Phosphene perceptions and safety of chronic visual cortex stimulation in a blind subject

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Stimulation of primary visual cortices has the potential to restore some degree of vision to blind individuals. Developing safe and reliable visual cortical prostheses requires assessment of the long-term stability, feasibility, and safety of generating stimulation-evoked perceptions.

A NeuroPace responsive neurostimulation system was implanted in a blind individual with an 8-year history of bare light perception, and stimulation-evoked phosphenes were evaluated over 19 months (41 test sessions). Electrical stimulation was delivered via two four-contact subdural electrode strips implanted over the right medial occipital cortex. Current and charge thresholds for eliciting visual perception (phosphenes) were measured, as were the shape, size, location, and intensity of the phosphenes. Adverse events were also assessed.

Stimulation of all contacts resulted in phosphene perception. Phosphenes appeared completely or partially in the left hemifield. Stimulation of the electrodes below the calcarine sulcus elicited phosphenes in the superior hemifield and vice versa. Changing the stimulation parameters of frequency, pulse width, and burst duration affected current thresholds for eliciting phosphenes, and increasing the amplitude or frequency of stimulation resulted in brighter perceptions. While stimulation thresholds decreased between an average of 5% and 12% after 19 months, spatial mapping of phosphenes remained consistent over time. Although no serious adverse events were observed, the subject experienced mild headaches and dizziness in three instances, symptoms that did not persist for more than a few hours and for which no clinical intervention was required.

Using an off-the-shelf neurostimulator, the authors were able to reliably generate phosphenes in different areas of the visual field over 19 months with no serious adverse events, providing preliminary proof of feasibility and safety to proceed with visual epicortical prosthetic clinical trials. Moreover, they systematically explored the relationship between stimulation parameters and phosphene thresholds and discovered the direct relation of perception thresholds based on primary visual cortex (V1) neuronal population excitation thresholds.

ABBREVIATIONS ECoG = electrocorticographic; FOV = field of view; PW = pulse width; RNS = responsive neurostimulation system; V1 = primary visual cortex.

Article Information

Correspondence Nader Pouratian, University of California, Los Angeles, CA. npouratian@mednet.ucla.edu.

INCLUDE WHEN CITING Published online May 31, 2019; DOI: 10.3171/2019.3.JNS182774.

Disclosures Dr. Patel has ownership in and works for Second Sight Medical Products Inc. Dr. Dorn works for, owns stock in, and holds a patent with Second Sight Medical Products Inc. Dr. Greenberg works for and owns stock in Second Sight Medical Products Inc. Dr. Pouratian is a consultant for Second Sight Medical Products Inc. This work was supported by an industry grant from Second Sight Medical Products Inc., which covered the cost of the device, the surgical implant, and personnel.

© AANS, except where prohibited by US copyright law.

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Figures

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    Phosphene mapping generated based on integration of the subject’s drawings and descriptions. Red areas correspond to the phosphene mapping experiment performed at month 8 of the study (three trials per channel), and blue areas correspond to the mapping experiment performed at month 19 (one trial per channel). Phosphene drawings are mapped to the polar space and connected to their corresponding electrode shown on the MRI slice of the right medial occipital lobe. Yellow line represents the calcarine sulcus.

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    Impedances measured at the beginning of each session during the 81-week study. A: Box plot of impedance values representing the median, the first and third quartiles, and the outliers (dots). B: Percent change of impedance for each contact compared to the first measurement (week 1) over time. Shaded areas indicate ± 20% change (chosen arbitrarily for scale). Values on the right axis represent the percent change of the last measurement (week 81) relative to the initial measurement. Figure is available in color online only.

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    Current amplitude thresholds for different PW values for contacts A1 (A) and B2 (B). The same data are presented in the form of charge per trial (C and D; i.e., total charge over the 250-msec burst duration). Curves in A and B as well as the lines in C and D were fitted to the data for better understanding of the relationship between stimulation parameters. Figure is available in color online only.

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    Current amplitude thresholds for different frequency values for contacts A3 (A) and B2 (B). The same data are presented in the form of charge per trial (C and D; i.e., total charge over the 250-msec burst duration). Curves in A and B as well as the lines in C and D were fitted to the data for better understanding of the relationship between stimulation parameters. Graphs of threshold testing results for different frequencies and burst durations (E and F). Controlling the burst duration allows for delivering a different number of pulses for each pulse frequency. Surface areas of the circles are proportional to the level of current thresholds. Figure is available in color online only.

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    Percent change of the threshold levels for current amplitude (A) and charge per trial (B) thresholds. Dashed lines correspond to the measurements during pulse interval threshold testing, and solid lines indicate the measurements during frequency threshold testing, at months 1, 7, and 19 of the study. The ± 20% change interval is indicated by the shaded area for each contact. Figure is available in color online only.

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    Subjective intensities of the perception based on the amount of charge delivered. Increasing the delivered charge was achieved by increasing the current amplitude (amplitude modulation [Amp mod]) or frequency (frequency modulation [Freq mod]).

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