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Jianing Wei, Varalakshmi Wuyyuru, Lyndon da Cruz, Gislin Dagnelie, Robert J. Greenberg, Argus II Study Group; The Effect of Frequency and Pulse Width on Perceptual Thresholds in ArgusTM II Retinal Implant Subjects. Invest. Ophthalmol. Vis. Sci. 2011;52(14):4948.
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The most common stimulation current used for retinal implants are biphasic rectangular pulse trains repeated at a specific frequency with a fixed pulse width. Depending on the electrode array-to-retina distance and other factors, thresholds may vary widely across and within subjects. Previous studies in Argus I subjects (Horsager et al, Inv. Ophth. Vis. Sci., 2009) established a model to predict the visual sensitivity for novel electrodes and temporal patterns of the stimulation. The purpose of this study was to investigate whether threshold levels will change if different frequency and pulse width were used in Argus II subjects.
A group of six electrodes were selected for each subject. Thresholds were measured with a fully automated procedure. After each stimulus, the subject used a keypad to respond "yes" if a phosphene was seen, or "no" if it was not. For the effect of frequency experiment, 19 subjects’ perceptual thresholds were measured in two different testing sessions. The stimulation parameters used in each session were: 1) Frequency=20Hz, Pulse Width = 450µs; 2) Frequency=6Hz, Pulse Width= 450µs. For the effect of pulse width experiment, 14 subjects’ thresholds were measured in these two conditions: 1) Frequency=20Hz, Pulse Width = 450µs; and 2) Frequency=20Hz, Pulse Width=1ms. The duration of the pulse train was 250ms.
The thresholds measured at two different frequencies (6Hz and 20Hz) were compared. 12 out of 19 subjects had average threshold increase when stimulation frequency decreased to 6Hz. 7 subjects’ average thresholds decreased at 6Hz. Overall thresholds measured at 6 Hz were significantly higher than thresholds measured at 20 Hz (p<0.005).
Perceptual thresholds increased significantly in Argus II subjects when stimulation frequency or pulse width decreased. These findings are important in providing optimal stimulation parameters for individual subjects.
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