What Can Eye Movements Tell Us About Reading?

I do believe something very magical can happen when you read a good book.

-- JK Rowling

A well written story can spark vivid mental images and intense emotions in a reader. Eye movements (EMs) associated with reading behavior generate small but detectable bio-signals that suggest how the brain perceives, integrates and comprehends information acquired through words printed on a page. Although personal experience tells us our eyes scan each line of text at a steady pace, recordings of EM activity reveal a different picture.

Saccades are rapid, horizontal movements that bring each word into visual focus. These movements are followed by fixations or attentive pauses. Individual words are recognized and given meaning during fixations according to a generally accepted theory of reading. At the end of a text line, the eyes perform a coordinated backsweep where the reader's focus shifts to the beginning of the line below. Both eyes continue their halting movements to produce a “stair step” signature when the amplified bio-signals (in volts) are graphed with respect to time (in seconds).

Leftward EMs within the interval between sequential backsweeps are regressions, which enable the reader to revisit parts of a sentence and re-inspect specific words. Languages read right to left or vertically produce their own distinct EM signatures. Text comprehension emerges from the integration of words across sentences and paragraphs.

Description: The proposed activity, which emerged from a pilot project, introduces high school students in an Advanced Placement Psychology course to experimental reading research. Each student will participate in a short reading task and generate their own electrooculograms (EOGs) or record of EM activity.

Key Concepts:Backsweep, EM Signature, EOG, Fixation, Regression, Saccade, Text Comprehension

Supplies:

ADC & Signal Display - Select an analog-to-digital converter (ADC) for the EM signals. Initially, I used a legacy Dataq digitizer (Model DI-190) – until I lost the unit while moving from one office to another. The converter had a signal sampling frequency of 240 Hz and a +/-5 V DC input range. I used the manufacturer's signal acquisition and display apps that require Windows 9X. The apps and user guides are available as a free download from Dataq.com, which also sells an updated version that runs on current operating systems. I addition, a post-acquisition analysis app that calculates signal derivatives is required. BTW, if you use Dataq's acquisition app, you must use their analysis app, Advanced CODAS. I tried substituting the DI-190 with a PC signal analysis app that collected data through a sound card, but experienced disappointing results.

Alcohol Wipes (disposable)

Connectors – Crimp-on or solder pin type

Electrodes – Use disposable electrodes to pick up bio-signals associated with the voltage difference between the cornea and the retina of each eye (Wallgreens; Item Code: 21055; ECG patches also work).

Monitor - In keeping with other reading researchers, the text sample should be readable on a computer monitor from a distance of about one meter using a basic font.

Power Supply - Eight, AA batteries to provide 12 VDC

Signal Acquisition & Processing – An electrooculograph is the writing instrument that amplifies positional changes of the eye and filters unwanted bio-signals as well as 60 Hz interference from nearby electric lights and appliances. An interpretable record of the analog voltage output associated with EM activity is an electrooculogram. The analog voltage correlates with the number of character spaces traveled by the eyes as they read a sentence.

Research grade EOGs sell for thousands of dollars; but this project will barely make a dent in your departmental budget! I discovered a student grade, dual-channel EOG at an estate auction for $25 and then invested some time to setup and test the unit. The battery operated, analog unit is ideal for this project. However, the model is discontinued and may require some effort to locate. Try electronic auctions or schools downsizing their biology departments. If you're really jazzed and have the tech skills, build your own battery-powered EOG. Watchmeflyy posted a detailed instructable that shows you how to do this.

Tools & Misc Items – Contact adhesive/sealant, small screw driver from repair kit for eye glasses, solder & soldering pencil, wire snips

USB Flash Drives - For storing each student's EOG file

Wire - Stranded, hook up wire to make jumper leads and electrode lead extensions

Teacher Notes

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Step 1: EOG Circuit Description

The battery operated, Heathkit EOG Module is compact, signal processor featuring channel-dedicated: 50/60 Hz rejection, baseline controls (Offsets 1-4), high and low pass filtering (HPF 1&2 = 0.05 Hz; LPF 1&2 = 30 Hz) and adjustable amplifier controls (Gains 1-2). I checked both sides of the circuit board for obvious physical damage. Aside from an improperly seated integrated circuit, everything seemed satisfactory.

Rejection of 60 Hz noise is achieved with differential amplifier, A1 that increases input signals that are dissimilar in voltage, while limiting electrical noise that is common to both inputs. Voltage output from a differential amp is proportional to the difference in voltage between each input.

The specific operation of the isolation amplifier, A2 isn't clear from the graphic, but my sense is that the circuit provides a safety barrier between the subject and possible current leakage from the battery pack.

