Orientation Discrimination Lesson 1

BASIC TASK

The aim of this experiment is to measure orientation discrimination thresholds with Gabor stimuli using a staircase method and a 2AFC procedure. To learn more about Orientation Discrimination, see:

Beaudot & Mullen (2006) Orientation discrimination in human vision: Psychophysics & Modeling, Vision Research 46(1-2):26-46

Step 1. Open the "Designer" panel

Launch Psykinematix, and select the Designer panel by clicking its icon in the toolbar.

Step 2. Add an Experiment Event

The Experiment Designer panel presents a hierarchical view of the experiments. Before going further, make sure to unselect any event by clicking an empty row or Apple-clicking the selected event to unselect it.

Now, create a new group event by clicking the FOLDER icon with the '+' symbol in the bottom toolbar.

Tip: a group event embeds one or severals events.

The new event appears at the top in the table with a default title (New Events). Change its title to Orientation Discrimination to reflect the experiment purpose (double-click the title to edit it).

Each event is characterized by a category that defines its function inside your experimental design. To change the default category (Undefined), use the popup menu to select the Experiment entry. Note how the small icon in front of the event changed from a small folder to a small red 'Psy' letter specific to the Experiment event.

Tips:
- each category of events has its own small icon to depict its function;
- the Undefined category can still be used to group several experiments under one roof (for example, all experiments related to the same study).

Step 3. Add a Staircase Method

Select the Experiment event 'Orientation Discrimination' you just created, and add a new group event by clicking the '+' FOLDER icon again.

A new Undefined event embedded in the Experiment event is automatically revealed with its title ready to be renamed.

Tip: click the small arrow in front of a group event to expand or collapse the first level of its hierarchical structure (option-click to reveal the whole hierarchy). The first level can be also revealed or hidden by selecting the event and pressing the space bar.

Change the title and category of the new event to Staircase and Method as illustrated.

Step 4. Add a 2AFC Procedure

Similarly to above, select the Method event, add a new group event, and change its title and category to 2AFC and Procedure.

Step 5. Add a Gabor Stimulus

Now, select the Procedure event, and add a new leaf event by clicking the FILE icon with the '+' symbol in the bottom toolbar. Change its title and category to Gabor and Visual Stimulus as illustrated.

Tip: as atomic events, leaf events do not embed other events hence the absence of the small arrow in front of their descriptive mini-icon.

We are done with the basic hierarchical structure of the experiment. Now, let's move to the customization of each event.

Step 6. Customize the Method

Click the Staircase event to select it, and click the INFO button (or press the Apple-i keystroke) to inspect the properties of the Method.

A properties sheet for the Method event appears with the Examples tab selected by default. Click on the Staircase tab to select the Staircase Method, and to view or edit its properties.

The default settings are suitable for a standard staircase method, but should be customized to better fit your experimental requirements. The mandatory changes are those associated with the stimulus parameter that drives the threshold estimation during the experiment, here the Gabor orientation which is the dependent variable of this experimental design.

The name of the stimulus parameter and its initial value (in the range specified by a uniform distribution, mean ± deviation) need to be set appropriately. Change these values to match those of the figure: we name the parameter orientation and set its initial value in the range 20 ± 5 (degrees).

Click the OK button in the properties sheet to validate these changes and return to the Designer panel.

Step 7. Customize the Procedure

Once you have returned to the Designer panel, select the Procedure 2AFC event, and click the INFO button to inspect its properties.

The properties sheet for the Procedure event consists of 3 parts:

1) a right side common to all procedures that specifies the properties to be applied on each trial,

2) a top-left part that specifies the type of procedure (Examples tab selected by default) and its properties,

3) a bottom-left part that provides more customization for some of the procedures (Reaction times, and pedestal).

1) First, in Trial Properties change the type of inputs used by the subject to provide his/her responses: select the Keyboard device instead of the default one (Mouse).

Uncheck the Fixation Mark option, so no fixation is presented during the stimulus presentation (pre- and post-stimulus fixations may be still present). This minimizes the possibility of interferences between vertical cue in the fixation and the orientation judgement.

2) In the procedure-specific part of the panel, click on the nAFC tab to select the nAFC Procedure, and view or edit its settings.

Make sure a 2AFC is specified by setting n to 2. n also corresponds to the number of inputs as there is one input for each stimulus interval in the nAFC Procedure.

Enter the subject response associated to each stimulus interval: select the first interval index with the stepper (1 in the above figure), and enter the keyboard key (m in the figure) followed by RETURN.

Similarly, set the response key z for the second interval.

Make sure the 2AFC is of temporal type, so the first interval (index 1) is selected when the subject presses the key m, and the second interval (index 2) is selected when the subject presses the key z.

Select the variable orientation from the popup menu (it appears in the menu because you entered in Step 6 above). This is the stimulus parameter the subject will be judging, so it is essential to make sure the method, procedure and stimulus are connected through this parameter.

Remember that in a nAFC, the correct response corresponds to the interval with the highest value for the stimulus parameter: the subject should indicate a right direction (key m) when the stimulus orientation is higher in the first interval than in the second interval (1>2), and vice-versa (left for 2>1, key z), so the correct response corresponds to the tilt direction of the stimulus (orientation increases in anti-clockwise direction).

