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Connectivity in the human retina is complex. Over one hundred million photoreceptors transduce light into electrical
signals. These electrical signals are sent to the ganglion cells through amacrine and bipolar cells. Lateral connections
involving horizontal and amacrine cells span throughout the outer plexiform layer and inner plexiform layer
respectively. Horizontal cells are important for photoreceptor regulation by depolarizing them after an illumination
occurs. Horizontal cells themselves form an electrical network that communicates by gap junctions, and these cells
exhibit plasticity (change in behavior and structure) with respect to glycine receptors. The bipolar and amacrine cells
transfer electrical signals from photoreceptors to the ganglion cells. Furthermore, amacrine cells are responsible for
further processing the retinal image. Finally, the ganglion cells receive electrical signals from the bipolar and amacrine
cells and will spike at a faster rate if there is a change in the overall intensity for a group of photoreceptors, sending a
signal to the brain.
Dramatic progress is being made with respect to retinal prostheses, raising hope for an entire synthetic retina in the
future. We propose a bio-inspired 3D hierarchical pyramidal architecture for a synthetic retina that mimics the overall
structure of the human retina. We chose to use a 3D architecture to facilitate connectivity among retinal cells,
maintaining a hierarchical structure similar to that of the biological retina. The first layer of the architecture contains
electronic circuits that model photoreceptors and horizontal cells. The second layer contains amacrine and bipolar
electronic cells, and the third layer contains ganglion cells. Layer I has the highest number of cells, and layer III has the
lowest number of cells, resulting in a pyramidal architecture. In our proposed architecture we intend to use
photodetectors to transduce light into electrical signals. We propose to employ wireless communication to mimic the gap
junction behavior among horizontal cells. These cells could communicate laterally to neighboring horizontal cells
through a network of spin wave transmitters and receivers that send magnetic waves over the surface of the first layer of
the synthetic retina. We discuss the tradeoffs for having point-to-point connections versus a network on chip in the
second layer. We examine the use of 3D CMOS technologies as well as nanotechnologies for the implementation of this
retina, considering size, interconnectivity capabilities, and power consumption. Finally, we estimate the volume, delay
and power dissipation of our architecture.
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