The graphic of the low frequency response curve (HPF1) gives a qualitative indication of the increase in amplifier output with respect to frequency. The graphic of the high frequency response curve (LPF1) on the right side of the board gives a qualitative indication of the drop off in amplifier output with respect to frequency. Together, the cutoff frequencies of 0.05 Hz and 30 Hz define bandpass for the EOG signals, which includes the expected range of EM frequencies associated with reading behavior. Because fixations have low frequencies, EOG units use minimal low frequency filtering. High frequency filtering reduces the presence of facial muscle activity as well as 60 Hz noise remaining in the signal.

Amplifier, A3 multiplies the input signal by a factor determined by Gain 1. Lastly, the (DC) Offsets 1&2 adjust the EOG baseline by adding voltage to or subtracting voltage from the signal. The 9-pin D connector on the far right edge of the board accepts 12 VDC to power the board.

Step 2: Assemble Jumpers & Electrodes; Connect Batteries

I assembled five jumper cables by soldering pin connectors to the ends of stranded hook up wires. Circuit wiring took seconds thanks to the block diagram printed on the circuit board. The second channel isn't needed for this activity.

After circuit components were connected; I cut three, 30 cm lengths of hook up wire to make electrode extensions and terminated the ends with a pin connector. I plugged one end of the extension into the terminal of each electrode and soldered battery pack leads directly to the +12 V and -12 V pins of the J2 connector on the board's component side. Leads were secured with several drops of Goop sealant. I attached the Channel 1 analog outputs at Vo1 and earth ground to the DI-190's inputs.

Step 3: Designing the Experiment

General Knowledge: This study uses an EOG to determine whether a distraction changes a student's EM signature with respect to a baseline condition. According to reading theorists, "the eyes go where the mind goes." If a distraction interferes with a reader's goal of attentively searching for the next word during a reading task, what might the eyes do? It's possible EMs will stop when a reader's attention disengages from the task. When attention re-engages, the need to re-read poorly understood words or sentences may trigger regressions. However, the pattern of attentional engagement followed by disengagement may compromise the integration of sentences necessary for text comprehension -- imagine trying to study in a library when seated near someone with a cell phone ringer that periodically interrupts your reading.

A within-subjects design where each student serves as their own control across the collection of two data sets is used to explore these possibilities. The first data set is collected in a designated quiet location of a computer lab to establish baseline EM activity. The second data set is collected in the same location while the student listens to a podcast. An EM signature consistent with a sound source disengaging a reader's attention should exhibit:

  • prolonged fixations with no EM activity indicating reading behavior has stalled; and,
  • frequent regressions between backsweeps indicating re-readings of sentences.

In addition, there should be a reduction in text comprehension when the reader is distracted.

Tech Note 1: Time constraints of a typical classroom session placed limits on the proposed experimental design. A better design would use random student assignment to one of two groups where each group is exposed to a counter-balanced presentation of the podcast across a longer reading session.

Step 4: Running the Experiment

Teacher: Obtain parental consent. Seat student in front of computer monitor and instruct student to prep left and right temples of the head as well as on the area directly behind the preferred ear with an alcohol wipe for placement of the reference or electrically neutral electrode. Have student place an electrode on each prepped location.

Tech Note 2: Brisk rubbing can reduce electrode impedance and improve signal quality.

Teacher: Insert leads into "R eye," "L eye" and "Head" inputs of circuit board. Begin data collection by launching signal acquisition app. Instruct student to perform bio-cals which simulate EMs and fixations by:

  1. focusing on left margin of screen and fixating for about one second;
  2. focusing on right margin of screen and fixating for about second;
  3. returning focus to left margin of screen for about one second.

General Knowledge: Because the cornea and retina of the eye are oppositely charged, the left electrode picks up a positive signal when subject looks left and the right electrode picks up a negative signal. When looking right, the left electrode picks up a negative signal and the right electrode picks up a positive signal. The differential amplifier, A1 interprets the eyes looking left as a downward signal deflection and as an upward signal deflection when the eyes look right.

Teacher: Instruct student to read first page silently. Administer quiz when data collection is finished. Repeat instructions for second page reading before collecting next data set. After student has positioned headset comfortably, begin streaming program. The experiment concludes following administration of second quiz. Prep, bio-cals and data collection should take about 5 to 8 minutes. Store EOG data on the dedicated flash drive provided to each student. Grade each quiz and instruct students to upload their data on their workstation computers.

