[open] Random dot motion

Rotem · April 2014

Hello, I heard there's an already implemented "Random dot motion" task in open sesame which i couldn't found in the standard tasks section. Is this task indeed available? Thanks, Rotem

sebastiaan · April 2014

What makes you think that this task has been implemented already? It could be, but I don't recall any mentions of it here on the forum.

At any rate, it shouldn't be too hard to implement this using PsychoPy's DotStim.

http://www.psychopy.org/api/visual/dotstim.html

All parameters are documented under the link above, but the following script shows the basic idea:

from psychopy.visual import DotStim
# Create a DotStim, which is automatically updated with every `draw()` call.
ds = DotStim(win, fieldSize=400, speed=10, dotLife=100, nDots=100, coherence=.5)
# Show 100 frames
for i in range(100):
    ds.draw()
    win.flip()

Cheers!
Sebastiaan

Rotem · April 2014

Thanks...I read about the phyton and inline scripts.
Should i copy the following script also to the prepare and run phases?

class psychopy.visual.DotStim(win, units='', nDots=1, coherence=0.5, fieldPos=(0.0, 0.0), fieldSize=(1.0, 1.0), fieldShape='sqr', dotSize=2.0, dotLife=3, dir=0.0, speed=0.5, rgb=None, color=(1.0, 1.0, 1.0), colorSpace='rgb', opacity=1.0, contrast=1.0, depth=0, element=None, signalDots='same', noiseDots='direction', name='', autoLog=True)

sebastiaan · April 2014

No, that's part of the definition of the class! That's part of the PsychoPy module, you don't need to include it yourself.

The only thing that you need to do to get a working example is copy the example script above into the run phase of an inline_script. However, there appears to be a problem with the fieldSize parameter of the DotStim class, as described here:

https://github.com/psychopy/psychopy/issues/554

This only affects recent versions of PsychoPy, which included with OpenSesame 2.8.1, but not 2.8.0. So if you this affects you as well, you can downgrade to 2.8.0 for now.

If you have no experience with Python, I would also recommend walking through one of the many tutorials. If you want to work with these kinds of stimuli, you will certainly need a basic knowledge of Python:

http://osdoc.cogsci.nl/python/about/#learning-python

Cheers!
Sebastiaan

Rotem · April 2014

Hey, I did the short basic python course plus now i have the random dot task in open sesame. The only thing is that i cant measure R.T cause the response is programmed after each frame. How can i change it so the subjects can response during the motion viewing?

sebastiaan · April 2014

Hi,

Could you please upload your experiment (for example to pastebin), or at least post the inline_script that is relevant to your question?

Cheers,
Sebastiaan

ambraferrari · April 2014

Hi all,

I'm trying to create an experiment with two circular areas filled with moving dots (RDK), on the left and right of a central fixation cross. See the picture I attached to get an idea of my ideal arrangement.

http://img.cogsci.nl/?q=5347ad6711830

I managed to create the two circles filled with the dots moving on the xy plane (e.g. orizontally) using dotStim.
BUT cannot get the same with the dot moving radially. I know there's the starField demo in Psychopy (I tried it in the stand alone version of that software). The problem is that it lets me create only a fullscreen scene of radially moving dots.
I tried to create two circular areas (using fieldShape and fieldSize of ElementArrayStim) on the left and right visual fields (using fieldPos), but running the code I only get the background (monitor units = norm).

Could you please give me a hint on this?
Looking forward to hearing from you.

Thanks in advance,
Ambra

Rotem · April 2014

Hey Sebastiaan,
Here it is
http://pastebin.com/RphaaFKX
Appriciate your help...

sebastiaan · April 2014

Hi @Rotem,

In your case, you need to alternate between showing the DotStim and polling the keyboard. So simply inserting a keyboard_response item after the inline_script won't do the trick. What you need to is poll for keypresses in the same loop that you use to present the DotStim, for example like so:

from psychopy.visual import DotStim
from psychopy import event
# Create a DotStim, which is automatically updated with every `draw()` call.
ds = DotStim(win, fieldSize=900, speed=4, dotLife=3, nDots=400, coherence= \
    self.get('coher'),dotSize=2.0,signalDots='different',dir=self.get('dir'), \
    noiseDots='walk')
# Show 100 frames
t0 = self.time()
while True:
    ds.draw()
    win.flip()
    # Get a list of pressed keys and break out of the loop as soon as this list
    # is not empty. Also note the timestamp of the response (t1)
    keys = event.getKeys(keyList=['s', 'l'])
    if keys != []:
        t1 = self.time()
        break
# The response is the first pressed key (and usually the only one)
response = keys[0]
# The RT is the difference between t1 and t0
response_time = t1 - t0
# Set these response so that they are logged, etc.
exp.set('response', response)
exp.set('response_time', response_time)

