The advantage of topographic prominence-adopted filter for the detection of short-latency spikes of retinal ganglion cells

Electrical stimulation through retinal prosthesis elicits both short and long-latency retinal ganglion cell (RGC) spikes. Because the short-latency RGC spike is usually obscured by electrical stimulus artifact, it is very important to isolate spike from stimulus artifact. Previously, we showed that topographic prominence (TP) discriminator based algorithm is valid and useful for artifact subtraction. In this study, we compared the performance of forward backward (FB) filter only vs. TP-adopted FB filter for artifact subtraction. From the extracted retinae of rd1 mice, we recorded RGC spikes with 8×8 multielectrode array (MEA). The recorded signals were classified into four groups by distances between the stimulation and recording electrodes on MEA (200-400, 400-600, 600-800, 800-1000 µm). Fifty cathodic phase-1(st) biphasic current pulses (duration 500 µs, intensity 5, 10, 20, 30, 40, 50, 60 µA) were applied at every 1 sec. We compared false positive error and false negative error in FB filter and TP-adopted FB filter. By implementing TP-adopted FB filter, short-latency spike can be detected better regarding sensitivity and specificity for detecting spikes regardless of the strength of stimulus and the distance between stimulus and recording electrodes.

Pulse count modulation based biphasic current stimulator for retinal prosthesis and in vitro experiment using rd1 mouse.

For a retinal prosthesis, retinal nerve cells are electrically stimulated by current pulses. Typically, the amplitude of the current pulses is modulated to control the amount of injected charges. However, a high spatial resolution can be difficult to achieve with this amplitude modulation method, because the neural response spreads more widely as the amplitude of the current pulses is increased. In this paper, a biphasic current stimulator integrated circuit (IC) using a new modulation method called, the pulse count modulation, is proposed. In the pulse count modulation method, the amplitude and the width of the current pulses are fixed, and the amount of injected charges is controlled by the number of applied current pulses in a base period. The proposed stimulator IC is fabricated by a 0.35 µm bipolar-CMOS-DMOS (BCDMOS) technology. The operation and performance of the stimulator IC are evaluated in an in vitro experiment environment with rd 1 mice. It is shown that a higher spatial resolution can be achieved compared with the amplitude modulation method.