Orientation Discrimination: Lesson 5

You will learn how to create time-varying stimuli and produce interleaved stimuli at the display frame rate.


This lesson makes the basic orientation discrimination task more elaborate by displaying a 1st-order drifting Gabor stimulus on a 2D noise background.

First, duplicate the original experiment, rename the copy Orientation Discrimination 5, and move it to the top of the Designer table. Reveal its whole hierarchy by option-clicking on its arrow.

Step 1: Creating a Drifting Stimulus

Rename the Gabor stimulus Drifting Gabor and edit its properties.

The drifting motion is created by specifying a time-varying spatial phase: enter 0:360(360) in the phase text field of the carrier, as illustrated. This tells the phase to go from 0 to 360 deg (range defined by minimum and maximum values separated by a colon) at a speed of 360 deg per second (speed defined between parentheses). This corresponds to a drifting speed of 1 cycle per second.

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

Check & run the experiment now to test it without the presence of a noise background.

Tips: There are alternative ways to create a drifting Gabor using the Phase parameter:
- Using an expression based on the [TIME] variable, for example 360*[TIME] instead of 0:360(360) to produce the same smooth motion
- Using 0:4:270(0.125) to create apparent motion so the spatial phase jumps in a discrete manner through 4 steps (0, 90, 180, 270), each presented for 0.125 second

Step 2: Adding a 2D Noise Stimulus

Similarly to what you did in Orientation Discrimination Lesson 3, create a 2D noise stimulus with a radius of 2 deg. Name it 2D Noise and move it above the Gabor stimulus.


Step 3: Adding a Dynamic Composing Event

Select the 2AFC procedure and insert a new group event using the '+' folder icon. Set the name and category of this new event to Interleaved Presentation and Composed Stimuli, respectively.

Select the two stimuli (2D Noise and Gabor) and drag & drop them onto Interleaved Presentation, so they appear indented, as illustrated.

Edit the properties of the Interleaved Presentation event and select the Dynamic tab.

Select the Fused option in the Flicker Mode pop-up menu and set the overall Duration to 0.5 s.

The table presents the visual stimuli to be interleaved. Due to the selected Fused mode, their individual duration should correspond to the duration of a single display frame based on the display settings applied to the currently edited Experiment event (0.017 seconds in this example corresponding to a frame rate of 60 Hz). Note that the Frequency text field reflects the equivalent flickering frequency.

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

Check & run the Experiment!


In this lesson you learned how to create time-varying stimuli and interleave stimuli at the display frame rate.

Now that you have completed the whole tutorial on orientation discrimination you should be ready to design your own experiments!

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