[open] Flickering stimulus generation (custom shape with checkerboard fill at a certain freq)

Comments

• sebastiaan

  April 2014 edited 6:15AM

  Is there a way to verify that the flicker rate on my system was close to 8Hz?

  If you use a back-end that uses a blocking flip (see Timing documentation), the timestamps that you get for your display presentation are fairly accurate. Below you see an example script that shows how you can determine the flicker rate of two alternating stimuli using the PsychoPy back-end. However, to be absolutely sure of your timing you will need to benchmark your system with a photodiode.

  Example script:

  import numpy as np
  from psychopy.visual import GratingStim
  
  # Create two stimuli
  stim1 = GratingStim(win, size=128, color=1)
  stim2 = GratingStim(win, size=128, color=0)
  
  # Alternately present the stimuli and log the timestamp after each window flip
  lT = []
  for i in range(100):
      stim1.draw()
      win.flip()
      lT.append(self.time())
      stim2.draw()
      win.flip()
      lT.append(self.time())
  
  # Use numpy to determine the difference between subsequent timestamps and print
  # this info to the debug window.
  aT = np.array(lT)
  aDiff = aT[1:] - aT[:-1]
  print 'M = %.2f, SD = %.2f' % (aDiff.mean(), aDiff.std())
  

  I just came across Opensesame with the psychopy backend but am unsure the best way to proceed or if I can access pre-built checkerboards and then just use an alpha mask of the shape to generate my desired stimulus

  That depends on your specific stimuli, of course. But if you want familiarize with the use of masks and textures in PsychoPy, you may want to take a look at this tutorial:

  • http://www.cogsci.nl/blog/tutorials/211-a-bit-about-patches-textures-and-masks-in-psychopy

  Cheers!
  Sebastiaan

  
  
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