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Alan R. Boate, Stuart G. Coupland, David Priest, Jeremy Gribben; A New Digital Micro-mirror Stimulus Generator For Visual Electrophysiology. Invest. Ophthalmol. Vis. Sci. 2012;53(14):2470.
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Cathode ray tubes (CRTs) have been the visual stimulators of choice for recording the pattern ERG, pattern VEP and multifocal ERG. CRT stimuli are painted as sequential series of horizontal raster scans from top to bottom of a screen. The key advantage is the relatively rapid impulse of light from each pixel (~1 to 2 ms). Commercial grade CRTs are no longer available and have been replaced by liquid crystal (LCD) displays. LCDs have their own inherent limitations as visual stimulators; pixel change is relatively slow - taking several milliseconds, the line sequential refresh results in a travelling band of luminance, and the stimulus ‘impulse’ is constrained to be the same as the frame rate (~13-16ms). Herein, we describe the first use of a digital micro-mirror device (DMD) based visual stimulator for investigation of electroretinal function which addresses many of the above problems. DMDs are arrays of micro-mirrors found in projectors and digital cinema. Currently DMDs are not used in research based visual stimulators, as available video drivers have been found to create timing irregularities and lighting artifacts.
A customized controller was developed to allow control of individual mirrors in a 480x320 DMD at variable frame rates up to 6800 frames/sec, while allowing the entire mirror array to be globally reset within 1 microsecond. High intensity LEDs were used as light sources and stimulating impulses can be as short as few microseconds. The customized DMD stimulator was integrated into a multi-spectral ophthalmoscope (RHA 2020, Annidis Health Systems) to enable simultaneous imaging and stimulation of the retina. In four human subjects (8 eyes) a 30 minute checkerboard pattern stimulus subtending 10x10 degrees was imaged on the fundus. Pattern reversal rate was systematically increased in decade steps from 10 to 60 reversals/second and PERG was recorded using Espion (Diagnosys LLC).
At 10 reversals/second the PERG had a truncated appearance with N35, P50, N95 components still visible. With increasing reversal rate the PERG waveform morphology changed to a sinusoidal appearance with a trough and peak. Peak implicit time appeared to increase with increasing reversal rate (p = 0.05) and trough to peak amplitude decreased significantly (p = 0.01).
Herein we have described a novel visual stimulation with simultaneous updating of all pixels. We have demonstrated that this device can display checkerboard pattern stimulation for recording rapid reversal pattern ERG. Unique custom designed focal and multifocal stimuli can be developed which take advantage of the high frames rates, global frame switching, spectral selectivity and high luminance levels now available.
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