[open] Need to define the location of the stimulus on the screen to the degree.. Please help!

Priya_m2092

Hi.. I'm conducting an experiment where I need to present the stimuli 2.5-3° left or right to the central fixation. I'm gonna need to use images and words. Please tell me its possible using Open Sesame!!

Edwin

Hi,

Recalculating visual angles (I assume you mean visual angle, not a rotation) to pixel distances is something you can do yourself, either using OpenSesame, or a calculator.

You can find the formula on how to calculate a distance on a flat screen based on a visual angle on Wikipedia.

The next thing you would need to do is calculate the amount of pixels per centimeters your screen contains, which is rather easy as well: measure the screen width, then divide the (horizontal) display resolution by your measurement.

You'll find an example below. Just fill in your own values when you want to do this yourself.

Good luck!

Edwin

Example

decided

visual angle (V): 3 degrees

measured

distance screen-participant (D): 57 cm

screen width: 33.8 cm

display resolution: 1280x1024 (pixels)

calculated

distance on monitor (S) = tangent of the visual angle multiplied by the distance screen-participant

S = tan(V) * D = tan(3°) / 57 cm = 2.98 cm

amount of pixels per centimeter = (horizontal) display resolution / display width

pix/cm = 1280 pixels / 33.8 cm = 37.9 pixels per cm

distance on monitor in pixels = distance on monitor in centimeters multiplied with amount of pixels per centimeter

S * px/cm = 2.98 cm * 37.9 pixels per centimeter = 113 pixels

Priya_m2092

Thank You Edwin!! That was a big help.
However, I still don't know WHERE to input these values in Open Sesame? Do I find the desired width and height of the stimulus based on these values and then input them as the (x,y) coordinates in the 'Grid' option given?

Please forgive the number of my questions. I'm quite new to this.

Edwin

Glad I could help!

If you're new to this, the tutorial might be really useful. These will learn you how to create stimuli. Just keep in mind that all of the numbers that indicate stimulus position and size in OpenSesame are in pixels. Use the method I posted earlier to recalculate all of your sizes in visual angle to pixels. Then use the values you've calculated in OpenSesame.

General tutorial: http://osdoc.cogsci.nl/tutorials/step-by-step-tutorial/

Video tutorials: http://osdoc.cogsci.nl/tutorials/video-tutorials/

Good luck!

booradley

Hi @Edwin,

Sorry if this question comes across a bit daft, but here we go.

I've been using your formula above to recreate stimuli size and position on a new monitor with a high resolution.

The resolution is as follows: 5120*2880 (which I've set to match in the experiment).

The width of the display screen is 59.4cm.

If I'm not mistaken the following calculation will give me pix per cm:

5120/59.4 = 86.19529 pix per cm

Therefore if I want to determine the size of the radius in pix of circle with a cm radius of 1.725 I'd do the following:

86.19529*1.725 = 148.6869 pix

I do something similar for the distance from fixation to the centre of the circle:

86.19529*12.93103cm = 1114.594 pix

However when I measure the size/distance in real terms via measuring tape they are way off when displayed via OpenSesame.

I'm guessing something is amiss in my calculation as when I divide the above values by 2 and re-input the divided values into OpenSesame the size/distance is correct.

I'm displaying on a 27" Mac.

Any thoughts?

Thanks,

Boo.

Edwin

Hi @booradley,

That might not actually be a very daft question at all! Because the resolution is quite high, I imagine there might be a layer of software in between OpenSesame and the actual display. What resolution does OpenSesame think the display is?

EDIT: Oh, I just realised you actually mentioned this in your question! Is there anything in your output that suggests the window is resized to a value you didn't set it to? PsychoPy will do this, but it will also print out a message to the Debug Window telling you that it did.

Cheers,
Edwin

booradley

Hi @Edwin,

I'll check that when I'm in the lab next and get back to you. Good to hear that I may not be totally bonkers after all.

Cheers,

Boo.