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.2008;37(10):1529-59.
doi: 10.1068/p6100.

Visual prosthesis

Affiliations

Visual prosthesis

Peter H Schiller et al. Perception.2008.

Abstract

There are more than forty million blind individuals in the world whose plight would be greatly ameliorated by creating a visual prosthesis. We begin by outlining the basic operational characteristics of the visual system, as this knowledge is essential for producing a prosthetic device based on electrical stimulation through arrays of implanted electrodes. We then list a series of tenets that we believe need to be followed in this effort. Central among these is our belief that the initial research in this area, which is in its infancy, should first be carried out on animals. We suggest that implantation of area V1 holds high promise as the area is of a large volume and can therefore accommodate extensive electrode arrays. We then proceed to consider coding operations that can effectively convert visual images viewed by a camera to stimulate electrode arrays to yield visual impressions that can provide shape, motion, and depth information. We advocate experimental work that mimics electrical stimulation effects non-invasively in sighted human subjects with a camera from which visual images are converted into displays on a monitor akin to those created by electrical stimulation.

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Figures

Figure 1
Figure 1
The retina: A. Head-on view of the rods and cones in the fovea and at an eccentricity of 5 degrees. B. Head-on view of the retina showing the fovea, the optic disk and the blood vessels. C. Head-on view of the retina showing the course of the axons to the optic disk. D. Cross section of the retina at the foveal pit demonstrating that this region is free of retinal cells other than the photoreceptors; this is achieved by having cell processes course away from this central area.
Figure 2
Figure 2
A: Schematic of the responses of ON and OFF-center ganglion cells to spots presented in the center of their receptive field and large spots that impinge on both the center and the surround of their receptive fields. The vertical lines represent the action potentials created by the stimulation. Center-surround organization in these ganglion cells is antagonistic as a result of which a more vigorous response is elicited when a small spot is presented than a large one. ON-center ganglion cells are excited by light increment (the white spot on the gray background) whereas OFF-center cells are excited by light decrement (black spot on the gray background). B. Demonstration of the effects of adaptation. Fixating on the little white square in the center between the two gaussian disks for 10–20 seconds will result in the disappearance of these disks due to the adaptation process that takes place in the retina. At this point, when one shifts the center of gaze to the lower little white square, two negative afterimages will appear as a result of the photons from the homogeneous background impinging on more and less sensitive areas in the retina at the adapted locations. C. The scaling of afterimages. Fixate on the white dot in the center of the black disk in the left for 20–30 seconds, then shift your gaze to the black cross in the center of the right. Once the afterimage appears move the sheet toward and away from you. The afterimage will scale in size.
Figure 3
Figure 3
The basic wiring diagram of the visual system. Ganglion cells from the nasal hemiretinae cross over at the optic chasm whereas those from the temporal hemiretinae project ipsilaterally. As a result, punctate images in the visual field activate corresponding points in the visual system when they are presented along the horopter where an image activates corresponding points on the retinal surface. Images from the left visual hemifield (in blue) project to the right hemisphere and images from the right visual hemifield (green) project to the left hemisphere. The major projection sites from the retina are the lateral geniculate nucleus, the superior colliculus, the terminal nuclei. In the cortex there are numerous visual area (shown are V1, V2, V3, V4, and MT). These areas send both feed forward and feed back connections to many areas of the brain. Images appear outside the horopter impinge on retinal non-corresponding points.
Figure 4
Figure 4
Layout of the lateral geniculate nucleus and primary visual cortex (V1). A. A Nissyl-stained cross section of the lateral geniculate nucleus which, for the central 17 degrees of the visual field representation, has six layers. The top four layers (3–6) receive input from the retinal midget cells. The bottom two layers (1–2) receive input from the parasol cells. The intralaminar layers receive input mostly from the retinal koniocellular. B: A Nissyl-stained cross section of area V1. The thickness of gray matter and density of neurons is quite constant thoughout this area. The layers in the gray matter are designated. The prime input of the midget and parasol cells from the LGN goes to layers 4cα and 4cβ. The inputs from the koniocellular cells terminate in layers 1 and 2. C: Layout of the central five degrees of the visual field. The left hemifield (blue) projects to the right hemisphere and the right hemifield (green) projects to the left visual field. D: A rear view of the monkey brain showing the central 6–8 degrees of the visual field layout. This region in the monkey is lissencephalic except for the external calcarine sulcus, which is quite shallow. The visual field is laid out in a topographic fashion with much more area allocated for central than peripheral representation. The visual field is laid out upside down in the cortex with the upper part of the visual field in the lower region of V1 and the lower visual field in the upper portion of V1 using the conformal mapping scheme of Schwartz (1994) for the macaque monkey.
Figure 5
Figure 5
