Matlab Class Chapter 6.pdf

Chapter 6 - 1 - 4/18/2007

Chapter 6 – Psychtoolbox

We don’t want to run this experiment on the Matlab figure window, since as you can see

it is slow and clunky. To do this we are going to need to start using a group of Matlab

programs developed especially for doing behavioral experiments. Many groups have

developed similar sets of programs, but the ones we are going to use are called

PsychToolbox.

You need to download Psychtoolbox from here:

http://psychtoolbox.org/

PsychToolbox is a collection of matlab functions written to make presenting visual

stimuli easier. Remember to cite the Toolbox.

"We wrote our experiments in MATLAB, using the Psychophysics Toolbox extensions

(Brainard, 1997; Pelli, 1997)."

Brainard, D. H. (1997) The Psychophysics Toolbox, Spatial Vision , 10:443-446.

Pelli, D. G. (1997) The VideoToolbox software for visual psychophysics: Transforming

numbers into movies, Spatial Vision 10:437-442.

You’ll have to download Psychtoolbox from the web site. Make sure you download the

right version (for MacOSX or PC). Once you have installed Matlab, then you can carry

on with the chapter.

Getting started with PsychToolbox Screen.m

1. Start with just 1 monitor

2. Try:

>ScreenTest

If you get the screen going blank and then something like the following then screenTest worked

***** ScreenTest: Testing Screen 0 *****

PTB-INFO: This is the OpenGL-Psychtoolbox version 3.0.8. Type

'PsychtoolboxVersion' for more detailed version information.

PTB-INFO: Psychtoolbox is licensed to you under terms of the GNU

General Public License (GPL). See file 'License.txt' in the

PTB-INFO: Psychtoolbox root folder for a copy of the GPL license.

PTB-INFO: OpenGL-Renderer is NVIDIA Corporation :: GeForce Go

7400/PCI/SSE2 :: 2.0.1

PTB-Info: VBL startline = 768 , VBL Endline = -1

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PTB-Info: Measured monitor refresh interval from VBLsync =

16.712593 ms [59.835118 Hz]. (50 valid samples taken,

stddev=0.044112 ms.)

PTB-Info: Reported monitor refresh interval from operating system

= 16.666667 ms [60.000000 Hz].

PTB-Info: Small deviations between reported values are normal and

no reason to worry.

PTB-INFO: Using NVidia's GL_TEXTURE_RECTANGLE_NV extension for

efficient high-performance texture mapping...

***** ScreenTest: Done With Screen 0 *****

If not, then Psychtoolbox isn't working right on your computer

Writing code using Screen.m

The next stage is also very simple. We are simply going to open an experimental window, making it black,

making it white, and then closing it again.

The script below uses a command called Screen , which one of the core functions of Psychtoolbox.

Screen is actually not an m file (like the ones you have been writing. It is a mex file. This means that it is

written in C (or C++) or some other programming language, and is then compiled to run in Matlab. The

reason this was done is because Screen does some pretty funky stuff that would be impossible in Matlab.

Oddly, under certain circumstances the Screen command likes to have a capital letter (it’s case sensitive

unlike most commands in Matlab).

% DarkScreen.m

% opens a window using Psychtoolbox,

% makes the window black, then white, and then closes

% the window again

% written for Psychtoolbox 3 on the PC by IF 3/2007

screenNum=0;

res=[1280 1024];

clrdepth=32;

[wPtr,rect]=Screen( 'OpenWindow' ,screenNum,0, …

[0 0 res(1) res(2)], clrdepth);

black=BlackIndex(wPtr);

white=WhiteIndex(wPtr);

Screen( 'FillRect' ,wPtr,black);

Screen(wPtr, 'Flip' );

HideCursor;

tic

while toc<3

;

end

Screen( 'FillRect' ,wPtr,white);

Screen(wPtr, 'Flip' );

Chapter 6 - 3 - 4/18/2007

HideCursor;

tic

while toc<3

;

end

Screen( 'CloseAll' );

ShowCursor

One weird thing about Screen is that you don’t get help about it’s commands in the

normal way.

If you type:

>help Screen

You don’t get a lot of info.

If you try

>Screen

You will get a list of all the subcommands that are contained within Screen. To get more information about a

particular command you do:

>Screen OpenWindow?

>Screen DrawText?

You should also bear in mind that you are entering the murky world of non-professional code. This

code is written by people like you, in the middle of trying to do real science. This means that

commands may not work as stated, help files may be out of date, commands may not even exist.

This is especially true in the case of Psychtoolbox, which is essentially a collaborative effort

between lots of people and has been through lots of reincarnations.

Line 9. If you are running more than one monitor on your computer then Screen tells you which one to use.

0 means the monitor with the menu bar, 1 means the other monitor. For now, if you have any difficulty I

would make sure you are only using one monitor. If you are cloning your monitor then Screen 1 becomes

the display screen.

Line 10-11. Check the resolution of your screen and the using Start, Control Panel, Display and write down

what the resolution of your monitor is in terms of the number of pixels and the color depth of the monitor. As

far as the resolution of the monitor in pixels is concerned the first number is the width of the screen, the

second number is the height of the screen. The resolution of your screen in terms of color depth will be 8, 16

or 32 bits, depending on the age of your monitor or computer. Set it to the highest setting the computer will

allow.

Line 12. This calls a function called Screen, which takes 5 arguments.

Argument 1 . a command telling Screen what to do – in this case you want Screen to open a window.

