Crowding

1. Stimuli and Design

1. Trials will involve all English consonants, written in black and shown in 36 pt. Helvetica font.
2. Each trial of the experiment will begin with a fixation cross presented in the center of the display for 500 ms, followed by stimuli presented to the right or left of the cross for 500 ms, and finally followed by a response screen. Figure 3 schematizes the experimental sequence.

Figure 3. Trial sequence. The sequence of events in a single trial of the experiment is as follows: The participant fixates the central cross and presses the spacebar to initiate the trial. After 500 ms, the test stimulus appears either to the right or left of fixation, including three letters. The participant should avoid making eye movements, but should try to identify the letter in the center of the group. The letters disappear after 500 ms, at which point the participant is prompted to enter a responses reporting the letter that they saw in the middle of the group.

Figure 4. Variables for manipulation. There are two crucial variables that can be manipulated in a crowding experiment. The first is called the eccentricity, referring to the distance between the fixation cross and the target stimulus, in this case, the letter in the center of a group of three. The second variable is called the inter-stimulus spacing, which refers to the distance between the target letter and each of its neighbors. In an experiment, these two variables will usually be manipulated together, by a third derived variable called the relative spacing, defined as the ratio of the relative spacing to the eccentricity.

3. The stimuli presented in each trial will always come in groups of three. Two variables will be manipulated in the experiment: (1) the distance between the fixation cross and the stimulus in the middle of the group of stimuli, a measure called the eccentricity; (2) the distance between the middle stimulus in the group and each of the other two, a measure called the inter-stimulus spacing. Figure 4 shows a sample trial and identifies these two variables.
   1. When preparing the experiment, these two variables will actually be controlled by a third variable derived from their ratio. We'll call that the relative spacing and define it as the inter-stimulus spacing divided by the eccentricity. Since it is a ratio defined over two variables with the same units, note that the relative spacing does not have units.
4. Now, to sequence the experiment, program a set of 200 trials.
   1. Each trial will belong to one of four relative spacing conditions, with values of 0.25, 0.4, 0.5, and 0.75. In pixels, the following are the values we will use for the inter-stimulus spacing and the eccentricity in each condition, respectively: (100, 400); (50, 125); (75, 150); (600, 800). But note, any values can be used so long as they produce the desired ratios.
   2. Randomly intermix an equal number of trials from the four conditions. On each trial, randomly choose the three consonants to display, with the restriction that no letter is repeated in a single trial.
   3. Also program the experiment to show the stimuli to the right of fixation in half of the trials, and to the left of fixation in the other half.
5. Be sure to program the experiment to report the relevant outputs, minimally for each trial: the trial number, the three letters shown, the relative spacing, the response supplied by the participant, and whether that response was right or wrong.

2. Running the Experiment

1. To run the experiment, recruit participants.
2. When a participant arrives in the lab, be sure to have her complete informed consent. Then seat the participant 60 cm from the computer monitor.
3. For this experiment, it important to also have a chin rest.
   1. Instruct the participant to place their chin in the chin rest and to resist moving during trials. Let them know that the experiment is self-paced, and that they are free to move or rest between trials as they please.
4. Explain the instructions to the participant as follows:
   1. "In this experiment, we are interested in investigating how easily we can recognize objects shown in the visual periphery. On each trial, I'd like you to fixate on the cross in the center of the screen, and do your best not to move your eyes. When you press the spacebar to start a trial, after 500 ms, three letters will appear on the screen either to the left or right of fixation. They will only remain on the screen for 500 ms, and they may be a little hard to see. Your task is to try your best to recognize the letter in the middle of the pack, that is, the letter presented between the two others. Remember, try not to move your eyes, even when the letters appear. After they disappear, enter the letter you saw in the middle using the keyboard. If you are unsure, just guess."
   2. Remain in the room with the participant while she completes a few trials. Then leave her to complete the experiment.
5. Data analysis in this experiment is straightforward. Simply compute average letter recognition accuracy as a function of relative spacing. So just count up the number of trials in which the participant accurately recognized the central letter in each spacing condition, and divide by 50, since there were 50 trials for each spacing condition.