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Takao Endo, Takashi Fujikado, Hiroyuki Kanda, Takeshi Morimoto, Kohji Nishida; Calibration of eye movements using reaching movements under simulated blindness conditions. Invest. Ophthalmol. Vis. Sci. 2014;55(13):4162.
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Eye movements can affect where a phosphene is perceived by patients with advanced retinitis pigmentosa (RP) who are implanted with a retinal prosthesis and equipped with an external camera (Fujikado, IOVS 2011). Because blind patients cannot fixate a target for calibration, we examined whether we can calibrate the eye position using the eye movements accompanied by reaching movement in subjects with simulated blindness.
The eye movements of ten normal subjects with right eye-dominance (4 men, 6 women; age, 22 to 50 years) were recorded with an eye tracker (Glass-type, Tobii®). Initially, the eye movements were calibrated using targets of known distances. Then, the eye movements were recorded with the subjects told to follow the index finger which pointed to the markers attached to an acrylic board located 40 cm in front of subjects (sighted condition). Nine markers were arranged in a lattice configuration with a 9.2 cm separation on the board. Then, the eyes were occluded by placing a piece of foil on the eye tracker glasses which decreased the visual acuity to NLP (blinded condition). Subjects were trained to explore and touch the marker in the blinded condition. Then, eye movements of the blinded subjects were recorded while the subjects were forced to fixate the index finger, which pointed the marker on the board. The distance between the gaze position to the central mark and that to each residual mark was measured and was divided by the distance between the central and each residual mark. The average of the 8 values was defined as the R value, and it was compared between the sighted and the blinded conditions.
In the sighted condition, R = 0.99±0.12 with a variance of 0.015, and in the blinded condition R = 0.76±0.76 with a variance of 0.053. R was significantly smaller under the blinded condition than the sighted condition (paired t test; P<0.001).
The distance of the gaze positions in the reaching movements was about 20% shorter in the blinded condition than in the sighted condition in normal subjects. These findings indicate that when we map the perceived position of phoshenes in advanced RP patients implanted with retinal prosthesis and with an external camera, the effect of eye movements on the location of perceived phosphene can be evaluated by an eye tracker which is calibrated by the eye movement following the reaching movement with a correction factor.
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