Perception Lecture Recording on March 4th 2025

Potency in Visual Processing

• Discussion on different forms of potency in the lateral geniculate nucleus and primary visual cortex.

• Importance of understanding receptive fields.

Types of Cells in Lateral Geniculate Nucleus

• Concomitant cells respond to pairs of cones.

  • Opposing cones: Red (l) or Green (m) vs. Blue (s).

  • Receptive fields can have l or m in center or surround.

Excitatory and Inhibitory Roles

• Center and surround roles vary:

  • Examples include ‘m’ in center or surround.

  • Identifying excitatory vs. inhibitory elements is essential in drawing correct receptive fields.

Conoponin Cells

• Maximally stimulated by uniform light.

  • Specific neuron example:

    • Excited by green light (when m is excitatory in the center).

    • Inhibited by uniform red light.

Double Opponent Cells

• Found in the primary visual cortex.

• Opponency occurs in both center and surround:

  • Green (excitatory) in center, inhibitory in surround.

  • Red (inhibitory) in center, excitatory in surround.

• Produces maximal firing for borders (edges) between colored regions (e.g. green and red).

Vergence as a Depth Cue

• Vergence: based on angle between two eyes while focusing.

• Feedback from extraocular muscles helps the brain gauge eye angles:

  • Larger angle when focusing on closer objects, uncrossing for distant objects.

• Effectiveness diminishes beyond two meters; angle remains fixed.

Pictorial Cues in Depth Perception

• Various monocular pictorial cues identified in images, discussed in class.

Common Monocular Cues

• Textured Gradient: From logs in an image; fine detail diminishes with distance.

• Linear Perspective: Observed in aligned structures like ladders; lines converge with distance.

• Relative Size: Objects (ladder rungs) appear smaller with distance.

• Relative Height: Position of objects relative to an implied horizon.

• Occlusion: When one object overlaps another, implying closeness.

• Shading: Variances in light create depth perception.

• Atmospheric Perspective: Distant objects appear hazier.

Motion Detection and Eye Movement

• Neurons tuned for motion detection depend on their retinal positions.

• Delays impact directionality of perceived motion:

  • Direction determined by which receptive field is stimulated first.

  • Speed of motion is inferred from the length of delay in circuits.

Eye Movements

• Two main types of eye movements:

  • Saccades: Rapid-fire eye movements; vision suppressed during.

  • Smooth Pursuit: Tracking a moving object smoothly; requires a moving subject to initiate.

Understanding Object vs Eye Movement

• Smooth pursuits can make stationary objects seem to move across the retina.

• Need for extra-retinal information (like eye position) to determine true versus perceived motion.

• Corollary discharge theory: duplicate commands sent to visual cortex for immediate adjustment based on eye movements.

Role of the Superior Colliculus

• Coordinates eye movement commands sent to the extraocular muscles and visual cortex.

• Traveling info to both locations is believed to enhance motion perception.

Akinetopsia (Motion Blindness)

• Rare condition where individuals cannot perceive motion, even though they can see other things normally.

• Can deduce motion through changes in static images over time.

Motion Aftereffects

• Perceptions after extended uniform motion (e.g., waterfalls).

• Opposite motion is perceived when stationary objects are viewed afterward due to fatigue of motion detectors.

Cycle of Action and Perception

• Interaction between perception and action through context, such as reaching for a cup.

• Action influences perception, emphasizing active nature of perception.

Optic Flow

• Key concept in understanding movement through environment.

• Points move across the retina in relation to focus of expansion during movement.

  • Closer objects move faster across retina.

• Important for navigation (e.g., while driving or walking).

• Changes in gaze influence how effectively we utilize optic flow.