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S.C. Chen, L. Hallum, G.J. Suaning, N.H. Lovell; Head Movement Dynamics of Prosthetic Vision: Virtual–Reality Simulation Study . Invest. Ophthalmol. Vis. Sci. 2006;47(13):3209.
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Although scanning the stimulus is a central strategy in low vision and retinal prosthetics alike, there has been little attention paid to the dynamics of head movement (HM) data. We report on HM velocity of subjects studied under simulated prosthetic vision (SPV).
In an earlier report (Chen et al., 2005, J. Neur. Eng., 2:S135–145), we studied visual acuity (VA) of SPV in an immersive virtual–reality environment. In the study, we also recorded HM (pitch and yaw) of subjects while undergoing testing. Subjects (n=12) were tested on the Landolt C optotype at random positions in their visual space. Subjects had to use HM to locate each symbol and identify its gap orientation. Images updated upon HM only; eye movements did not present subjects with additional information. Subjects were encouraged to use HM and suppress any eye movements. Each subject participated in one control (normal vision) session and ten SPV sessions. SPV was modeled as fixed–spaced (1.6o) lattice of spots of light (phosphenes). Phosphene size was modulated.
During the control sessions, subjects’ averaged HM velocity (HMV) was minimal (∼2o/s). For SPV sessions, HMV was as much as 20o/s. HMV was positively correlated with VA performance and plateaued at ∼8o/s (25% phosphene spacing). The top six learners of the study demonstrated an averaged 2.4x performance improvement in conjunction with a 4.4x HMV increase over the bottom six. Increased HM was also observed for tests involving smaller optotype sizes and phosphene generation processes that retained more high frequency content of the original image.
HM enriches SPV perception, which can be observed in its close correlation to VA performance. However, most sessions and most subjects, failed to reach 8o/s (plateau threshold (PT)), thus may not have achieved maximum performance. Projection showed that if subjects were deprived of HM, performance would reduce to 60% of averaged performance at plateau. Also, if HM data scales linearly, one would expect PT to coincide with the averaged control session HM (2o/s) at phosphene spacing of 0.4o or 24 arcmin (normal vision VA is 1 arcmin). Lastly, there is an indication that increased HM is associated with "deciphering" higher frequency content in prosthetic vision.
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