Parameters for Gabor patch in comparision to Matlab

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asked Apr 9, 2016 by Marcello
We would like to create 2 types of Gabor stimuli, one with a spatial frequency of 6cpd and one with a spatial frequency of 1cpd. In both cases, the sigma of the Gabor should be equal to the wavelength (so sigma of the gabor = 1/spatial frequency), meaning that increasing the spatial frequency the size of the stimulus decreases.

In EventIDE the codification of spatial frequency and gaussian spread is a bit different, specifically because the size of the stimulus influences the spatial frequency where in Matlab the sf remains constant.

Can you tell us which are the Gabor parameters in EventIDE that could lead to the stimulus we want, so 1cpd with sigma = lambda and 6cpd with sigma = lambda?

1 Answer

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answered Apr 9, 2016 by Ilia Korjoukov (7,460 points)
 
Best answer

EventIDE treats all Gabor parameters relatively to the normalized size of the patch. If you set the ‘Grating Frequency’ to 6 and the element size to 1 dva (the angular size of the viewport on the screen), then you have 6cpd physically.  If you increase the patch size to 6 dva without changing the gabor parameters, the physical frequency will be 1cpd. Thus, the physical spatial frequency can be calculated as:

Freq=GaborFrequency/GaborSize.aWidth;

You defined the physical sigma in dva as:

Sigma=1/Freq=1/(GaborFrequency/ GaborSize.aWidth)= GaborSize.aWidth/GaborFrequency;

Since the EventIDE sigma (Gaussian Spead property) is also normalized to the patch size, the reverse conversion is necessary to adjust the property in EventIDE:

GaussianSpread= Sigma*GaborSize.aWidth= (GaborSize.aWidth^2)/GaborFrequency;

I attached below a screenshot with 4 patches generated with the above expressions:

The used gabor parameters are (same order as on the screenshot):

Grating Frequency=6,

aWidth=1

Gaussian Sigma=0.16

Grating Frequency=1,

aWidth=1

Gaussian Sigma=1

Grating Frequency=12,

aWidth=2

Gaussian Sigma=0.16

Grating Frequency=2,

aWidth=2

Gaussian Sigma=4

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