2nd Year Practical Semester 2 2011

Selective attention & the flanker paradigm

Jonathan Stirk

Introduction

2nd year practicals run for 5 week periods. Within these 5 weeks you will be expected to design and run an experiment which you will present both verbally and in written report form.

This practical will examine selective attention and the flanker compatability effect

  • Read the relevant literature.
  • Create a research hypothesis.
  • Program an experiment (using E-Prime).
  • Collect data.
  • Analyze your data.
  • Present your data to the class.
  • Write a report about the findings.

 

Flanker Paradigm

The flanker paradigm has been used to investigate what factors may affect selective attention and the extent to which processing of irrelevant information occurs. There are lots of variants of the classic flanker paradigm (one is shown above), some of which we will discuss in class.

The classic flanker effect

Flankers are stimuli which are presented spatially close to target stimuli and which have been sometimes shown to interfere with target processing/responses.

The classic flanker effect shows differences in reaction times for compatible and incompatible response mappings on a choice reaction task.

Eriksen and Eriksen (1974) had subjects respond with one hand if a central target was an H or a K and with the other hand if it was an S or C. The flanking items were either from the same response set as the target (compatible, e.g. H flankers with K target), from the other response set (incompatible, e.g. S flankers when the target was a K), or neutral (e.g. X flankers when the target was a K). Responses to targets with compatible flankers are generally faster than responses to targets with incompatible flankers. This paradigm examines to what extent irrelevant information is processed in a visual task. You will be manipulating factors to see what effects they have on the level of interference in the flanker task.

Designing and running an experiment using EPrime

Running the software

You were introduced to E-Prime in your 1st year. You will use E-Prime to test your experiments. The best way to learn about E-Prime is to try out an example experiment. An example experiment is available here (classic flanker) and here (colour flanker)

*** It is important that you save any programs to your HARD DISK and not to your network space. Running E-Prime experiments from the network will result in inaccurate timing and other possible problems. ***

The example experiment (classic flanker - FCE-original.es)

In this experiment the task of the subject is to respond to the central letter (known as the target) whilst ignoring the outside letters (distracters) which 'flank' the target. The target stimuli must be responded to by pressing the 1 key if it is an S or C (rounded/curved features), and the 0 key if it is a K or H (straight features). Speed of responses is affected by whether the target is surrounded by response compatible or incompatible flankers (see presentation). We will also manipulate the spatial distance (eccentricity) between the target & distracters, at 2 levels (near flankers & far flankers). This leads to a 2 x 2 design.

Creating a unique experiment

To run the experiment, start EStudio (select Start/Programs/E-Prime/E-Studio). You can then select the "Open an existing experiment" option and choose the experiment that you want to modify. You will see a screen that looks like this (this is from the 'FCE-original' example):

 

Here is a quick reminder of the components that you can see on the screen:

  • "Toolbox" panel: here are new items that you can drag and drop on your experiment if you want to add a new image display, text display, etc.
  • "Structure" panel: a flow diagram of how the experiment will run. Clicking on items here will cause their properties to be displayed in the next two panels. Click on the "TrialProc" icon two see the following:
  • The "Timeline for trialproc" window appears when you click on the "TrialProc" procedure. Here you can organize the flow of events during each procedure: e.g. each trial in TrialProc will start with a fix[ation], followed by a stimulus and conclude with error feedback. We can drag items from the toolbox to this window to modify the events.
  • Properties window: Shows properties for currently selected item of the "Structure", in this case we can modify details of TrialProc.

Lets examine the structure window

:The structure window starts with some general instructions and is then followed by a list which runs 2 blocks, a practise procedure and an experimental procedure. Both of these procedures (Practise & Expt) give the sequence of events for the practise and experimental trials respectively. Each of these two procedures has some instructions and a trial list controlling the presentation of stimuli. The actual trial procedure (TrialProc) determines what happens in an idividual trial.

