Electrical Stimulus Parameters for Visual Perception in Blind Humans with Retinal Prosthetic Implants

Abstract

Abstract: : Purpose: To study the threshold stimulus parameters with respect to the stimulus length Methods: The FDA and USC-IRB approved the study protocol. Once the informed consent was obtained and the subject met the qualifications in electrophysiological and psychophysical tests, a Second Sight Model 1 prosthesis was implanted in the worse eye. A multiconductor wire extended from an extra ocular stimulator case terminating in a 4x4 array-stimulating electrodes. The array was secured in to the retina with a single retinal tack. Electrode impedance(using a 1 KHz sine wave) was measured before and after the stimulation study carried out. The threshold of the perception was determined by increasing the stimulus current until the subject was able to see the reliable visual percept (patient reports flash of light, 2 in 3 times). As a control, false positive tests were conducted (no stimulus, ask patients perception). Strength duration curves were obtained from 3 different electrodes by varying the pulse width of the current impulse and the amplitude of the current. Results: The stimulus threshold for subject 1(S1) ranged from 28-949µA and for subject 2(S2) ranged from 43-679µA. The electrode impedance was stable during the stimulation study(3-50kΩ). The strength duration curves were obtained by determining threshold current for various pulse. From this response it is noticed that, as the duration of the impulse is increased, the magnitude of the input current reduced, (exponentially) for example from 2 ms at 150µA to 8 ms at 45µA, indicating less charge at lower pulse width. The values of rheobase / chronaxie for 3 electrodes tested are 26µA/0.975ms, 45µA/2ms and 108µA/2ms respectively. Subjects reported false positive less than 5%. Conclusion: The impedance of the electrodes was stable after electrical stimulation of the retina for 8 months. Shorter pulses use less charge.  

Keywords: electrophysiology: clinical • retinal pigment epithelium • age-related macular degeneration