See also:

http://www.psychopy.org/api/event.html#psychopy.event.getKeys

Cheers!
Sebastiaan

sebastiaan · April 2014

Hi Ambra (@ambraferrari),

The DotStim only supports coherent motion in one direction. If you want to create radially moving dots, you need to implement this manually using the ElementArrayStim. It's actually not that difficulty, and all options are described in the PsychoPy documentation:

http://www.psychopy.org/api/visual/elementarraystim.html

To get you started, here's a working example, modified from the starField demo. Just walk through the code line-by-line and try to understand the logic, which is less complicated than it may seem. If you are not familiar with Python, I would also recommend walking through a basic tutorial first:

http://osdoc.cogsci.nl/python/about/#learning-python

(Note this bug related to the fieldSize keyword.)

Cheers,
Sebastiaan

# Modified from
# https://github.com/psychopy/psychopy/blob/master/psychopy/demos/coder/stimuli/starField.py

from psychopy import visual, event
from psychopy.tools.coordinatetools import pol2cart
import numpy

# The number of dots
nDots = 500
# The maximum speed of the dots
maxSpeed = 10
# The size of the field. Note that you also need to modify the `fieldSize`
# keyword that is passed to `ElementArrayStim` below, due to (apparently) a bug
# in PsychoPy
fieldSize = 200
# The size of the dots
dotSize = 2
# The number of frames
nFrames = 1000

# Initial parameters
dotsTheta=numpy.random.rand(nDots)*360
dotsRadius=(numpy.random.rand(nDots)**0.5)*200
speed=numpy.random.rand(nDots)*maxSpeed

# Create the stimulus array
dots = visual.ElementArrayStim(win, elementTex=None, fieldShape='circle', \
    elementMask='circle', nElements=nDots, sizes=dotSize, units='pix', \
    fieldSize=10000)

# Walk through each frame, update the dot positions and draw it
for frameN in range(100):
    #update radius
    dotsRadius = (dotsRadius+speed)
    #random radius where radius too large
    outFieldDots = (dotsRadius>=fieldSize)
    dotsRadius[outFieldDots] = numpy.random.rand(sum(outFieldDots))*fieldSize
    dotsX, dotsY = pol2cart(dotsTheta,dotsRadius)
    dots.setXYs(numpy.array([dotsX, dotsY]).transpose())
    dots.draw() 
    win.flip()

ambraferrari · April 2014

Hi Sebastiaan,

thanks for your help, your example works well. I now have my two experiments with:

dots moving horizontally (left/right)
dots moving radially (in/out)

I'm now facing a new problem: how can I compare the dots speed across the two experiments? My aim is to make the dots move at the same speed, but in two different directions (horizontal vs. radial). So, I need to have equal speeds between the two experiments.
In both I set units='pix' in the stimuli generation (inside DotStim() in 1. and inside ElementArrayStim() in 2.) so I guess speed is expressed in pixels/frame (or pixel/sec ...?)
Is there a way to check for this?

Empirically I see this: if I set the same speed in both experiments, the dots move much faster horizontally, so I have to decrease by a factor of 10 to gain perceptually comparable speeds. I am wondering what is the proper method to gain physically comparable speeds.

Note that I changed your script above:
from

speed=numpy.random.rand(nDots)*maxSpeed -> different values of speed
to
speed=k (now set to 2) -> same value of speed for all the dots

Maybe of interest:
I'm working on Windows
My monitor resolution: 1280 x 800
My refresh rate: 60 Hz

Thanks again,
Ambra

sebastiaan · May 2014

Hi Ambra,

Provided that you change the speed of the 'starfield' to a fixed value, which you appear to have done, both speeds are expressed in pixels per frame. And they do in fact move equally quickly for me, if I specify the same speed. Perhaps the framerate differs between the two stimuli? To check this, I would time the frame durations of both stimuli with a structure like this:

t0 = self.time()
for frameN in range(100):
    dots.draw() 
    ds.draw()
    win.flip()
    t1 = self.time()
    print 'frame duration: %.2f' % (t1-t0)
    t0 = t1

If the frame duration of the starfield is higher than that of the dotStim then you will have to take this into account when specifying the speed. (or get a better system!)

Is that it, you think?

Cheers!
Sebastiaan

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