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Hiroshi Horiguchi, Jonathan Winawer, Brian Wandell, Robert Dougherty; Novel MR safe stimulator with six color channels at accurate high temporal frequencies. Journal of Vision 2011;11(11):1236. doi: 10.1167/11.11.1236.
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© ARVO (1962-2015); The Authors (2016-present)
Commercial displays have three significant limitations for temporal and chromatic neuroscience measurements: (a) severe contrast attenuation for frequencies over 50 Hz, (b) only three color primaries, and (c) modest maximum intensity and dynamic range. To study temporal processing and adaptation within isolated color channels, we developed a new, uniform-field display apparatus, suitable for both psychophysics and magnetic resonance (MR) imaging. The display consists of an Arduino Mega micro-controller board, six high-intensity LEDs (LUXEON® Star), precise constant-current controllers (LuxDrive™ BuckPuck), optical fiber light pipes and a MR-compatible eyepiece. The micro-controller runs a custom (open-source) firmware that receives simple commands from a host computer over a USB connection. The waveform generation, temporal envelope, and gamma-correction are computed on the microcontroller. The waveform play-out and pulse-width modulation (PWM) are precisely controlled by the microcontroller's 16-bit timers. The LED intensity is refreshed at about 2000 Hz with 12-bit PWM intensity control. Therefore, the device produces accurate sine wave flicker at temporal frequencies over 100 Hz. Photons from the LEDs go through plastic optical fiber bundles and are homogenized by two diffusers (LSD®, Luminit™) at the eyepiece. Through an aspheric lens attached with the eyepiece, a subject can observe spatially uniform flickering stimuli at a visual angle of over 45 degrees radius. The spectral peaks of the six LED colors are 447.5 nm (Royal Blue), 470 nm (Blue), 505 nm (Cyan), 530 nm (Green), 590 nm (Amber) and 627 nm (Red). Mean luminance through the eyepiece is 720 cd/m2. (For more details, see:http://vistalab.stanford.edu/newlm/index.php/LedFlicker) With six color channels we can modulate photopigments in the rods, the three cone classes or the intrinsically photoreceptive ganglion cells either separately or in combination across a wide range of temporal frequencies and mean intensity levels. The apparatus overcomes temporal and chromatic limitations of commercial displays to generate novel psychophysical and neuroimaging measurements.
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