The basic manner in which images activate various regions of area V1 in the monkey. Dotted figures are presented in the visual field in accordance with the fact that electrical stimulation of area V1 produces star-like images. A: A dotted arrow is placed in into the visual field centered on the vertical meridian. The areas activated in V1 are shown below. The tail of the arrow projects into the right and the head of the arrow to the left hemisphere. Due to the magnification factor that results in more space allocated per unit area for central than for peripheral vision, the size of the area activated by each dot is progressively larger the closer the dots are to the foveal representation. Due to center/surround antagonism in retinal and LGN cells and due to the greater responses elicited to edges in the cortex, the dots drive neurons more vigorously at the outer periphery of each dot than in the center, which is depicted by the shading of the dots on the cortical surface. B: The dotted circle placed in the right visual hemifield activates the marked regions in the left hemifield; the dot in the center of the fovea activates the foveal representation in both hemispheres. The activation in the left hemifield forms a pretty good circle but the size of each dot representation changes as a function of eccentricity. C: The same circle is presented in the visual field centered on the vertical meridian. The activation in the cortex forms two crescents yet what we perceive is a perfect circle. The size of the areas activated is constant as these locations are equidistant from the fovea.
Figure 6
Figure 6
A: An array of 256 dots arranged in the shape of a square and the corresponding brain regions activated in area V1. B: An array of 256 electrodes placed proportionally on the cortical surface taking magnification factor into account, with 128 in each hemisphere. Electrical stimulation under these conditions presumably activates an array or star-like images whose size increases with increasing eccentricity. The white lines show schematically the correspondence of points in the visual field and with the electrodes in the cortex.
Figure 7
Figure 7
The presumed effects of electrical stimulation using a proportional square array. In B is shown the rear view of a monkey brain with the electrodes placed bilaterally as had been shown in Figure 6B. When electrical stimulation is applied to all electrodes, as indicated by the red-centered electrodes, it presumably yields the square array of dots shown in A, above. C shows a digital camera that looks at the display FIAT LUX centered in the visual field. The camera is hooked up to a computer and a stimulator arranged to activate the appropriate points in the cortex. The regions activated by the letters are shown by the red-centered dots in E. The resultant image created by the selective activation of the subregions in the camera unit is shown in D.
Figure 8
Figure 8
Examination of the percept yielded when a proportional circular array is placed on the cortical surface, shown in B. When all electrodes are activated a radial display comprised of an array of near-perfect circles is produced, shown in A. When the same words, FIAT LUX are presented, shown in C and E, this arrangement yields an image depicted in D.
Figure 9
Figure 9
The images created when equally spaced electrode arrays are placed onto the visual cortex. The arrays consist of 256 elemen per hemsiphere, eacht spaced 1.3 mm apart thus making for twice the overall number of electrodes as in the previous figures. When all electrodes are activated, shown in B, the image created is presumed to have the appearances of a butterfly (shown in A) due to the manner in which the visual field is laid out on the cortical surface. Presenting the words FIAT LUX, shown in C and E with the electroces activated in red, produces a rather distorted image shown in D.
Figure 10
Figure 10
A. Post stimulus time histograms of multiunit data obtained from a fixating alert monkey when the receptive fields of the V1 cells, located in the lower visual field at an eccentricity of 3.2 degrees, were stimulated with a small white spot, a small black spot and a white spot larger than their receptive fields. The data are based on 40 trials for each condition. The small light and dark spots elicited similar responses whereas the large spot elicited no response at all. Standard glass coated platinum/iridium electrodes were used with tip diameters of 1–2 μm and exposed shaft of 10–15 μm, having 40–100 picofarad capacitances. B. The activation of a single cell in the recording system that is set up to have the same number of units as the number of implanted electrodes. Each unit is activated only when a difference in activation occurswithin its elements, as shown in d. Changes in illumination that affect all elements within a unit equally, as in b, produce no activation. This arrangement mimicks the basic characteristics of V1 neurons.
Figure 11
Figure 11
An enhanced proportional display system. A: Ninety-six electrodes are added to each array (shown in Figures 6 and 7) thereby making for a total of 448 elements. The added electrodes are placed in-between the central 8 by 8 portion of the electrode array shown in Figure 6 yielding a 16 by 16 array. The image elicited by stimulating all sites appears on top. B: The images created by the 256 and 448 element arrays when the words FIAT LUX are confined largely to the central two degrees of the visual field appear demonstrating the higher resolution reaped by the addition of the 96 electrodes. C: Evenly spaced electrode array that allows for proportional activation using a program that corrects for magnification factor. The arrangement of 610 electrodes per side provides approximately the same resolution as the proportional array in A that has 224 electrodes per side.
Figure 12
Figure 12
Procedures for mimicking a prosthetic device non-invasively. A video camera is attached to the head with a laser so head movements can be tracked. The image in the camera is converted as in Figure 7 and 8. The resultant image is displayed on a small monitor inside a bezel that is attached to the head, which is the only visual signal provided to the observer. Shown are the layouts for the proportional square and circular arrays when a small central square and a large circle are presented in the visual field. The small square is reproduced well with the proportional array; the large circle is produced extremely well with the circular array.
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