Electrical stimulation of rat retina elicits retinotopic cortical electrophysiology activity with a dose response characteristic

Purpose: The purpose of this study is to characterize electrophysiological response in the visual cortex elicited by electrical stimulation of rat retina. Methods: Three Long Evans female rats under anesthesia were used for this study. A 75 µm diameter cylindrical Pt-Ir electrode was inserted into the left eye.The tip of the electrode was placed 50-100 µm from the ventral temporal area of the retina. Charge balanced biphasic stimulus current pulses from 30 to 100 µA and 0.5 ms duration were delivered to the retina at 1 Hz. A craniotomy was performed to expose the right visual cortex. A tungsten recording electrode was advanced into the visual cortex to capture elicited electrophysiological signals. The electrically evoked response (EER) was amplified (gain 2000) and low-pass filtered (cutoff frequency 8 kHz), and sampled at 20 kHz. EERs were recorded at multiple sites in the visual cortex of each rat. For each stimulus condition, the EER was averaged over 25 stimulus pulses. Digital filtering was applied to remove high frequency and 60 Hz noise before analysis. The root mean square (rms) value of the EER after stimulus pulse over 40 ms (signal) was compared with the rms value before stimulus (noise) to calculate signal to noise ratios (SNR) for each stimulus condition. Results: We measured detectable EERs (defined as > 10 dB SNR with 100 uA stimulus) 3.25 - 4 mm lateral and 0.75 mm anterior - 0.5 mm posterior to lambda. Previous studies have shown that light stimulus of ventral temporal retina evokes cortical activity in the region 3 - 5 mm lateral and 2 mm anterior - 0.5 mm posterior to lambda. No detectable EERs were measured 0.25 mm away from this cortical region. The measured EER SNR (n=17 locations in 3 rats) showed a dose response relationship with respect to stimulus current with some locations showing monotonic behavior (n = 6) and some locations showing peak SNR at intermediate current (n = 8). At 100 µA stimulus current, the mean SNR was 17.3 dB with a standard deviation of 3.3 dB (n=15) in the cortical regions that showed detectable EERs. Conclusions: Our results show that focal electrical stimulation of rat retina elicits visual cortex activity in the same region where light stimulus generates activity. This study will form a baseline to evaluate long-term stimulation of rat retina