I was wondering whether anyone knows the current conversion from stimulus size in pixels to degrees of visual angle when viewing stimuli through the mirror at the 1.5T.
That is, how many screen pixels per degree are there?
The answer will likely vary with the screen's resolution, which is 640 X 480 in my particular case.
The conversion depends on where the screen is placed as well as the pixel resolution. In CIGAL I convert stimuli from visual angle to pixels using screen width and height (cms), screen width and height (pixels), an distance to screen (cm). alpha = 2 * atan(scrwidcm/scrdist); factor = scrwidpixels/alpha factor = scrwidpixels/(2 * atan(scrwidcm/scrdist)) The trouble is that the distance to the screen is not fixed and the active screen area changes with the screen position and zoom. But, if you assume the screen is 67 cm from the eyes and the active viewing area is 33 cm across, then the conversion factors will be: 640x480 23 pixels/degree 800x600 29 pixels/degree That's close; if you want to be precise you should put up one of your stimuli with the screen in the bore and measure for yourself.
visual angle at the 1.5T
Posted - Jan 16 2004 : 2:21:06 PM
