Flickering Stimulus for SSVEP-Experiment

[Markus]

Hi,

as stimulus I use squares of pixels. The pixels can be in one of two colors and thier position is rendomized on each trial (see Code). In PsychoPy I present this Square with visual.PatchStim. For a Study with steady-state visual evoked potentials (SSVEPs) I want to present all pixels of one color at a different rate (e.g. 12.5 Hz vs. 10 Hz).

I have no idea how and where I can enter this Information in psychopy. Ist there a possibility to write a "simple" modification for visual.PatchStim in the Builder or in the Coder. Or do I have to program a new modified version of visual.PatchStim?

Could it be necessary to handle all pixels of one color as different stimuli (forming a square), so that each stimuli can be presented at a certain Hz?

THANKS!

Markus


Code for Square:

def generate_px_image(sizex=128, anteil_l = 0.5, anteil_r = 0.5):

    size = sizex **2

    # n_white_px = round(size * anteil)

    n_black_l = round(size/2.0 * anteil_l)

    n_black_r = round(size/2.0 * anteil_r)



    n_l = np.ones(size/2) * -1

    n_r = np.ones(size/2) * -1

    n_l[0:n_black_l] = 1

    n_r[0:n_black_r] = 1

    n_l = np.random.permutation(n_l).reshape(sizex,sizex/2)

    n_r = np.random.permutation(n_r).reshape(sizex,sizex/2)

    return np.append(n_l,n_r,1)

[dani]

Not sure it could help:

http://dlinares.org/stimuli-in-psychopy/repetitive-stimuli.html

dani

[Jonathan Peirce]

Markus,

Just to be clear, in case it wasn't obvious from Dani's demo,
presentation rates are not something that PsychoPy handles for you; on
every frame of your monitor you simply have to decide what to draw to
the screen. To have stimulus flicker at 10 Hz (on a 60Hz monitor) you
would draw your stimulus for three frames and then not draw it for 3
frames (the period of the stimulus is 60/10=6 frames and you want half
of that period 'on' and half 'off').

You need to keep this in mind when choosing what rates you're aiming for
as well. On a 60Hz monitor you can't achieve 12.5 Hz because that would
require a period of 4.8 frames; you'd need to have 2.4 frames on and 2.4
frames off, which obviously isn't possible. On a 100Hz monitor this rate
is fine because 100/12.5=8 so you can have 4 frames on, 4 frames off.
(Then again 30Hz is possible on a 60Hz monitor but not a 100Hz monitor!)

These aren't things that PsychoPy, or any other software, can get around.

all the best,
Jon

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Jonathan Peirce
Nottingham Visual Neuroscience

http://www.peirce.org.uk


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