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COGS 119
MATLAB for Experimental Research
Fall 2013 Week 7
Basic Vision Science with Psychtoolbox
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Gabor
! Simple cells in the primary visual cortex (V1) of theprimate brain can be modeled using Gabor filters, which
is a product of sine waveand a Gaussian.
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Gabor
! A gabor is characterized by several parameters:! Orientation (theta)! Spatial frequency (sf)! Width (sd)! Size
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Example gabors
theta = 90 theta = 45
sf = 5 sf = 10
sd = 0.2 sd = 0.2
size = 100; size = 100
theta = 90 theta = 45
sf = 10 sf = 10sd = 0.2 sd = 0.4
size = 100; size = 100
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Creating a gabor in Matlab
Gabor = Sinewave * Gaussian
Step 1: Create Sinewave
Step 2: Create Gaussian
Step 3: Multiply
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Creating a gabor in Matlab
prep_gabor.m
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Creating a gabor in Matlab
prep_gabor.m
sinewave andgrating
gaussian
multiply sinewave and gaussian
inputs
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Step-by-step
prep_gabor.m
sinewave andgrating
gaussian
multiply sinewave and gaussian
inputs
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Meshgrid
!" $ %&'( !) $ *%( +,-+. $ '/( 0.+!12- $ *%%(
33 45678 $ 0-!,9:1";.?-;@*6*60.+!12-A*BB(
33 10.9-!?;5B( .51! C))( ?C;9:.7B(
33 10.9-!?;7B( .51! C))(
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ramp33 :.0> $ ?C!;+,-+.D>1E*F%BD;5@5?B A !1=;+,-+.D>1E*F%BD;7@7?B(
33 10.9-!?;:.0>B( .51! !GH.:-( .51! C))(
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Sinewave (grating)
33 9:.+1=9 $ !1=;ID>1D!)D:.0>B(
33 10.9-!?;9:.+1=9B( .51! !GH.:-( .51! C))(
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Gaussian
33 9.H!! $ -5>; @;;5@5?B&JI A ;7@7?B&JIBE;ID!"JIBB(
33 10.9-!?;9.H!!B( .51! !GH.:-( .51! C))(
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Gabor
33 9.KC: $ 9.H!!&D9:.+1=9(
33 10.9-!?;9.KC:B( .51! C))(
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Orienting gabor
! Download gabor_sweep_orient.mfrom the classwebsite and run it.
!Now, lets look at the code closely to understand howthe gabor changes orientation.
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Displaying gabor in PTB
! Download disp_gabor.mfrom the class website andrun it.
!disp_gabor.mprepares a gabor (callingprep_gabor.m) and displays it in Psychtoolbox.
! Inputs:! sd, sf, theta (gabor parameters)! offx, offy (offset from the center of the screen)
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Displaying gabor in PTB
disp_gabor.m
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Displaying gabor in PTB
disp_gabor.m
Call prep_gabor.m
Make gabor texture
Draw gabor texture
inputs
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Experiment with gabors
! Lets display multiple gabors with various properties (sd,sf, theta) on various locations on the screen.
!Download gabor_experiment.mfrom the classwebsite and run it.
! Lets look at the code closely to understand how itdisplays gabors.
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gabor_experiment.m
Experiment has the following trial types:
! gabor with sd = 0.1, sf = 10, theta = 5;! gabor with sd = 0.1, sf = 10, theta = 355;! gabor with sd = 0.1, sf = 16, theta = 5;! gabor with sd = 0.1, sf = 16, theta = 355;
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gabor_experiment.m (trial set-up)
Open a file to write
Set-up the trials gaborparameters
!! #$%& '()
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rng
:=9;L!,H))
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gabor_experiment.m (open window
and set screen parameters)
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gabor_experiment.m (prepare and
display gabors) Call prep_gabor.m
Make a texture for
the gabor and then draw itas in disp_gabor.m
Get user response and rt
Write into a file
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Exercise
! Read Fine and Boynton chapter 5 and do the exercisesthere in Matlab.
! Submit project choices instructions via E-mail