1. Stimuli and design

1. To run this experiment, you will need a programming environment such as MATLAB or experimental sequencing software such as E-Prime.
2. Each trial will consist of four basic components: A fixation cross, a shape stimulus display (called the target display), a mask (four dots), and a response display. Figure 1 depicts the four primary elements in each trial.
   1. The display background will always be white.
   2. The fixation cross is a small cross in the center of the display, with each arm of the cross measuring 0.25 cm. The fixation cross will be present at all times, except for when the response display appears.
   3. The response display will consist of a black outline of a square, a circle, a diamond, and a triangle arranged horizontally and in the center of the screen. The shapes should each be inscribed within an invisible circle with a radius of 0.75 cm.
   4. The target display will include eight shapes, randomly selected on each trial from the set of four: a black outline of a circle, square, diamond, and triangle. Not all shapes need to appear in each trial, and of course, shapes can (and will be repeated) in a given trial. Again, the shapes will each be 0.75 cm in radius. They will appear in random positions on an invisible circle with a radius of 1.5 cm from the fixation cross in the center of the screen.
   5. The mask will be four black discs (dots) with a radius of 0.25 cm. The dots should be arranged to form the four corners of a square just large enough to include all of the four target shapes within.

Figure 1: Primary elements of an object substation display. Every trial will begin with a fixation display, and participants will be instructed to fixate the cross before initiating a trial. Every trial will end with a response display, in which the participant will select the shape she remembers seeing between the four dots. Between the fixation and response display, a target display will show a ring of eight shapes and a mask of four dots will also appear, in a position surrounding one of the shapes. As described in more detail in the procedure, the mask and target display can appear in different orders, but each will remain present for 30 ms.

3. The sequence of events in each trial will begin with a fixation screen that will remain present until the participant presses the spacebar, and for 200 ms after.
4. A trial will always end with a response display that will remain present until the participant enters a response (by clicking on the shape she believes she saw between the four dots on that trial.)
5. Between the fixation and response displays a target display and a mask will each be shown for 30 ms. The mask will surround the position of a randomly selected target shape in each trial.
6. The critical independent variable in this experiment is the stimulus onset asynchrony (SOA). This is defined as the time difference between the onset of two stimuli, in the case of this experiment, the mask and the target display. Thus, the SOA is the time of onset of the target display minus the time of onset of the mask display. Figures 2-4 schematize several trials of the experiment with different SOAs.
   1. If the SOA is 0, it means that the stimuli appear together (and since each last for 30 ms, they disappear together as well).
   2. If the SOA is negative, it means that the mask appeared before the target display.
   3. If the SOA is positive, it means that the mask appeared after the target display.
   4. The experiment will include 15 different SOAs, 0 ms as well as positive and negative: 10, 30, 50, 70, 90, 150, and 300 ms.

Figure 2: Stimulus onset asynchrony of 0 ms. In a trial with an SOA of 0 ms, the mask and the targets appear simultaneously. Since each is programmed to remain present for 30 ms, they are present and expire together as well.

Figure 3: Stimulus onset asynchrony of 50 ms. An SOA of 50 ms, whether negative or positive, means that one stimulus will onset 50 ms after the other. But these stimuli are programmed to last only 30 ms, meaning that an SOA of 50 will leave 20 ms with an empty display (except for the fixation cross). For this experiment, we've defined SOA as target onset-mask onset, meaning that negative SOA values are associated with the mask appearing first, and positive values are when the mask appears second.

Figure 4: Stimulus onset asynchrony of 10 ms. With masks and targets programmed to remain present for 30 ms, SOAs of 10 ms leave 20 ms during which the mask and the target overlap.

7. Sequence the experiment to include 20 trials with each of the 15 SOAs, randomly intermixing all the different kinds of trials.
8. A critical factor is that the participant does not know where the dots will appear in each trial, so be sure that the dots are appearing unpredictably from trial to trial.
9. Be sure that the experimental program outputs sufficient information about each trial to allow you to analyze the results later. The relevant information for each trial is: the trial number, the SOA on that trial, the shape shown between the mask, and the shape response given by the participant.

2. Running the experiment

1. To run this experiment, you'll want to recruit 10 participants, testing each individually in a quiet room.
2. When a participant arrives for the experiment, seat them 60 cm away from the monitor of the testing computer.
3. The image on the screen should be of a target display with a mask. Use this image to explain the instructions as follows:
   1. In each trial of this experiment you will see a ring of shapes, like the one shown here [pointing]. Additionally, you will see four dots, like the ones here. Sometimes, they will overlap, as in this image. But sometimes, one might precede the other. Regardless of their order of appearance, your task is simply to try to see and remember the shape that appears in the space between those four dots. Does this make sense?"
   2. Great. Sometimes, it might feel hard-you might feel like you really don't know the answer. Just guess in those cases. A few other things. Each trial will start with a display of just a cross, like the one in the middle, here. Before you start a trial, make sure you are fixating the cross, and try your best not to move your eyes. When you are ready, you'll press the spacebar to start a trial."
4. After explaining the instructions to the participant, start the experiment, and observe the participant for four or five trials to make sure they have understood the instructions. Then leave them to complete the experiment.

3. Analyzing the results

1. To analyze the results, you'll first want to compute response accuracy individually for each subject and SOA.
2. Next, you can average together response accuracy by SOA, across subjects.
3. To determine whether there is a significant effect of SOA on performance, perform a repeated measures ANOVA on accuracy as a function of SOA using MATLAB or a statistical package such as SPSS.