Skip to main page content
Access keys NCBI Homepage MyNCBI Homepage Main Content Main Navigation
Randomized Controlled Trial
. 2012 Jan 20;53(1):205-14.
doi: 10.1167/iovs.11-8401.

Frequency and Amplitude Modulation Have Different Effects on the Percepts Elicited by Retinal Stimulation

Affiliations
Free PMC article
Randomized Controlled Trial

Frequency and Amplitude Modulation Have Different Effects on the Percepts Elicited by Retinal Stimulation

Devyani Nanduri et al. Invest Ophthalmol Vis Sci. .
Free PMC article

Abstract

Purpose: In an effort to restore functional form vision, epiretinal prostheses that elicit percepts by directly stimulating remaining retinal circuitry were implanted in human subjects with advanced retinitis pigmentosa RP). In this study, manipulating pulse train frequency and amplitude had different effects on the size and brightness of phosphene appearance.

Methods: Experiments were performed on a single subject with severe RP (implanted with a 16-channel epiretinal prosthesis in 2004) on nine individual electrodes. Psychophysical techniques were used to measure both the brightness and size of phosphenes when the biphasic pulse train was varied by either modulating the current amplitude (with constant frequency) or the stimulating frequency (with constant current amplitude).

Results: Increasing stimulation frequency always increased brightness, while having a smaller effect on the size of elicited phosphenes. In contrast, increasing stimulation amplitude generally increased both the size and brightness of phosphenes. These experimental findings can be explained by using a simple computational model based on previous psychophysical work and the expected spatial spread of current from a disc electrode.

Conclusions: Given that amplitude and frequency have separable effects on percept size, these findings suggest that frequency modulation improves the encoding of a wide range of brightness levels without a loss of spatial resolution. Future retinal prosthesis designs could benefit from having the flexibility to manipulate pulse train amplitude and frequency independently (clinicaltrials.gov number, NCT00279500).

Figures

Figure 1.
Figure 1.
The prosthesis system. (A) Overview of the implant, (B) 4 × 4 electrode array, and (C) biphasic pulse train. Pulse trains were varied by changing either pulse amplitude (dashed arrows) or pulse frequency (solid arrows).
Figure 2.
Figure 2.
Phosphene drawings from three different electrodes (a single trial for each electrode). (A) The array, showing the example electrodes in BD. For all three electrodes, stimulation was 0.45 ms biphasic and 20-Hz pulse train, for a duration of 500 ms. (B, C) For electrodes D2 and C4 the pulse train was at 1.25× threshold. (D) For electrode B3 the pulse train was at 3.00× threshold.
Figure 3.
Figure 3.
The effects of current amplitude and frequency for electrode D2. Left: individual drawings for a set of trials (each drawn in a different color) aligned using the reference grid. Right: the average of these trials, aligned at the mean centroid of each individual drawing. (A, B) Ten trials with a pulse train of current amplitude 1.25× threshold and frequency of 20 Hz (one of the trials is a replication of the data shown in Fig. 2B). (C, D) Five trials at a current amplitude of 4.0× threshold and a frequency of 20 Hz. (E, F) Five trials at a current amplitude of 1.5× threshold and a frequency of 80 Hz.
Figure 4.
Figure 4.
The effects of amplitude and frequency on apparent brightness and size. (A, C) Brightness and apparent size as a function of normalized (relative to threshold) amplitude for nine electrodes. (B, D) Brightness and apparent size as a function frequency for the same nine electrodes. Each electrode's data are fit with the best-fit linear regression.
Figure 5.
Figure 5.
Normalized size and brightness as a function of amplitude and frequency averaged across nine electrodes. Data are fit using linear regression.
Figure 6.
Figure 6.
The study model. BOX 1: the time stimulus, f(t), was transformed into a spatiotemporal representation, on the basis of the measured electrophysiological thresholds from a disc electrode. BOXES 2–5: the output was then passed through a modified version of the perceptual sensitivity model incorporating threshold and suprathreshold parameters. The resulting output corresponds to a spatial brightness response, B(r).
Figure 7.
Figure 7.
Predicted percepts with increasing amplitude (top row) and increasing frequency (bottom).
Figure 8.
Figure 8.
Comparing brightness rating and apparent size psychophysical data to model predictions for the effects of changing amplitude and frequency. Experimental data from amplitude modulation are shown in dark gray, and data from frequency modulation are in light gray. Dotted and solid lines: model predictions.

Similar articles

See all similar articles

Cited by 29 articles

See all "Cited by" articles

Publication types

Associated data

Feedback