experiment hp-18: posner’s attention test - iworx · experiment hp-18: posner’s attention test...

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Experiment HP-18: Posner’s Attention Test

The Posner Cueing Task, also known as the Posner paradigm, is a psychological test used to assess

attention and attention deficit. This task, formulated by Michael Posner, attempts to assess an

individual’s ability to perform an attentional shift. Attentional shift occurs when attention is directed to

a focal point to increase the efficiency of processing and includes inhibition to decrease attention

towards unwanted or irrelevant input. Posner's spatial cueing task is a timing task used to measure

manual and/or eye-movement reaction to target stimuli in order to determine the effects of covert

orienting of attention in response to different cue conditions.

Subjects sit in front of a computer screen situated at eye level. They are instructed to look at a central

point on the screen, marked by a cross. To the left and the right of the cross are 3 boxes. For a brief

period, a brightened stimulus is presented on the screen; this is followed by a target stimulus, usually a

shape, that appears in either the same location as the stimulus or in another box. The observer must

respond to the target stimulus immediately after detecting it. Only the correct response counts.

Two major cue types can be used to analyze attention based on the type of visual input.

• An endogenous cue is presented in the center of the screen, usually at the same location as the

center of focus. It is usually an arrow pointing to the left or right boxes on the screen. This cue

relies on input from the central visual field.

• An exogenous cue is presented outside of the central focal point, usually a brightening of the

left or right boxes presented. An exogenous cue can also be an object or image in the periphery,

a number of degrees away from the center, but still within the visual field. This cue relies on

visual input from your peripheral vision. This particular version of Posner’s experiment relies

on the use of these exogenous cues.

Dr. Posner devised a method of using valid and invalid cues for the trials.

• Valid trials have the stimulus is presented in the same area as the cue. For example, if the cue

was a brightening of the first box to the left of center, then the target will also be the first box to

the left of center.

• Invalid trials have the stimulus presented on the side opposite the cue. In this case, the cue

would be on the right but the stimulus on the left.

Posner used a ratio of 80% valid trials and 20% invalid trials in his original studies.

Eye movements can also be tracked using an electroculogram or an eye-tracking system. By adding this

additional recording, we can differentiate between overt and covert attention. Overt attention involves

directed eye movements, known as saccades, to consciously focus the eye on a target stimulus. Covert

attention involves mental focus and attention to an object without any significant eye movement, and is

the predominant area of interest when using the Posner cueing task for research. Using the same 80%

valid and 20% invalid parameters, Posner tried to encourage covert shifts of attention to take place in

response to cueing. When we attend to a location, even without directly looking at it, it facilitates

processing and decreases the time we need to respond to information occurring in that given space.

This results in decreased reaction times in Posner's spatial cueing task for validly cued targets, and

slower reaction times in response to invalidly cued targets.

Human Psychophysiology – Attention – Background HP-18-1

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Equipment Required

PC or Mac Computer

IXTA, USB cable, power supply

EM-220 Event Marker

iWire-B3G and 3 electrodes for EOG

Optional – iTrax Eye-Tracking System

Start the Software

1. Click on LabScribe

2. Click Settings → Human Psychophysiology → PosnerAttentionTest

3. Once the settings file has been loaded, click the Experiment button on the toolbar to open any

of the following documents:

• Appendix

• Background

• Labs

• Setup (opens automatically)

Sensor Setup

1. Locate the EM-220 Event Marker and plug it into the EM1 input on the back of the IXTA

(Figure HP-1 8 -S1).

2. Plug the iWire-B3G into the iWire 1 port on the front of the IXTA and attach the red, black and

green leads (Figure HP-18-S2).

NOTE – Plug the iWire-B3G into the IX-TA before turning the unit on.

Figure HP-17-S1: EM-220 Event Marker.


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Figure HP-18-S2: iWire-B3G. You will use the red, black and green electrodes.

Figure HP-17-S3: The IX-TA shown with both the EM-220 and iWire-B3G attached.

For the Subject:

1. Use an alcohol swab to clean and scrub the areas where the electrodes will be placed (Figure

HM-3-S3). Let the areas dry before attaching the electrodes.

2. Trim the electrodes and then remove the plastic disk. Snap the leads onto the electrodes before

applying them to the subject’s head.

3. Snap the recording lead wires onto the electrodes, so that:

1. the red (+1) lead is on electrode next to the left eye.

2. the black (-1) lead is on electrode next to the right eye.

3. the green (C) lead (the ground) is on electrode below the right ear.


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4. Place the electrodes to each of the other areas.

Figure HM-18-S4: Placement of electrodes for recording an electroculogram (EOG).

5. Have the subject hold the event marker in his or her dominant hand.

6. When the subject is ready, click the Record button and click the “Instructions” sequence to learn

what needs to be done for this experiment.

• Measure the distance from the screen the subject needs to be sitting.

• Make sure the subject understands to ONLY click the event marker when the cue and the

target are the same box.

OPTIONAL - Eye Tracker Setup

You must chose the iTRAX View from the View menu on the LabScribe toolbar.

1. Open “External Devices” on the main menu bar (Figure HP-18-S5).

Figure HP-18-S5: External Devices set up window.


