gms | German Medical Science

Background: Retinal implants can evoke visual percept in blind patients after successful implantation and efficient contact to the residual inner retina. Their stimulation performance with regard to spatial or temporal discrimination can be easily assessed using ex vivo retinas. Here we present how to evaluate the spatial sensitivity by using flexible microelectrode arrays which record simultaneously from multiple ganglion cells.

Methods: Whole mount retinas were interfaced in subretinal configuration to either (i) a capacitive stimulation array (Neurochip prototype; electrode size and pitch: ~ 50 µm) or (ii) to a retinal implant chip (Retina Implant AG, electrode size: 7 µm, pitch: 70 µm). The superfused retinal ganglion cell layer was contacted by a flexible, transparent and perforated microelectrode array comprising 16 densely - spaced recording electrodes (150 µm electrode spacing). Stimulation with the capacitive electrodes is performed using monophasic constant current pulses (amplitude: 3 mA/cm2), while stimulation with the retinal implant is performed using constant voltage pulses (amplitude: 1.6V). In the latter experiments the stimulation current is monitored separately.

Results: Our protocols comprised stimulation with different electrode areas sequentially shifted by (i) 50 µm for the capacitive electrodes or (ii) by 70 µm in case of the retina implant chip. In both protocols we recorded retinal ganglion cells which change their spike pattern for minimally shifted stimulation areas. In both protocols we identify response patterns which mimick the receptive field of a retinal ganglion cell.

Conclusion: In addition to the high sensitivity to stimulus position the presented experiments demonstrate efficient stimulation with regard to stimulation strength (current amplitude or voltage amplitude respectively). This suggests that stimulation may be feasible with smaller electrodes. This work is funded by a BMBF grant (FKZ 1312038).