Eye-tracking to Distinguish Comprehension-based and Oculomotor-based Regressive Eye Movements During Reading

This method can help answer key questions related to eye movement control during reading such as the role of inhibition of return in regressive saccades. The main advantage of this technique is that it allows the researcher to distinguish between different types of regressive eye movements. Demonstrating the procedure will be Cassidy Campbell, Mackenzie Nalven, and Brittany Thomas who are undergraduate research assistants in my lab.

Cassidy will control the eye tracker as the researcher, Mackenzie will be the participant, and Brittany will complete the closed task. Begin by preparing the experimental stimuli for oculomotor corrective regressions. To do this, select words from an English language corpus such as SUBTLWF which provides a list of words and their frequencies in counts per million.

First select a set of at least 30 target words which are the stimuli for which word skipping and regressive eye movements will be measured. To facilitate skipping, select three-letter content words with frequencies lower than 30 counts per million. Also select pre-target words that the reader is likely to fixate before skipping or fixating the target word.

These should be high frequency with at least 30 counts per million and have five to seven letters. Next, select the post-target words that the reader is likely to fixate on after skipping or fixating the target word. Select high-frequency prepositions with at least four letters.

Now using the words selected in the previous steps, create sentence contexts in which these three words will be embedded. Place all words stimuli in the middle of the sentence. An example sentence is seen here.

In this sentence green is the pre-target word, gem is the three-letter low frequency unpredictable target word, and around is the post-target word. After sentence frames are created, use a separate set of participants to norm the stimuli using a closed task to ensure that the target word is not predictable from the sentence context. Have these participants view each sentence context up until the target word without including the target and produce a word that they think best fits the sentence.

This can be done using pen and paper or software for behavioral research. Next, prepare stimuli for comprehension based regressions. For this experiment, obtain sentence context by referencing appendix A in the article seen here which provides neutral sentence frames that end with an ambiguous homophone.

Then determine the subordinate meaning of the target word using the University of South Florida homograph norms and write the second half of the sentence that disambiguates the homophone to its subordinate meaning. For example, the subordinate meaning of the homophone grade is hill incline while the dominant meeting is evaluation scale. Thus for the sentence seen here, add of the hill they were driving down to complete the sentence.

Use an eye tracker with a sampling rate of 1, 000 Hertz meaning eye positions will be measured 1, 000 times per second. Eye movements will be measured from the right eye for this experiment. Use a 21 inch or smaller size monitor and have participants sit about 60 centimeters away from the screen.

Provide a chin rest to stabilize head movements and allow them to adjust the height of both the chin rest and chair until they are comfortable. Also provide a keyboard and instruct them to rest their fingers on yes and no buttons that will be used to answer comprehension questions during the experiment. Now instruct the participant to gaze at a white circle in the center of a black screen to calibrate their eye positions.

They should keep their eyes on the circle and follow it as it moves around to a total of nine positions on the screen. Next, validate eye positions by having the participant gaze at a white circle on the left side of the screen in the location of the first letter of the first word. Then press a button that controls the onset of reading.

Each sentence should appear in random order in the center of the screen. Use a monospaced font so that all characters take up the same amount of space on the participants retina. Intermix filler sentences and comprehension sentences followed by questions with the experimental sentences.

Be sure to perform a drift correct prior to each sentence and recalibrate the participants eye positions if they are off by more than one degree of visual angle. This figure shows reading times on target words from which the regression was launched. The regression was either made to a word that was previously fixated or previously skipped.

Regression to the previously fixated words are expected to be longer if there is an inhibition of return effect demonstrating the latency to move the eyes back to a previously fixated target. Once mastered, this technique can be done in about 30 minutes per participant if it is performed properly. While attempting this procedure, it's important to remember that skipping rates tend to be low so a large number of stimuli and participants are needed to have sufficient power to detect differences.

After watching this video you should have a good understanding of how to create stimuli that will result in a regressive eye movements to correct for oculomotor error and stimuli that will result in regressive eye movements caused by comprehension difficulty.