Visual stimulation

Visual stimulation and eyetracking

 

General remarks

The most important requirements for a visual stimulation system are related to a controlled and repeatable presentation of the visual stimulus with respect to latency, luminance values and rise times, color, and image geometry. In the MEG community video projectors are the most popular visual stimulation devices. We will therefore give some examples of the typical problems for these  devices with respect to the above list of requirements. Typical problems for video projectors are slow rise times, frame drops (some images in a rapid stream not being presented at all), and color drifts over time. The severity of these problems not only depends on the device used, but also on the settings used to drive the device, i.e. frame drops typically occur when the video projector is not driven with its native resolution and repetition parameters. In depth discussions on these topics maybe found at the visionlist (http://visionscience.com/mailman/listinfo/visionlist).

Please always consider possible safety issues: When using custom build LED stimulators or commercial video projectors that were retrofitted with custom projection lenses, extreme light intensities may be reached that could be harmful to the subject's or patient's eye. Therefore, sufficient testing and the application of safety regulations is a must.

 

Creating the image

Video Projectors

Currently there are two competing technologies. there are LCD projectors that use three colored TFT-like mini panels to filter the white light from the lamp so colors are all produced at the same time and pixel-management and color go togther. Then there are DLP projectors where pixels are created by an array of micro-mirrors that divert light either into the line of vision or out of it. In cheaper models color is realized by a fast rotating color wheel in front of the mirror and the (single) mirror creates the brightness profile for each color sequentially. This often leads to the so called rainbow effect, where subjects see a flickering of colors when stimuli change rapidly. A better (and way more expensive) solution are 3-mirror DLP projectors that have a mirror for each color.

 

Model: Sanyo 41 XP / 41 XP-L (lenseless)

Used at: BIC Frankfurt, Donders Centre Nijmegen

Technology: LCD

Specs and Comments: Runs without too many framedrops (<0.1%) at 1024x768 at 60Hz. Other resolutions and refresh rates lead to massive framedropping

 

Special graphics cards and software bundles

Devices based LEDs and fiberoptics

Getting the image into the magnetically shielded room (MSR)

Waveguides, front silvered mirror systems, TCO screens

MEG compatible eyetrackers

Model: SR Reserach eyelink 1000 (2000)

Used at: BIC Frankfurt, CCNI Glasgow (?)

Specs and Comments: Monocular tracking @1000Hz (2000Hz with upgrade), binocular tracking  @500Hz (1000Hz with upgrade)