Orientation Discrimination Lesson 3

TEMPORAL MASKING

This lesson makes the basic orientation discrimination task more elaborate by presenting 2D noise masks before and after the target stimulus (aka sandwich paradigm with forward and feedback masking stimuli).

First, duplicate the original experiment and rename the copy to Orientation Discrimination 3 as you have learned in Orientation Discrimination Lesson 2.

Step 1. Add a Timeline event

Option-click the arrow in front of the Orientation Discrimination 3 experiment to reveal its whole hierarchy.

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

Move the Gabor stimuli INSIDE the Timeline event, so it appears indented to the right as illustrated.

Select the Timeline event, and click the INFO button to inspect its properties. Click on the Timeline tab to edit its settings (1).

The Timeline properties present a list of embedded stimulus events: so far we have only one there, the Gabor stimulus.

Click the OK button to close the properties panel (2). We are going to add masking stimuli presented before and after the Gabor stimulus.

Step 2. Add Masking Stimuli

Select the Timeline event, and insert a new stimulus event using the '+' FILE icon. Set its name and category to Forward Mask and Visual Stimulus, respectively (you could also duplicate the Gabor event, rename the copy, and move it just above the original Gabor).

Select the Forward Mask event and edit its properties by clicking the INFO button (or press Apple-i).

Select the Grating tab if necessary.

Set the carrier type to Noise using the popup menu, and select the 2D, Gaussian and White noise options.

Set the envelope to Hard-Edge and its radius to 2 deg (make sure this is larger than the sigma used for the Gabor stimulus).

Click the OK button to validate the changes.

Duplicate the Forward Mask stimulus, rename the copy to Backward Mask, and move it just after the Gabor stimulus inside the Timeline event as illustrated.

We are going to edit the Timeline properties to specify the temporal relationship between the masks and the target stimulus.

Step 3. Customize the Timeline

Select the Timeline event, and click the INFO button to edit its settings.

We have now 3 stimuli presented in the Timeline table. The green area in the table indicates when and how long each stimulus is presented. By default, as illustrated, they are presented simultaneously for the same duration.

Note that above the table are indicated the total duration of the timeline, as well as the onset and duration for the selected stimulus.

Select the Gabor entry in the table, and set its Onset column to ISI using the popup menu (ISI stands for inter-stimulus interval, and SOA for stimulus onset asynchrony).

Click NA in the to column, and select the entry 0: Forward Mask from the popup menu. This selection indicates that the ISI for the Gabor stimulus is made relatively to the Forward Mask stimulus.

The Gabor timeline is now split in two: a black zone corresponding to the period before the ISI is applied, and the green zone during which the stimulus is presented.

Note the ISI onset is set to 0 ms by default, which means that the Gabor onset occurs immediately at the offset of the Forward Mask, and that the total duration has been also automatically updated to account for the extra duration introduced by the Gabor ISI relative to the Forward Mask.

Select the Backward Mask entry in the table, and set its Onset column to ISI, and set its to column to 1: Gabor. This specifies an ISI (of 0 ms by default) between the Gabor stimulus and the Backward Mask stimulus.

The Gabor stimulus is now embedded in a sandwich masking paradigm.

Click the OK button to validate the changes and return to the Designer panel.
Check & run the Experiment !

Conclusion

In this lesson, you learned how to create noise masks, and how to use a timeline to specify precisely the relative timing of the stimuli, in particular their presentation order through the use of ISI.