Step 5: Computer Lab Prep

Teacher: Make arrangements to have each workstation in your computer lab run a signal analysis app that calculates changes in eye position over time on the vertical axis (defined as velocity or reading speed) with respect to time displayed on the horizontal axis. The calculated channel should display momentary changes in reading speed as well as changes in EM direction indicating a backsweep, saccade or regression. During a fixation when the eyes pause for word recognition and meaning assignment, reading speed for practical purposes drops to zero.

General Knowledge: Here is a screen grab of eye position (upper channel) and calculated velocity (lower channel) from the pilot study's bio-cal procedure. Notice how the velocity waveform peaks and then returns to the baseline as expected when the eyes reach the fixation point. Although both channels feature clean waveforms that exhibit correct shapes, the amplitude of the position signal is too small. The span between the scale endpoints is only 0.360 volts! There should be a full-scale deflection (FSD) that approximates the input span of your A/D converter when a student moves their visual focus to opposite margins of the screen. The desired span using the DI-190 should approach +/-5 VDC as displayed in the bottom channel. When bio-cals do not achieve FSD, fixations recorded during the reading tasks will be difficult to identify. Dataq has a helpful tutorial on digital data conversion here.

Tech Note 3: Adjust Gain 1 with eye glass screwdriver to produce the largest deflection possible within the span of your ADC during bio-cals. Don't exceeds specified limits of your ADC or signal will "hit the rails" causing signal distortion.

Step 6: Pilot Study Results

General Knowledge: The interval defined by Backsweeps 1 and 2 in this screen from the pilot shows eye speed during the inspection of a text line while subject listened to a podcast. I needed the highlighted velocity band to identify fixations when the eyes were stationary because of the background noise in the channel. Fixations, F1 - F9 are labeled above those intervals between EMs and within the velocity thresholds. Upper and lower limits of the band were eyeballed for the pilot (pardon the pun :>). The downward dip following the 750 msec pause is a regression.

Tech Note 4: A more rigorous approach to fixation detection should consider: accuracy, sensitivity and specificity when determining thresholds for the velocity band. These terms are addressed here.

The second screen details a pause in EM activity followed by a regression consistent with distracted attention requiring re-inspection of a word. Digitized voltages corresponding to the regression’s start and peak amplitude appear in sidebar. Because the equipment doesn't map words on to fixations, the particular word associated with this pause can't be identified -- but overall, not too shabby for a $25 investment!

Step 7: Class Exercise

General Knowledge: The average fixation time for silent readings of grade-relevant material is 200 to 250 msec with a standard deviation of about 100 msec. Assuming these fixations are normally distributed, 99+ % of measured fixations should fall within three standard deviations (SDs) of the average. In the proposed project, fixation times beyond three SDs are defined as longer than expected pauses consistent with stalled reading behavior. For example, using an average fixation of 250 msec with a SD of 100 msec, minimum duration for a prolonged fixation would be: 250 msec + (3 x 100 msec) = 550 msec.

Teacher: For the baseline reading task, instruct class on how to measure the duration of each fixation within the signal analysis app that you're using. Given the number of words skipped versus those re-fixated, the text should produce a ball park figure around 250 measurements; or about one fixation per word. Instruct students to: calculate their average fixation duration, SD and minimum duration of an outlier fixation. In addition, have them tally the number of regressions and report quiz scores on their worksheets.

For the distracted reading task, instruct students to: calculate their average fixation duration, SD and scroll through their data to identify outlier fixations; tally the number of regressions and report their quiz scores on their worksheets. Have students compare records of their performance across the two tasks.

Home Assignment: Ask students prepare talking points for in-class, group presentations describing their personal experiences involving divided attention, such as listening to a TV newscast while simultaneously trying to read and understand captions scrolling across the bottom of the screen.

Step 8: Applications & Expected Learning Outcomes

The re-direction of attention due to a distraction presented during a reading task can lead to comprehension deficits. Reading scientists use EOG data coupled with sophisticated computational methods to uncover episodes of mindless reading where the eyes advance across a page even though the reader is not attending to the text. The use of such assistive technologies has the potential to promote mindful reading and consequently increase the impact of teaching materials by alerting students when their attention no longer text-focused.

The proposed classroom activity aims to expand general knowledge as well as performance opportunities for students in AP Psych. Expected learning outcomes are:

  • a familiarity with elements of reading theory; and,
  • the development of skill sets needed to pursue empirical research.

Specifically, students are introduced to important EM variables in reading theory, the relationships among these variables and the use of research methods to test these relationships. A lab exercise offers students a unique perspective on their individual reading styles. A take home assignment reinforces concept learning by encouraging students consider and share their personal experiences with distracted visual attention.

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    WeTeachThemSTEM

    7 weeks ago

    Thanks for sharing this. It looks like a very interesting activity for your AP Psych students.

    1 reply