3) In the bottom-left, select the Pedestal tab. In each trial, the pedestal level is added to the value of the parameter indicated in the nAFC properties, so the subject's judgment is made relative to the pedestal level.

Set a uniform distribution of 90 ± 10 for the pedestal, so the orientation threshold is measured relatively to the vertical. The random variation is required to prevent the use of the absolute vertical as a reference by the subject.

4) Click the OK button to validate the changes and return to the Designer panel.

Step 8. Customize the Visual Stimulus

From the Designer panel, select the Visual Stimulus Gabor event, and click the INFO button to inspect its properties. In the properties sheet, click the Grating tab to select the Grating-like Stimulus, and view or edit its settings.

The properties sheet for the Visual Stimulus event consists of 4 parts:

1) a top part common to all types of visual stimulus that specifies their position, duration and appearance,

2) a middle part that specifies the carrier properties,

3) a bottom-left part that specifies the envelope properties,

4) a real-time preview in the bottom-right corner that can be zoomed in and out.

1) Set the stimulus contrast to maximum: 100%.

2) To create the Gabor stimulus, select in the mid-panel the Grating type from the Carrier popup menu, select the Sinusoidal modulation, and set some values for the spatial frequency and phase.

3) In the bottom-left panel, select a Gaussian envelope and set some value for its sigma, and select the circular shape with an aspect-ratio of 1.

4) Note how the preview is automatically updated with your changes (use the slider to zoom the preview in and out).

Finally and more important, connect the Carrier orientation to the parameter used by the Method and Procedure by selecting the variable orientation that appears in the contextual menu when control-clicking the text field as illustrated. Make sure to select the whole content of the text field before selecting the variable in the menu so the selection entirely replaces the previous content.

Note that, once selected, the variable name appears between brackets, which indicates the variable provides a value to the stimulus parameter.

When using a variable-based expression to define a stimulus parameter, the preview is generated assuming a value of 0 for each variable. To get a preview for a different value of the specified variable, add the desired value followed by a colon before the closing bracket ( [orientation:90] ). This value is use only for preview purpose and has no effect on the experimental design.

We are done with the experimental design, click the OK button to validate the changes and return to the Designer panel.

Step 9. Check & Run the Experiment

To run an experiment, simply do the following:

1) Select the Experiment event Orientation Discrimination which is at the root of your experiment design;

2) Check the correctness of your design by clicking the CHECK button in the button toolbar: if correct, all indicators in the check column turn into green indicators;

3) Toggle the experimental session in windowed or fullscreen mode: the windowed mode is appropriate for testing purpose only as nor Gamma correction or timing check are performed during the experiment;

4) Click the PLAY button to run the experiment.

Tips:
- Experiments can also be run through the Experiments menu which contains an up-to-date list of all experiments present in the Designer panel;
- If the Check button is disabled, then Auto-Checking is activated so an event status is automatically updated every time the event is modified (see related option in Preferences panel);

You are presented with this warning message when no calibration configuration is specified for the running experiment (see the end of the Calibration Tutorial to learn how to specified a calibration configuration). However you can still run the experiment using a default calibration, click the Continue button.

Before the session starts, you are presented with a panel summarizing the Session Information. You can either run the session in testing mode which does not require to specify the subject and group, or in real mode if you specify them. In testing mode, no session data is actually saved in the results database.

If you already have entered some subjects and groups in the Subjects panel (see the Subjects Tutorial), you can select them using the two provided popup menus. Their age, gender and handedness are shown.

Click the START SESSION button.

Press ESC (escape keyboard key) to stop the session at any time.

Step 10. Visualize the Results

Upon termination of an experimental session, results data are created and saved in Psykinematix database. The session data can be also visualized right away from the Plotter panel:

1) click the Plotter icon in the toolbar to access the panel,

2) option-click the small arrow in front of the session entry inside the table to expand the whole hierarchy of the results data,

3) select one of the entries in the last level of the hierarchy to plot its attached dataset (each entry contains different dataset),

4) you can customize the appearance of the graph using the "Properties" tab,

5) control-click the graph to print, copy or save it,

6) click the arrow to reveal the spreadsheet containing the selected data (3) used to plot the graph (control-click the spreadsheet to print or export the data).

See the Subjects Tutorial and the Plotter Tutorial to learn how to select, import and plot data collected during previous Psykinematix sessions.

Conclusion

In this lesson, you learned Psykinematix basics: how to create the hierarchical structure of an experiment, how to customize the events, how to use variables to connect procedural and stimulus events, and how to run a session.

The next few lessons in this tutorial elaborate this experimental paradigm by introducing various improvements over this single static stimulus version:

- Lesson 2 interleaves several staircases to investigate the effects of several experimental conditions, spatial frequency and size;
- Lesson 3 implements a sandwich paradigm with forward and feedback masking noise;
- Lesson 4 adds spatial and temporal context;
- Lesson 5 adds motion & dynamics through the use of 1st-order drifting Gabor.