Focal Activation in the Visual Cortex by Electrical Stimulation of Retina

Abstract

Retinal prosthesis uses multichannel electrical stimulation at the retina to generate a response in the visual cortex in order to provide partial vision for blind patients. In this study, response to the activation of the retina by a single channel epiretinal electrical stimulation was characterized. The aim was to identify an optimum set of parameters, i.e., frequency, pulse duration, and interphase gap, that decreases current threshold while increasing spatial activation. To collect the following experimental data, past members of the lab conducted experiments on LE and F1 strains of rats. A sixteen-channel recording device was placed in the visual cortex to record the generated voltage response as a result of single-channel electrical stimulation of the retina. Voltage response was recorded for blind rats and normally sighted rats for proper comparison. Response was collected by varying two parameters at a time, i.e. frequency and pulse duration or pulse duration and interphase gap. For each experiment specified by a set of two parameters, ten trials were conducted at twenty current levels, starting at 10 μA and incremented by 10 μA till 200 μA. It was observed that spatial activation increased with decreasing frequency, increasing pulse duration, and increasing interphase gap. The highest activation in blind rats was achieved at a frequency of 1 Hz combined with a pulse duration of 1.5 ms, when comparing data collected by varying frequency and pulse duration, keeping the interphase gap at 0 ms. While studying the combination of pulse duration and interphase gap, readings were taken for only two pulse durations (0.5 ms and 1 ms) at 5 Hz. Activation was highest at an interphase gap of 1.5 ms and a pulse duration of 1 ms. Similar trends could be observed in normally sighted rats. The current threshold generally showed a decreasing trend with decreasing frequency and increasing interphase gap. When studying the combination of pulse duration and interphase gap, the best results for current threshold were achieved with a frequency of 5 Hz, an interphase gap of 1.5 ms, and a pulse duration of 1 ms. The highest spatial activation could be best achieved with a frequency of 5 Hz, pulse duration of 1 ms and interphase gap of 1.5 ms. Adding the interphase gap increases spatial activation by nearly 96%. Current threshold also shows very similar results in terms of parametric combinations. The study of optimum parametric combinations is important to determine the best set of stimulating conditions for different blind patients with differing degrees of blindness.