Visual Modality

• The visual modality is crucial for human sensory perception.

• Visual processing involves a complex, multistep procedure.

Foveal Area

• Highest density of cones (color-sensitive photoreceptors) is found here.

• Provides the best visual acuity.

Brain and Visual Field

• The brain aims to keep the most important parts of the visual field on the fovea during normal lighting conditions.

Visual Tracking

• Fixation on an object often requires head and eye movement due to external changes.

Eyeball Movements

• Movements include:

  • Microsaccades: Involuntary, minor, repetitive quick movements.

  • Controlled voluntary eye movements: Facilitated by six extrinsic eye muscles.

Eye Muscles

• Lateral Rectus Muscle: Controlled by cranial nerve VI (abducens nerve).

• Superior Oblique Muscle: Controlled by cranial nerve IV (trochlear nerve).

• Other Muscles: Controlled by cranial nerve III (oculomotor nerve).

Synchronized Movement

• Both eyeballs move in coordination to provide slightly different images, which are crucial for:

  • Stereo vision: Depth perception.

Electrooculography (EOG)

• The human eyeball acts as a dipole with a positive terminal (cornea) and a negative terminal (behind the retina).

• Voltage generated (0.4 - 1.0 mV) is based on retinal cellular activity and can be measured during eye movements.

Electrode Placement

• Pairs of electrodes:

  • Horizontal measurements: Placed on the sides of the orbits.

  • Vertical measurements: Above/below the right eye.

Signal Interpretation

• Movement towards the positive electrode results in a positive voltage change.

• Voltage is proportional to movement (approximately 20 mV/degree).

• Typical accuracy is 1.5-2º; measurable range: ±70º both vertically and horizontally.

EOG Signal Characteristics

• Ranges from 0.05 to 3.50 mV; frequency of interest: 0-15 Hz.

• Baseline readings consistently return to zero after several minutes under constant fixation.

• Artifacts from facial/jaw movements can cause noise in data.

Other Eye-Related Measurements

• Pupillography: Measures changes in pupil size controlled by the autonomic nervous system.

  • Sympathetic Activation: Causes dilation via trigeminal nerve control.

  • Parasympathetic Activation: Causes constriction through cranial nerve III.

• Task Evoked Pupillary Response: Pupils respond in size to stimuli.

  • Dilation for attractive stimuli, constriction for unappealing ones.

• Startle Reflex: Eye blink response to unexpected stimuli can be measured with EMG.

Applications of EOG

• Useful in psychophysiological research for tracking eye movement on visual stimuli, relevant in studies of attention and cognitive processes.

• Recent applications:

  • Used to identify rapid eye movements (REM) in sleep studies.

  • Pupillary responses are indicators of cognitive effort and emotional states (e.g., dilation linked to novel stimuli/fatigue).

  • Magnitude of the startle response reflects high arousal and negative emotional states (anger, fear, sadness).

Goal of the Class

• Demonstrate basic EOG phenomena using Biopac Student Lab (Lesson L10