EM 2 Measuring EM

Measuring EM Notes

What is the principle behind measuring eye movements by direct viewing?

• Tracks a visible landmark on the eye (e.g., limbus) as the eye rotates

• Converts rotational movement → linear displacement of the landmark

What is the detection threshold and accuracy of direct viewing for eye movements?

• Detects movements of about 1°–2°

• Equivalent to ~2–3 prism diopters

• Limited sensitivity → not useful for very small movements

How are eye movements quantified in direct viewing methods?

• Measure linear movement of a landmark (e.g., limbus)

• As the eye rotates, the landmark shifts position

• Movement is estimated visually, not instrument-based

What is the mechanism and main advantage of videocamera-based eye tracking?

• Uses real-time video imaging of the eye

• Typically tracks the pupil center (± first Purkinje image)

• Data can be analyzed computationally → higher precision & automation

How is the eye visualized in videocamera eye tracking systems?

• Eye is illuminated with infrared (IR) light

• Allows clear pupil detection without visible light interference

What is the resolution and accuracy of videocamera eye-tracking methods?

• Resolution ≈ 15 minutes of arc (~0.25°)

• Much more sensitive than direct viewing

What are common videocamera eye-tracking systems or examples?

• ISCAN, ASL, SMI, Arrington, Tobii

• All track pupil center (± corneal reflection/Purkinje image)

What is the mechanism of photoelectric eye movement recording (e.g., Visagraph)?

• Uses a photocell to detect light intensity

• Eye illuminated with infrared light (not visible to subject)

• Measures reflected light changes as eye moves

• Tracks limbus position

What is the resolution and accuracy of photoelectric eye movement methods?

• Resolution < 5 minutes of arc

• More precise than videocameras (~15 min arc) and direct viewing

What are the key features and advantages of photoelectric (Visagraph) eye tracking?

• High sensitivity (small eye movements detected)

• Uses infrared illumination → subject unaware

• Objective measurement via light detection (photocell)

• Tracks limbus rather than pupil center

What is the mechanism of ENG/EOG (electronystagmography/electrooculography)?

• Eye behaves like a dipole:

  • Cornea = positive, retina = negative

• Electrodes placed at outer canthi + reference (between eyes)

• As eye rotates → change in electrical potential detected

What are the key advantages and capabilities of ENG/EOG?

• Measures large eye movements (up to ±30°)

• Detects horizontal + vertical movements

• Works with eyes open AND closed

What are the limitations and resolution of ENG/EOG?

• Resolution ≈ 1° (poor precision)

• Electrodes pick up facial/blink muscle activity (noise/artifact)

• Less accurate than optical methods

How are electrodes arranged in ENG/EOG and what do they detect?

• Electrodes at lateral canthi + central reference electrode

• Detect voltage differences as cornea moves closer/fartherElectrodes at lateral canthi + central reference electrode

• Detect voltage differences as cornea moves closer/farther

• Signal reflects eye position via bioelectric potential changes

• Signal reflects eye position via bioelectric potential changes

What is the mechanism of the scleral search coil method for measuring eye movements?

• Subject placed in a magnetic field

• Wears a contact lens (silicone annulus) with embedded wire coil

• Eye movement → coil moves → voltage induced

• Voltage ∝ sine of angle between coil plane and magnetic field

• Signal reflects precise eye position

What is the accuracy and role of the scleral search coil method?

• Detects seconds of arc (extremely high resolution)

• Most accurate / gold standard for eye movement research

What are the advantages and limitations of the scleral search coil method?

Advantages:

• Highest precision available

• Detects very small eye movements

Limitations:

• Invasive/uncomfortable (contact lens coil)

• Coil may slip at large gaze angles

Why are continuous recording methods ideal for eye movements, and which systems use them?

• Eye movements are continuous in nature

• Some devices produce continuous (analog) signals

• Examples: Search coil, EOG, limbal trackers

• Can be recorded with a strip recorder or digitized later

What is the difference between analog and digital eye movement recordings?

• Analog: continuous signal (true eye movement over time)

• Digital: sampled at discrete time points

• Digital may miss information between samples

What are key limitations of video-based (sampled) eye tracking?

• No info between frames (temporal gaps)

• High-frequency eye movements may be missed

• Can cause aliasing if movement frequency > sampling rate

Why are eye movements important in reading?

• Reading is one of the most eye movement–demanding tasks

• Critical in early education (learning to read)

• Inefficient eye movements → impair academic & vocational performance

What did Javal (1879) discover about eye movements in reading?

• Eyes move in discrete jumps (saccades)

• Fixation pattern: jerk from word to word, not smooth motion

• Reading = saccades + fixations, not continuous flow

What factors increase the number of fixations during reading?

• Increased difficulty of text

• Poor reading ability
  → More fixations = slower, less efficient reading

How does word size influence fixation patterns in reading?

• Larger words → more likely to be fixated

• Smaller words are often skipped

What are interfixation movements (saccades) and their role in reading?

• Rapid eye movements between fixations

• Move eyes across text (word-to-word)

• No visual processing during movement

What is the typical magnitude of saccades in reading?

• ~1.5° (range: 0.5–4°)

• ≈ 8 characters (range: 1–18)

• Includes spaces + punctuation

What is a return-sweep saccade in reading?

• A large saccade from right → left

• Moves eyes from end of one line → beginning of next line

• Starts ~6 characters before the end of a line

• Lands around the ~6th character of the next line

• Rarely lands exactly at margins → involves systematic undershoot

What is a key accuracy feature of return-sweep saccades?

• Often undershoot the intended target

• Requires corrective saccades to reach optimal fixation

• Reflects limits in long-range saccadic accuracy

How much of total reading time is spent on eye movements?

• ≤ 10% of total reading time

• Majority of time spent during fixations (processing), not movement

What is the average span of recognition in reading?

• Amount of text processed per fixation

• Defines how many words/characters can be perceived at once

What is fixation duration and what factors affect it?

• Time eye remains on a word (~225 ms average)

• Text dependent:

  • Shorter for easy text

  • Longer for difficult text

What is reading rate and how is it measured?

• Reading rate = words read per unit time

• Typically measured in words per minute (wpm)

What is the typical reading rate for average vs. exceptional readers?

• Average college student: ~200–350 wpm

• Exceptional readers: up to ~1000 wpm

What is the limitation of “speed reading” claims?

• Claims: 2000–10,000 wpm

• Achieved by skimming (very few fixations, e.g., ~1 per line)

• Results in poor comprehension → not true effective reading