Conventional stimulation strategies assume evenly distributed, round greyscale phosphenes, whose intensity can be modulated to reflect different brightness levels in the image that is being presented to the implantee. However, it has become increasingly apparent from psychophysical studies with retinal implant recipients that phosphenes can be very complex. Here, we argue that a user can benefit from a strategy that uses more phosphene properties than just intensity to convey information. In particular, we are investigating ways to harness the phosphene shape and orientation in order to facilitate reading with a retinal implant and increase reading speed.
To test stimulation strategies, normally sighted participants participated in psychophysical experiments using simulated bionic vision. A first study is based on the assumption that we can deliberately change the orientation of phosphenes by using current steering techniques. Results show that participants perform significantly better in a letter recognition task, even when confronted with large phosphene dropout and phosphene location shift.
In the present work, phosphene shape and direction are modelled based on in vitro data, which allows only for a minimal control over the phosphene appearance. We investigate reading speed to compare different stimulation strategies operating under this model.
To date, we have shown that our strategies improve letter discrimination in simulations. In the long term, we hope that this will improve the reading experience of retinal implant recipients.