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Model (a): cone outer segments (CO), cone terminals (CT), horizontal cells (HC), bipolar cells (BC), narrow- and wide-field amacrine cells (NA,WA), transient ganglion cells (OnT, OffT), sustained ganglion cells (OnS, OffS). Model (b): Transistor synapses

Chipdesign and human photoreceptor mosaic: phototransistor (P), outer plexiform (synaptic) layer (OPL), bipolar cells (BC), inner plexiform layer (IPL), ganglion cells (GC), narrow-field amacrine (NA)
One step closer to creating the cybernetic organism

Researchers from the University of Pennsylvania and Stanford University have made a breakthrough in the field of vision. Kareem Amir Zaghloul and Kwabena Boahen have proposed a silicon retina that reproduces signals in the optic nerve, a technology which could be used to provide vision to those who suffer from blindness-related diseases, such as retinitis pigmentosa.


Unlike previous attempts to create an artificial retina, which relied on external cameras and processors, the silicon retina integrates many functions of the mammalian retina in a package that could be suitable for implantation.


“Here, we present a silicon retina modeled on neural circuitry in both the outer and the inner retina,” Zaghloul and Boahen introduced in their paper for the Journal of Neural Engineering. “It is constructed at a scale comparable to the human retina and uses under

a tenth of a watt, thereby satisfying the requirements of a fully implantable prosthesis.”


It is estimated that the silicon retina will maintain sensitivity over at least 15 years of average use relating to vision.


The mammalian retina was used as the basis for the design of the artificial retina. 13 neuronal types were made into transistor form, each mimicking the function of its biological equivalent.


“We morphed our retinal model into a silicon chip by replacing each synapse or gap junction in our model with a transistor,” Zaghloul and Boahen revealed. “One of its terminals is connected to the pre-synaptic node, another to the post-synaptic node and a third to the modulatory node. By permuting these assignments, we realize excitation, inhibition and conduction, all of which are under modulatory control.”


Conveniently, the binary nature of transistors lends itself well to replicating the functions of neurons, which operate in a similar “all-or-none” fashion. The silicon retina uses “on” and “off” signals similar to those found between amacrine cells in the mammalian retina.


Another aspect of the silicon retina that’s copied from the real retina is that it filters out all the useless, unchanged and redundant data from a scene, which reduces the bandwidth required to produce an image.


While much of this may sound like science fiction, the researchers already have working silicon. The die measures 3.5 × 3.3 millimeters and has 5760 phototransistors, which mimic photoreceptor cells. The phototransistors are then connected to another group of 3600 transistors which act as ganglion cells.


“Our chip design was fabricated in a 0.35 µm minimum feature-size process, with its cell mosaics tiled at a scale similar to the mammalian retina,” the paper reads. “Phototransistors are tiled triangularly 40 µm apart; this spacing is only about two and a half times that of human cones at 5 mm nasal eccentricity.”


One major difference the researchers had to deal with when designing the artificial retina is that silicon micro-fabrication technology cannot produce three dimensional structures found in the real retina.


Power consumption is an area where the researchers are still currently trying to improve.


“Our artificial retina satisfies the requirements of a neural prosthesis by matching the biological retina in size and weight and using under a tenth of a watt,” the researchers stated. “Although this energy consumption is 1000 times less efficient than the mammalian retina, it still represents a 100-fold improvement over conventional microprocessors.”


Zaghloul and Boahen are currently working on improving energy efficiency, spatial resolution and dynamic range. According to the researchers, advances in chip fabrication will be of great aid to this technology.


Further development of the silicon retina not only benefits those suffering from impaired vision, as the technology can also be applied to artificial neural systems and robots.

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