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2. Enter the license information provided and set up the parameters for eye tracking (Figure HP-

18-S6) and open the Setup:

• Gaze Duration (msec): The amount of time, in msec, before the current time that fixation

information will be displayed in the Eye Tracking window.

• Outlier Filter (msec): The amount of time, in msec, that the eye has to be fixated at a single

spot. If the fixation duration is less than the Outlier Filter value, then the fixation will not

be displayed in the Eye Tracking window.

• Time Offset (msec): This can be used to take care of any time offset between the eye

tracking data and LabScribe data.

• Display Scaling (%): The Eye Tracker tracks the complete screen. We want to analyze

LabScribe data as well as eye tracking data. Display scaling allows for the eye tracking

window to show a scaled version of the screen.

• Fixation Size: The “fixation” data is shown as a circle whose radius is proportional to the

fixation duration. The circle size is the radius of the circle in pixels corresponding to a 1

second fixation duration.

Figure HP-18-S6: Eye tracking set up dialog window.

3. Set up the Area of Interest (AOI) if there is a specific location on the image you wish the

subject to focus on. Note that some experiments will just want to see where the subject is

focusing rather than knowing when the subject focuses on the “designated” area.

• Choose AOI#, click the drop down to have the AOI shown as a circle or rectangle


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• Open the image that the subjects will be looking at in an image editing program such as

Paint, Photoshop, Lightroom

• Hover over the AOI, note the X and Y pixel coordinates – and enter these in the boxes to the

right of AOI#

• When you choose to “Show the EyeTracking Window” - the image the subject is looking at

will be shown on one monitor with the AOI in blue. The actual image the subject will be

looking at on the second monitor will not show the AOI.

NOTE: If you want to know when the subject looks at a specific location, you must set the AOI.

4. To acquire eye tracking data, the GazePoint module has to be enabled in LabScribe and the

GazePoint application has to be running.

5. Calibrate the GazePoint Eye Tracking sensor (Figure HP-18-S7).

Figure HP-18-S7: Eye tracking calibration window.

5. Choose the screen that the subject will be using. Then calibrate the Eye Tracker.

6. Refer to the GazePoint manual for more instructions to calibrate the system.

7. The image window in LabScribe should be moved over to a second monitor if using multiple

monitors. Note that 2 monitors is preferable in this experiment especially when looking at AOI

data.

8. Enable the Eye Tracking window by choosing External Devices → GazePoint Eye Tracker →

Show Eye Tracking Window (Figure HP-18-S8).


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Figure HP-18-S8: Eye tracking window and GazePoint Eye Tracker.

General Directions:

• Subjects will be shown a series of 150 cues and 150 targets.

• Each cue will have a fixed center cross (for vision fixation) with 3 square boxes on each side.

One of the boxes will be white, the other 5 will be gray. The cue will be shown for 150 msec.

• Once the cue is shown, a target will appear.

◦ It will appear 100 to 600 msec after the cue.

◦ The target will have the same center fixation cross and square boxes. One of the square

boxes will have a “target star” in it, the other boxes will be gray. The target will be shown

for 150 msec.

• The subject will click the event marker ONLY if the “target star” shows up in the SAME box as

the white cue.

Procedure

Warning: It is important to press the event marker immediately upon recognizing if the target and

cue match.


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1. Click on the Record button.

2. Click the Instructions sequence on the toolbar and run the sequence. Follow the instructions to

situate your subject at the proper distance from the computer screen.

3. Click Stop once you have followed the measurement procedure.

4. When ready to start the experiment, click the Record button.

5. Choose the Attention sequence on the toolbar and run the sequence.

6. Follow the general directions as outlined above.

7. After the last cue and target have been shown, click Stop to halt recording.

8. Click on the Save button to save the data file.

Figure HP-18-L1: Sample of what the data may look like. This sample only shows one graph of data.

For this experiment, there will be two graphs of data, one representing the Reaction and one for the

EOG.

Note – if using the I-TRAX, there will be 3 graphs: Reaction, AOI and Velocity.


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Data Analysis

Figure HP-18-L2: LabScribe toolbar.

REACTION TIME:

1. Click the Double Display time button until approximately 10 responses show on the screen.

2. Move one red cursor line to the mark made when the TARGET was shown and the 2nd to the

start of the click of the event marker or response pad. Make a notation if the Target shown

actually matched the cue shown previous to it.

3. Look at T2-T1 (upper right corner) and record that number.

4. Repeat for the next 9 responses.

5. Repeat the procedure for the next set of 10 targets, keep repeating for a total of 150.

6. Enter the data into a table.

EOG:

1. Move back to the data where the Attention sequence began.

2. Look at the EOG channel to see where saccadic eye movement has occurred.

3. Place the cursors on either side of the time the cue was presented to when the subject reacted.

Look at the EOG signal. Measure the Mean value for EOG during this time period.

4. Repeat the EOG measurements for 20 correct responses, and calculate the average EOG.

5. Repeat step 4 for 20 non-responses by placing the cursors at the start of the cue and 150 msec

after the target. Calculate the average EOG.

Questions:

1. What was the mean reaction time of the subject?

2. What was the total percentage correct?

3. What was the average EOG when the subject answered correctly? When there were non-

responses?


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4. Is there a different in average EOG? Why?

5. Explain the Posner Attention Paradigm.


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