I am using fMRI imaging in my study (Flanker Task) where I am lost at how to calculate the Stimulus size. I have the visual angle information for my stimulus but I am unable to convert that information into pixel size. In the fMRI setting, the projector screen is placed at the back of the participant's head behind the scanner and the images are projected to an angular mirror on the head coil which then gets reflected in the participant's eyes.
For conversion of visual angle to pixel size, we generally need the visual distance which in this case cant be just the distance between the screen to the eye. I am not sure how to take into account the angular arrangement of the head coil mirror into this calculation or if we need to account for mirror dimension as well.
Following is the information we have,
Given Stimulus Visual angle= 3.28 on x-axis, 0.41 on y-axis
Screen to mirror distance = 160 cm
The mirror is at a 45-degree angle to the head
Mirror to eye distance = 14 cm
Screen dimention: 1920x1080(resolution)
87.5 cm (Width), 48.3 (Height)
Mirror dimentions = 15.5 cm (Width), 12 cm (Height)
Question: What would be the pixel size or dimensions(height/width) for the Stimulus?
Any help would be greatly appreciated. Thank you.
Hello, Tripti Verma
It seems using the distance to the mirror (not the screen) is a good idea in this case. If your aim is to know the visual angle subtended by the stimulus shown on the screen, I don't see why you would be concerned about the screen size. I hope I have understood that correctly from your description.
If you want a given visual angle, you'd change the image size projected at the given viewing distance to meet that visual angle size requirement. If you have a given image size and are not changing that or the viewing distance, then your visual angle would be calculated based on those two points, not necessarily on the initial size of the image you created on the screen.
I love this tool for converting from pixels to degrees:
https://www.sr-research.com/visual-angle-calculator/
There is a similar tool here:
https://michtesar.github.io/visual_angle_calculator/
I hope this helps.
Warmly,
Brooke Hallowell
First, calculate the required size of your stimulus on the mirror: 8cm x 1cm (https://elvers.us/perception/visualAngle/). Next, go into the scanner with a ruler, and adjust the stimulus pixel size until the dimensions are the correct size on the mirror
I’ll make a couple of assumptions, namely that the mirror is flat (if it is curved then all kinds of complications ensue!), and that the screen is angled perpendicular to the line of sight (rather than being tilted) such that the screen itself appears rectangular to the participant (not trapezoidal). If these are true, then things are pretty simple. You just calculate the visual angle/pixel size using the total path length for the light as the viewing distance (i.e. Screen to mirror distance + Mirror to eye distance, in this case 160 + 14 = 174cm). If the 2 assumptions above are correct, then the size of the mirror and the angle of the mirror are not relevant.
I hope I haven’t misunderstood anything, and that this answers your question.
Cheers,
K
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