P3Speller using Icons (size problem)

[ikara]

Hi there,
I have a 3x3 matrix on which I'm displaying different pictures of fixed size (420 by 540 pixels).
The only problem is that I can't find a way to maintain the image proportions when I try to display them on the matrix,
since I need to give a percentage of the screen width/height they occupy (TargetWidth and TargetHeight respectively)

So, what I'm asking for is a way (if there's any) to maintain the aspect ratio of the .bmp files that I'm using
and also, if it's possible to add padding in the .bmp files through BCI2000 (if not, I can do it in some Image Editing Software)

Thank you in advance

[mellinger]

Hi,

it's not possible to specify individual sizes for target icons. If all icons have the same aspect ratio, you can use TargetHeight and TargetWidth values with the appropriate ratio. Otherwise, you will need to add padding in an image editor, as you suggest.

Juergen

[ikara]

Thank you for the fast reply.
Another question concerning the calibration of the P3Speller using Icons instead of letters is whether I should change the settings for
StimulusDuration, ISI Min/Max Duration. Would that make any sense (since an image is visually more complex than a character) and what are the timeframes (in ms) that you suggest?

[mellinger]

I don't think visual complexity will influence optimal presentation times, as the subject will be aware of the image's content before it is presented. I suggest a stimulus duration of between 30 and 40 ms, and a fixed ISI of 125ms.

[ikara]

Changes in my experiment setup will require a matrix of 2x3. (I will be displaying 6 pictures in the matrix)
Are there any suggestions concerning the settings, (other than training the p300 classifier for the specific
matrix) or should it work optimally with the current settings of the p300 speller application?

[mellinger]

A small matrix means a high frequency of target flashes, so it might not work as well as the larger matrices do. However, check:
Sellers et al. A P300 event-related potential brain-computer interface (BCI): the effects of matrix size and inter stimulus interval on performance. Biological Psychology 73 (2006), pp 242-252.
They found good accuracy results with a 3x3 matrix, using an ISI of 175ms.