Random presentation of stimuli equally on left and right side of the screen

Hi Everyone,

I need to present gray oval randomly on the left and right side of the screen from the centre for equal number of times (105 each), thus total will be 210 number of trials . How can i do this ? and how to ensure when it was presented on left and when right ?

Please help me as i am stucked badly onto this.

[Jonas]

you can generate a series of 0 and 1

e.g.

series=[zeros(1,105) ones(1,105)];

and shake it randomly at each experiment

currentSeries=series(randperm(210))

then you go through the vector and display e.g. left if 0 and right if 1

[ANKIT MAURYA]

Hi Jonas,

First of all, thank you so much for providing me the logic. However I am facing issue while running the code below.

Actually I am presenting the ovals for seven durations (50,100,150,200,250,300,350 milliseconds) randomly. Each of these seven durations will be presented 15 times on the left (15*7=105) and 15 times on right side (15*7=105) of the screen, thus total for 30 times for each duration and making up total number of trials 210 (30*7 = 210; for both left and right side).

I don't know exactly where in the code given below, I should be using this-----------

series=[zeros(1,105) ones(1,105)];
currentSeries=series(randperm(210))

The code is running perfectly but the oval is presented in center only.

Kindly have a look at the code below and provide me the relevant suggestion. Thanks again for your time and effort in advance.

% Clear the workspace and the screen
Screen('Preference', 'SkipSyncTests', 1);
sca;
close all;
clear;
%PsychDebugWindowConfiguration;
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% Creating Variables %%%%%%%%%%%%%%%%%%%%%%%%%%%%
series=[zeros(1,105) ones(1,105)];
currentSeries=series(randperm(210));
% Delay_time is a variable that will take up 10 random values among
% (0.05,0.1,0.15,0.2,0.25,0.3,0.35s- 7 random duration values)
Delay_time = zeros(1,210);
Delay_time (1:30) = 0.05;
Delay_time (31:60) = 0.1;
Delay_time (61:90) = 0.15;
Delay_time (91:120) = 0.2;
Delay_time (121:150) = 0.25;
Delay_time (151:180) = 0.3;
Delay_time (181:210) = 0.35;
Delay_time = Delay_time (randperm(210))*1;
%%%%%%%%%%%%%%%%%%%%%%%%k Display Oval on PTB screen %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% Here we call some default settings for setting up Psychtoolbox
PsychDefaultSetup(2);
% Get the screen numbers
screens = Screen('Screens');
% Draw to the external screen if avaliable
screenNumber = max(screens);
% Define black and white
white = WhiteIndex(screenNumber);
black = BlackIndex(screenNumber);
% Open an on screen window
[window, windowRect] = PsychImaging('OpenWindow', screenNumber, black);
% Get the size of the on screen window
[screenXpixels, screenYpixels] = Screen('WindowSize', window);
% Set up alpha-blending for smooth (anti-aliased) lines
Screen('BlendFunction', window, 'GL_SRC_ALPHA', 'GL_ONE_MINUS_SRC_ALPHA');
%Set the TextFont & TextSize of the screen window
Screen('TextFont', window, 'Ariel');
Screen('TextSize', window, 40);
% Get the centre coordinate of the window
[xCenter, yCenter] = RectCenter(windowRect);
% Here we set the size of the arms of our fixation cross
fixCrossDimPix = 30;
% Now we set the coordinates (these are all relative to zero we will let
% the drawing routine center the cross in the center of our monitor for us)
xCoords = [-fixCrossDimPix fixCrossDimPix 0 0];
yCoords = [0 0 -fixCrossDimPix fixCrossDimPix];
allCoords = [xCoords; yCoords];
% Set the line width for our fixation cross
lineWidthPix = 4;
% Make a base Rect of 200 by 250 pixels
baseRect = [0 0 200 250];
% For Ovals we set a miximum diameter up to which it is perfect for
maxDiameter = max(baseRect) * 1.01;
% Center the rectangle on the centre of the screen
centeredRect = CenterRectOnPointd(baseRect, xCenter, yCenter);
% Set the color of the rect to gray
rectColor = [0.5 0.5 0.5];
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%TEST EXPERIMENT PHASE BEGINS%%%%%%%%%%%%%%%%%%%%%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% INSTRUCTIONS %%%%%%%%%%%%%%%%%%%%%%%%%%%%
DrawFormattedText(window,['Welcome to the Experiment\n\n'...
    'When you get ready, press any key to continue'], 'center', 'center', white);
Screen('Flip', window);
KbWait;
start = GetSecs;
for i_trial = 1:210
    %     curr = GetSecs;                                   %Gets current time
    %     disp(curr-start);                                 %Displays time since
    %     start=curr;
    % Draw the fixation cross in white, set it to the center of our screen and % set good quality antialiasing
    Screen('DrawLines', window, allCoords,...
        lineWidthPix, white, [xCenter yCenter], 2);
    % Flip to the screen
    Screen('Flip', window);
    % Wait for 500 ms
    WaitSecs (0.5);
    % Draw the oval to the screen
    Screen('FrameOval', window, centeredRect, rectColor, maxDiameter);
    % Flip to the screen
    Screen('Flip', window);
    WaitSecs (Delay_time(i_trial));  % random durations for which oval to be presented on screen
    DrawFormattedText(window, 'Closer to Shorter or Longer duration?', 'center', 'center', white);  % Question asked to Participants
    Screen('Flip', window)
    %     KbWait;                     % Wait for taking responses from participants
end
% Clear the screen
sca;

