Abstract
Commercially available computer monitors (e.g. CRTs and LCD) and projectors have been used to display visual stimuli in vision experiments for some time. These displays produce different colours by mixing red, green and blue lights, and this is normally sufficient to stimulate one cone class while keeping the other two constant (silenced). However, to isolate a cone response also from rods and intrinsically-photosensitive retinal ganglion cells (ipRGC), we need a stimulator with as many channels as the number of photoreceptor types that need to be modulated/silenced. Several labs have built devices with more than three channels but often these systems have drawbacks. For example: they cannot produce complex spatial patterns, or the light output is not bright enough, or they require complex calibration procedures and time-consuming routine maintenance. We developed a multi-channel visual stimulator based on a digital micromirror device (DMD) and five spectrally differently LEDs, suitable for commercial production. While DMD projectors are common, a system using five channels is unusual. The device allows more control over the DMD and LEDs than is possible with commercial three-channel displays. When programming stimuli, the experimenter can choose how to trade-off chromatic resolution, number of channels used, temporal precision, palette size, and radiance, as appropriate. From spectral measurements, we calculate that we can produce modulations in ipRGC contrast of up to 28% while silencing cones. Cone contrast modulation can be as high as 36%. This is achievable from a compact device producing a projected image approximately 18 inches diagonal, with a peak white output that measures up to 250cdm^-2 (although this will be less for most stimuli). The flexibility of control over the DMD also allows high temporal frequency presentations (e.g. square-wave modulations of over 1000Hz) or finely-controlled (>16bit) greyscale, which makes it suitable for a large variety of vision experiments.