Proposal of Application of Pulsed Vision Chip to Retinal Prosthesis

In this paper, we propose and discuss an application of a CMOS vision chip using pulse frequency modulation to sub-retinal implantation to partially recover human vision. In sub-retinal approaches, a photo-sensor buried underneath the retina converts light distribution of the image projected on a retina surface to electric signals, and stimulates the horizontal cells or neurons behind them [1], [2]. So far in the sub-retinal approach, a simple photodiode array without any bias voltage has been used as a photo-sensor mainly due to its simple configuration [2]. The photocurrent is directly used as injection current into cells. However, the amount of the photocurrent is so small not to stimulate cells due to its low photosensitivity. To increase the sensitivity it is effective to integrate the photo-generated charge in a capacitor, and this technique, integration mode, is commonly used in conventional image sensors. We use a pulse frequency modulation (PFM) [3], [4] instead of this conventional integration method as following reasons. First, PFM operates asynchronously and independently, and thus each pixel could be separated each other in space. This makes it easy to flow nutrition penetrating the chip. On the contrary, a conventional integration mode requires timing sequence for reset and read-out. Second, PFM outputs pulse streams, which would be suitable to stimulate the cells. In addition, PFM can operate in a very low voltage without decreasing signal-to-noise ratio. Finally, its dynamic range is relatively large, which is very effective to replacement of photoreceptors. We have designed and fabricated PFM pixel circuits using standard CMOS technology and demonstrated its fundamental characteristics. In addition, we discuss future issues to apply the PFM to retinal prosthesis device.