[dpb]

At least format the code -- better would be to pare it down to the pertinent section that controls when a new object is to be displayed --

BUT, it appears

    % Draw the oval to the screen
    Screen('FrameOval', window, centeredRect, rectColor, maxDiameter);

must be the place -- we don't have the Screen function, but somehow it has to be able to position the object where it's wanted -- the specific place would be determined by walking through the previously generated set of random positions -- I didn't try to parse the longish description as to whether it's a random choice between left and right as well as location within the half; that's would determine whether one permutes the entire sequence or splits it by half and what bookkeeping you'll have to do to know which half it is that you're drawing from each iteration.

[ANKIT MAURYA]

Hi Jonas,

I didn't understood exactly what you said. Can you just simplify it ? May i know if I am not able to make you understand question properly?

[Jonas]

the scond comment was dpb nit me ;)

your code looks promising. you say you want to place the oval at one of the screen halfs, where exactly? just in the middle at normalized x =0.25 or 0.75 respectively?

as dpb said, we dont know how exactly your drawing function works but it may look like

if currentSeries(i_trial)==1
    Screen('FrameOval', window, centeredRect, rectColor, maxDiameter); % change to draw right/left
else
    Screen('FrameOval', window, centeredRect, rectColor, maxDiameter); % chamge to draw left/right
end

[ANKIT MAURYA]

Kindly download the attached pic. This is what exactly i need to do

[ANKIT MAURYA]

I think this 'centeredRect' in ---------Screen('FrameOval', window, centeredRect, rectColor, maxDiameter);

is drawing the oval in center. If i just remove it , it is showing the 'Screen' usage error

[Jonas]

we dont know. show us the variable, what size they are, ehat numbers are in. maybe just adding a value to the centeredRect is enough to move. we dont know how your Screen function works

probably just centeredRect.*[1.5 1 1.5 1] to move to the right or centeredRect.*[.5 1 .5 1] to move to the left

[ANKIT MAURYA]

Here is the code conntaining the variable------

% Open an on screen window
[window, windowRect] = PsychImaging('OpenWindow', screenNumber, black);
% Get the size of the on screen window
[screenXpixels, screenYpixels] = Screen('WindowSize', window);
% Get the centre coordinate of the window
[xCenter, yCenter] = RectCenter(windowRect);
% Make a base Rect of 200 by 250 pixels
baseRect = [0 0 200 250];
% For Ovals we set a miximum diameter up to which it is perfect for
maxDiameter = max(baseRect) * 1.01;
% Center the rectangle on the centre of the screen
centeredRect = CenterRectOnPointd(baseRect, xCenter, yCenter);

centeredRect.*[1.5 1 1.5 1] to move to the right or centeredRect.*[.5 1 .5 1] to move to the left ------------- its working but the dimensions are abrupt

[dpb]

Would you PLEASE format the code snippets you post so they are readable as code???!!!????!!!!!

You continue to not provide any links to the functions being called and we don't have the toolbox -- I can't find the doc for any of these functions and I don't see any Psych Toolbox on the TMW MATLAB product site. Where does all this stuff come from so we can see the doc on how to use it?

To solve your problem well, you'll need to dive into the doc for how those drawing methods are used; certainly the ratios given by @Jonas of < and > 1 appear to move the object on the display, but certainly by the law of ratios, 1.5*X and X/1.5 do NOT equate to the same absolute change in X so the number of pixels moved either direction isn't the same which is the effect one would presume you're looking for.

But, we don't know the actual use; we're just guessing because we don't have the needed doc...and you're not providing any really useful information by simply describing what you don't like...

[Jonas]

@dpb apparently i lost my brain before suggesting those values. of course thats could not be correct.

if you got your resolution

screenXpixels, screenYpixels

and you keep the width of the rectangle and the the 4 values are the values of the 4 borders left top right bottom (not like in matlab x y width height, and also also not like on Latex {screw you psych toolbox }), then use as left rectangle

w=200;
h=250;
[screenXpixels screenYpixels screenXpixels screenYpixels].*[0.25 0.5 0.25 0.5]+[-w/2 +h/2 +w/2 -h/2];

and for the right

[screenXpixels screenYpixels screenXpixels screenYpixels].*[0.75 0.5 0.75 0.5]+[-w/2 +h/2 +w/2 -h/2];