Blind subjects with severe to profound RP were implanted with the Argus II prosthesis as part of a clinical study at multiple clinical sites worldwide. Those centres were: Doheny Eye Institute at the University of Southern California (Los Angeles, California, USA); Retina Foundation of the Southwest (Dallas, Texas, USA); Moorfields Eye Hospital (London, UK); University of California at San Francisco (San Francisco, California, USA); Wilmer Eye Institute at the Johns Hopkins School of Medicine (Baltimore, Maryland, USA); Wills Eye Institute at the University of Pennsylvania (Philadelphia, Pennsylvania, USA), the Edward S. Harkness Eye Institute at Columbia University (New York, USA); Puerta de Hierro Centro Medico (Guadalajara, Mexico); Centre Hospitalier National d’Ophtalmologie des Quinze-Vingts (Paris, France); Hôpitaux Universitaires de Genève (Geneva, Switzerland). All data presented here were collected at these clinical sites, with the exception of sites in Philadelphia and New York, where psychophysical testing and data collection were done at the Scheie Eye Institute at the University of Pennsylvania and Lighthouse International, respectively.
All implanted subjects had some level of bare light perception (BLP) prior to surgery, and at the time of the latest clinical follow-up. No subjects had a recordable visual acuity prior to surgery (worse than 2.9 logarithm of the minimum angle of resolution (logMAR)) (ie, worse than 20/15887) as determined by an adaptive four alternative force choice (4-AFC) square wave grating test.
The experiments described in here were run on all Argus II subjects who were currently available for regular testing and who had been implanted before 1 July 2009 (27 subjects). Nineteen of the 27 subjects (70%) included in this study were male. At the time of publication the mean subject age was 58±10 (range 27–77) years; the mean duration of implantation was 430±314 (range 60–857) days.
In the experiment, white square stimuli (5.85 cm2, 200 pixels2, 221 cd/m2) were displayed in random locations on a 19″ (48.3 cm) LCD touch screen monitor (ELO Accutouch or IntelliTouch Surface Wave LCD Touch Screen Flat Monitor 1915L; Menlo Park, California, USA) located 12″ (30.5 cm) in front of the subject (subtending a visual angle of 10.9°). After the onset of the square and an auditory prompt, the subjects were instructed to locate and touch the square centre. The program provided verbal feedback after subject’s responses indicating ‘correct’ for a touch anywhere on the square and ‘close’ for a touch within 100 pixels (2.9 cm) of the edge of the square. Corrective feedback was given after ‘close’ and incorrect responses. For example, a response 6 cm to the left of and 4 cm below the stimulus square prompted the response ‘It was higher and right.’
Before the test was administered, 10 trials were presented in ‘training mode’ to familiarise the subject with the position and vertical and horizontal extent of the monitor. In this mode subjects defined the location of the square centre by pressing the blank touch screen monitor; a 5.85 cm square appeared at that location until subjects touched another part of the monitor. The subjects’ cameras were aligned with respect to the monitor centre. Experiments consisting of 40 trials were run with both the system on and off (80 trials total per subject). The subjects’ eyes were not patched in either condition; in system off testing the subjects did not wear the Argus II system glasses.
Subjects were instructed to move their head while keeping their eyes centred relative to their head position. (Since the stimulating array is fixed on retina, the perceived phosphene location depends on eye and head position.) The x–y pixel coordinates of the square centre and of the location touched were recorded for off-line analysis and plotting using Matlab 7.1, Microsoft Excel 2007 and Igor Pro 6.1.