrangements of phosphenes in space.

1.3 Functional mapping and retinotopy

The striate cortex has been subdivided into 11 identifiable laminar divisions (25, 26) rather than the customary six layers described in most cortical areas. Human V1 was also the focus of several early functional studies, including electrically induced phosphenes (e.g., ref. 32), retinotopic lesion defects (e.g., ref. 21), and positron-emission tomography retinotopy (33). The localization of visual areas in the human cortex is typically based on mapping the retinotopic organization with functional magnetic resonance imaging (fMRI). This retinotopic visual field topography is particularly clear in the early visual areas V1, V2, and V3. The retinotopic organization is the main criterion for delineation of several visual areas in the human cortex. Retinotopy is most commonly mapped using a periodic visual stimulus that moves across the visual field and produces a travelling wave of activity along the retinotopic cortex 4 5 6.

1.4 Learning and plasticity

The visual cortex has the capacity for experience-dependent change, or cortical plasticity, that is retained throughout life. Plasticity is invoked for encoding information during perceptual learning, by internally representing the regularities of the visual environment, which is useful for facilitating intermediate-level vision—contour integration and surface segmentation. The same mechanisms have adaptive value for functional recovery after CNS damage, such as that associated with stroke or neurodegenerative disease. A common feature to plasticity in primary visual cortex (V1) is an association field that links contour elements across the visual field. The circuitry underlying the association field includes a plexus of long-range horizontal connections formed by cortical pyramidal cells. These connections undergo rapid and exuberant sprouting and pruning in response to removal of sensory input, which can account for the topographic reorganization following retinal lesions. Similar alterations in cortical circuitry may be involved in perceptual learning, and the changes observed in V1 may be representative of how learned information is encoded throughout the cerebral cortex.

2 From electrode to perception

Lorem ipsum dolor sit amet, consectetur adipiscing elit. Sed pellentesque ullamcorper ante quis vehicula. Aenean massa odio, placerat eget vehicula id, porta eget mauris. In hac habitasse platea dictumst. Aenean suscipit pellentesque ante. Etiam vel nunc a justo laoreet congue. Phasellus at ullamcorper quam. In mattis sem blandit lectus pharetra ac feugiat nibh ornare. Class aptent taciti sociosqu ad litora torquent per conubia nostra, per inceptos himenaeos. Suspendisse a risus a lacus feugiat tempor. Morbi sed risus turpis, quis viverra arcu. Fusce vel erat libero. Nam nunc ante, laoreet at tincidunt quis, tempor vel lorem. Integer dapibus pulvinar est, sit amet ultricies orci posuere et. Mauris euismod sagittis sapien quis vulputate. Maecenas mollis massa ac nibh ultricies egestas. Etiam tincidunt, lorem sed bibendum ultricies, orci metus feugiat lacus, sit amet gravida ante mi sit amet metus.

2.1 Generating phosphenes

For a cortically-based neuroprosthetic device to function, it obviously must be capable of evoking neural activity It does so by injecting current into the visual cortical tissue, causing the subject to perceive a phosphene in his or her visual field Brindley's and Dobelle's pioneering experiments with surface electrodes have shown that phosphenes are stable over time.
Repeated stimulation of the same location in the visual cortex evoked a phosphene at the same location in visual space (provided the eyes of the subject did not move) The amount of cur- rent required to evoke each phosphene was also fairly stable These phosphenes generally have unchanging dimensions and colors Further, stimulation of a number of points on the cortex caused the subject to see multiple phosphenes

3 The system components and prosthetic implantation

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3.1 Hardware and power

Lorem ipsum dolor sit amet, consectetur adipiscing elit. Sed pellentesque ullamcorper ante quis vehicula. Aenean massa odio, placerat eget vehicula id, porta eget mauris. In hac habitasse platea dictumst. Aenean suscipit pellentesque ante. Etiam vel nunc a justo laoreet congue. Phasellus at ullamcorper quam. In mattis sem blandit lectus pharetra ac feugiat nibh ornare. Class aptent taciti sociosqu ad litora torquent per conubia nostra, per inceptos himenaeos. Suspendisse a risus a lacus feugiat tempor. Morbi sed risus turpis, quis viverra arcu. Fusce vel erat libero. Nam nunc ante, laoreet at tincidunt quis, tempor vel lorem. Integer dapibus pulvinar est, sit amet ultricies orci posuere et. Mauris euismod sagittis sapien quis vulputate. Maecenas mollis massa ac nibh ultricies egestas. Etiam tincidunt, lorem sed bibendum ultricies, orci metus feugiat lacus, sit amet gravida ante mi sit amet metus.

4 Signal processing and visual architectures

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5 Unresolved problems

Lorem ipsum dolor sit amet, consectetur adipiscing elit. Sed pellentesque ullamcorper ante quis vehicula. Aenean massa odio, placerat eget vehicula id, porta eget mauris. In hac habitasse platea dictumst. Aenean suscipit pellentesque ante. Etiam vel nunc a justo laoreet congue. Phasellus at ullamcorper quam. In mattis sem blandit lectus pharetra ac feugiat nibh ornare. Class aptent taciti sociosqu ad litora torquent per conubia nostra, per inceptos himenaeos. Suspendisse a risus a lacus feugiat tempor. Morbi sed risus turpis, quis viverra arcu. Fusce vel erat libero. Nam nunc ante, laoreet at tincidunt quis, tempor vel lorem. Integer dapibus pulvinar est, sit amet ultricies orci posuere et. Mauris euismod sagittis sapien quis vulputate. Maecenas mollis massa ac nibh ultricies egestas. Etiam tincidunt, lorem sed bibendum ultricies, orci metus feugiat lacus, sit amet gravida ante mi sit amet metus.

Bibliography
1. Pearson R.A. et al. (2012). Restoration of vision after transplantation of photoreceptors. Nature. 485 (7396): 99-103.
2. Tehovnik E.J., Slocum W.M., Smirnakis S.M., Tolias A.S. (2009). Microstimulation of visual cortex to restore vision. Prog Brain Res. 175: 347-75.

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