The response of retinal neurons to high-frequency stimulation

High-rate pulse trains have proven to be effective in cochlear prosthetics and, more recently, have been shown to elicit a wide range of interesting response properties in axons of the peripheral nervous system. Surprisingly, the effectiveness of such trains for use in retinal prostheses has not been explored.

Using cell-attached patch clamp methods, we measured the in vitro response of two rabbit retinal ganglion cell types, OFF-brisk transient (OFF-BT) and ON-OFF directionally selective (DS), to trains of biphasic pulses delivered at 2000 pulses per second (PPS).

For OFF-BT cells, response onset occurred at ~20 µA, and maximum response occurred at ~40 µA. Interestingly, spiking levels decreased for further increases in amplitude. In contrast, DS cells had a spiking onset at ~25 µA and maintained strong spiking as stimulus amplitude was increased, even at the highest levels tested. Thus, a low-amplitude stimulus train at 2000 PPS (~25 µA) will activate OFF-BT cells strongly, while simultaneously activating DS cells only weakly. In contrast, a high amplitude train (~75 µA) will activate DS cells strongly while suppressing responses in OFF-BT cells.

The response differences between cell types suggest some forms of preferential activation may be possible, and further testing is warranted. Further, the scope of the response differences found here suggests activation mechanisms that are more complex than those described in previous studies.