Video System

Much of the video system was provided either wholly or partially by Nancy Kanwisher and her lab at MIT. Special thanks to Nancy and her father, and Liu Jia; thanks also to Dr. Alison Harris, formerly an undergraduate working in Nancy’s lab, and now a PhD recipient from Harvard University in the Vision Sciences Lab.

IMAGE GENERATION

Our visual stimuli are computer-generated. The images generated on the computer monitor are mirrored by an InFocus LP850 projector. The LP850 uses Digital Light Processing (DLP) technology developed by Texas Instruments (Warren, NJ). This technology immediately transmits the image drawn on the LCD within the projector to its mirror-and-lens system, meaning that there is very little lag between the receipt of image information and transmission of the image (a large concern for the temporal accuracy of MEG experiments). The native resolution of this projector is 800 x 600, and our experiments either employ this resolution or most likely a 1024 x 768 resolution.

IMAGE RESOLUTION

We have replaced the original lens system with several wide-angle camera lenses to produce a focused image size of approximately 18cm x 18cm (7″ x 7″) at a distance of 24cm (9.5″) from the participants. The distance of the image from the projector is approximately 2.4m (8′). The image also enters the MSR horizontally, and must be reflected down. We accomplish this using a first surface mirror from Edmund Scientific (Barrington, NJ). This mirror is suspended from the ceiling and is fixed at a 45° angle; the incoming image hits the mirror and is reflected 90° straight down.

The image comes to rest on a ground glass “screen.” The screen is held in a frame built by Nancy Kanwisher’s father out of old fiberglass supports from a windsurfer. It isn’t pretty, but it’s functional. What is slightly unusual about the setup is that the participant is essentially viewing the back of the screen, rather than the front of the screen (and what we usually think of when we see anything project onto a screen). This means that the image is natively “backwards”; in order to fix this we use the rear projection option on the LP850 to “flip” the image.