This shows the order for events in the experiment. It runs something like this:

  1. Show initial instructions
  2. Run Practise Block followed by Experimental Block
  3. Repeat trials [TrialProc] for the repetitions specified in PracTrialList & ExpTrialList
  4. For each trial, show a stimulus on the screen (target and flankers), then wait for a response, folloiwed by visual feedback.
  5. End by showing the goodbye message (EndThanks)

Trial List Section

Lets now take a look at trial list - this is the bit that you will have to modify to adapt the experiment for your designs (you may also need to adapt other components, depending on your design). Double clicking on the TrialList icon in the structure window shows us a window that looks like this:

The buttons on the top allow us to add/delete new columns or rows to our list. Beneath that we are told the total number of trials in this list (64 trials). Beneath this is the list, with the following columns:

  • ID: row number
  • Weight: How many times does this stimuli appear per block. For example, the stimulus described in row 1 is presented 2 times per block, while condition 5 is only presented once. The reason for this difference will become clear when we get to the "Stimulus" and "F_Compatability" column.
  • Procedure: Which procedure (stimuli order) will be used for this condition: in this study, all conditions use "TrialProc".
  • Nested: ignore this for now (not required in this particular design).
  • F_Compatabilty: Whether a trial uses response compatible/incompatible flankers.
  • Stimulus: Contains the text stimuli or name of bitmap(if a graphic is used) to be used.
  • Correct : what should the subject press to be scored as a correct answer? In this example, the subject is supposed to press a '1' when a target is a K or H, and '9' if it is an S or a C.
  • F_distance: is whether a trial uses close flankers (near) or more distant ones (far)
  • handresp: contains which hand should be making the response. You might want to run half your subjects in a reverse mapping.
  • target: target stimulus
  • flanker: flanker stimulus

You will see that the weightings column assists in presenting equal numbers of compatible and incompatible trials (8 of each) for each of the 2 distance conditions. One complete set of trials would be 32 samples (although this has been doubled to present 64 trials in total!).

You may modify or redesign the e-prime files to test your own hypothesis. Depending on your design/hypothesis, you may need to construct your program in a different way to my example! e.g. you may need to use nested lists or different states of a slide object.

After modifying your experiment, make sure to save your new experiment. You may want to choose "Save as" from the "File" menu to give the program a new name.

NOTE:

If you use any images as stimuli they must be saved as windows bitmaps (.bmp's) and E-Prime files MUST be in the SAME directory on the C: drive of the PC (create a temporary storage space off C:\).

Piloting your study

Before collecting data, it is important to test your design to validate and verify that everything works correctly. Choose "Run" from the "E-Run" menu to start the experiment.

To examine your pilot data, choose "E-Data-Aid" from the "Tools" menu. Then open your data file (it is saved in the same folder as your experiment, and the name shows the subject number).

Collecting Data

After you have thoroughly tested your experiment, you are now ready to begin collecting data. For this practical, you will want to test at least 6 people - 12 would be better. Consider whether you can test yourselves. When you run each individual, give them a different subject number so that the old data will not be overwritten.

IMPORTANT: If each member of a group is collecting data, decide before hand what subject number you will each start with! (e.g. person 1 in group uses subject numbers 1-25, person 2 uses 26-50. This is vital to prevent duplicating the same subject number for different subjects! Make sure to keep a backup copy of your data after each subject (copy it to your network drive).

Analysing Data

You can analyse data using SPSS for Windows

Helpful Reminders:

  1. To abort an experiment partway through, press <Ctrl> <Alt> and <Shift> simultaneously.
  2. Run your experiment from the computers hard disk - running on the network will lead to poor timing and other problems with running programs. (Though you should backup your experiment and data to the disk).
  3. Putting in feedback messages may help you test that you have assigned the correct keys and also inform subjects of their mistakes. However, they can also be distracting.

Summary of time-plan (remember, you should be working on your practical between classes too!):

Week 1 Lecture, example exp’ts, literature search
Week 2 Develop hypothesis, select project
Week 3 Pilot study, collect data
Week 4 Data analysis

Week 5 Presentations

Please present your experiment to the class as a 15 minute presentation. Use PowerPoint.


Week 6 Hand in written report (Deadline: 3rd May, 2011 by 4pm)

 

Click for Introduction Lecture (PowerPoint file-available on WebCT)

Click for Statistics/Analysis Lecture (PowerPoint file-available on WebCT)

Click for Presentation skills/report writing (PowerPoint file-available on WebCT)

Click for class 1 hand out (available on WebCT)

Click for research paper by Eriksen (1995) (available on WebCT)

 

Need HELP? See Tim or me in our drop-in hours (advertised in class)

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