Argument 2. which monitor you want the window opened inside. In this case you want the Screen opened in

monitor 0 – the one with the menu bar.

Argument 3. The color you want to fill the window with. This currently doesn’t seem to work in Psychtoolbox.

Argument 4 . This tells Screen how big you want the window to be – in this case you want it to be a rectangle

the size of the entire screen. The order in which you describe this rectangle can be remembered as

ox (Left, Top, Right, Bottom). We want the rectangle to start 0 pixels from the Left, and 0 pixels

from the Top, and go to 1280 pixels towards the Right and 1024 pixels towards the Bottom.

(Why does it start with 0 instead of 1? Why because Screen is a mex file and was written in C which is an 0-

based language.) Currently Psychtoolbox always uses the whole screen, no matter what you enter into rect.

Argument 5 is the color depth of the monitor.

Arguments 3-5 don’t actually need to be specified – Matlab will default to certain values: a random

background color, the whole screen and the color depth of the monitor as described in the control panel

Lines 13-14 Find the colormap values that will give you black and white.

Line 15 Draw a black rectangle the size of the screen. Psychtoolbox automatically assumes that you have

an offscreen window and an onscreen window. Every time you use a drawing command like DrawRect it will

automatically draw on the offscreen window. So the rectangle won’t yet be visible. The advantage of this

Chapter 6 - 4 - 4/18/2007

approach (as you will see later) is that you can spend some time drawing several things offscreen without

them showing up one by one on the screen. Once you have finished drawing you can move the offscreen

window to the front.

Line 16 You need to Flip the screen so the offscreen window you drew the black rectangle on comes to

the onscreen window – the one that is actually on the monitor.

Lines 18-22 Hide the cursor and wait 3 seconds

Lines 24-30 Draw a white rectangle on the offscreen window, flip it to the front and wait 3 seconds

Lines 34-35. Close the screens and re-appear the cursor

****************************************************************************

NOTE FOR IF YOUR SCREEN FREEZES

If you display stimuli on the main screen, as we often do, then the Screen window will

hide the main menu bar and obscure Matlab’s command window. That can be a problem

if your program stops (perhaps due to an error) before closing the window. The keyboard

will seem to be dead because its output is directed to the front most window, which

belongs to Screen not Matlab, so Matlab won’t be aware of your typing.

Remain calm.

Typing Ctrl-C will stop your program if hasn't stopped already. Typing:

command-zero (on the Mac)

Alt-Tab (on Windows)

Will bring Matlab’s command window forward. That will restore keyboard input. The

screen might still be hard to make out, if you’ve been playing with the lookup table.

Typing:

clear Screen

will then cause Matlab to flush Screen.mex. Screen.mex, as part of its exit procedure,

cleans up everything it did, closing all its windows and restoring the lookup table of all

its displays. And everything will be hunky dory again.

****************************************************************************

Here’s another more elaborate example of how you can use Screen

% FunkyScreen.m

% opens a window using psychtoolbox,

% makes the window do some funky things

% written for Psychtoolbox 3 on the PC by IF 3/2007

screenNum=0;

flipSpd=13; % a flip every 13 frames

[wPtr,rect]=Screen( 'OpenWindow' ,screenNum);

monitorFlipInterval=Screen( 'GetFlipInterval' , wPtr);

Chapter 6 - 5 - 4/18/2007

% 1/monitorFlipInterval is the frame rate of the monitor

black=BlackIndex(wPtr);

white=WhiteIndex(wPtr);

% blank the Screen and wait a second

Screen( 'FillRect' ,wPtr,black);

Screen(wPtr, 'Flip' );

HideCursor;

tic

while toc<1

;

end

% make a rectangle in the middle of the screen flip colors and size

Screen( 'FillRect' ,wPtr,black);

vbl=Screen(wPtr, 'Flip' ); % collect the time for the first flip with vbl

for i=1:10

Screen( 'FillRect' ,wPtr,[0 0 255], [100 150 200 250]);

vbl=Screen(wPtr, 'Flip' , vbl+(flipSpd*monitorFlipInterval));

% flip 13 frames after vbl

Screen( 'FillRect' ,wPtr,[255 0 0], [100 150 400 450]);

vbl=Screen(wPtr, 'Flip' , vbl+(flipSpd*monitorFlipInterval));

end

% blank the screen and wait a second

Screen( 'FillRect' ,wPtr,black);

vbl=Screen(wPtr, 'Flip' , vbl+(flipSpd*monitorFlipInterval));

tic

while toc<1

;

end

% make circles flip colors & size

Screen( 'FillRect' ,wPtr,black);

vbl=Screen(wPtr, 'Flip' );

for i=1:10

Screen( 'FillOval' ,wPtr,[0 180 255], [ 500 500 600 600]);

vbl=Screen(wPtr, 'Flip' , vbl+(flipSpd*monitorFlipInterval));

Screen( 'FillOval' ,wPtr,[0 255 0], [ 400 400 900 700]);

vbl=Screen(wPtr, 'Flip' , vbl+(flipSpd*monitorFlipInterval));

end

% blank the Screen and wait a second

Screen( 'FillRect' ,wPtr,black);

vbl=Screen(wPtr, 'Flip' , vbl+(flipSpd*monitorFlipInterval));

tic

while toc<1

;

end

% make lines that flip colors size & position

Screen( 'FillRect' ,wPtr,black);

vbl=Screen(wPtr, 'Flip' );

for i=1:10

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