In-Depth Notes on Motion Perception and Cognition

Overview of Motion Perception

• Course: PSYU2239/PSYX2239

• Instructor: Kevin Brooks

• Key Focus: Understanding how we perceive motion in visual stimuli.

What is Motion?

• Definition: Change of position (or displacement) over time.

  • Mathematical Representation:
    $ext{Speed} = rac{ ext{Distance}}{ ext{Time}}$

  • Alternatively, in terms of grating:
    $ext{Speed} = rac{ ext{Temporal Frequency (TF)}}{ ext{Spatial Frequency (SF)}}$

• Temporal Frequency (TF): Refers to how often cycles of light-dark transition occurs in one second.

Perception of Motion

• Direct vs. Indirect Sensing:

  • We don't simply judge position and time separately for motion perception.

  • Specialized mechanisms allow for direct motion sensing:

  • Adaptation Experiments: Show that motion perception doesn't rely on precise judgment of position and time.

  • E.g., Motion Aftereffect (MAE): After adapting to motion in one direction, stationary objects appear to drift in the opposite direction.

Motion Detectors

• Reichardt Detector Model:

  • Designed to signal motion direction.

  • Mechanism:

  • Receptors delay and compare signals from neighboring detectors.

    • One receptor's output is delayed; signals coincide at the "AND" unit if they reach it simultaneously for motion detection.

  • Null Direction Response: No firing occurs for motion in the opposite direction (null direction).

  • Multiple detectors with varying delays are necessary to represent different speeds.

Example of Reichardt Detector for Motion:

• Leftward Motion Example:

  • Configuration allows for detection in both opposing directions using the same receptors.

Stroboscopic Motion

• Definition: Illusory motion created by a series of discrete positions viewed in sequence (e.g., on a TV).

• Mechanism:

  • The motion appears real because it stimulates motion detectors similarly to real motion.

  • Persistence of Vision: Not the cause; blurring doesn't explain the realness of the perceived motion.

Wagon Wheel Effect

• Observation: Wheels can appear to spin backward or stationary in film.

  • Explained through Frame Changes:

  • At specific frame rates, a wheel’s movement may coincide with its displayed image, leading to perceived backward motion.

• Examples: Different spoke displacements create varying perceptions of motion.

Motion Aftereffect (MAE) Explained

• Concept: Prolonged exposure to a moving stimulus alters perception of motion.

  • Adaptation: Results in increased thresholds for detecting subsequent similar stimuli.

  • MAE occurs when stationary objects seem to move opposite to the adaptational motion direction.

Physiological Mechanisms in Motion Perception

• Types of Cells:

  • Magnocellular Pathway: Involved in motion detection; connections include:

  • LGN → V1(4Ca) → V1(4B) → V2(Thick Stripes) → MT+

  • Different cellular layers in LGN (M, P, K) are involved in specific stimulus processing.

• V1 (Primary Visual Cortex): Key for direction tuning in motion perception.

• MT+ (V5): Specialized for motion detection, with microstimulation influencing motion judgment.

• Akinetopsia: A condition caused by MT lesions affecting the perception of motion.

Eye Movements and Motion Perception

• Motion on the retina can result from either object or eye movement.

• Distinguishing the source requires comparing retinal motion with eye velocity.

Theories of Eye Movement Signals

• Inflow Theory (Sherrington): Proprioceptive feedback from eye muscles.

• Outflow Theory (Helmholtz): Efference copy of the eye movement command sent to the comparator.

Applications of Theories in Motion Perception

• Afterimages: Demonstrates interplay of the two theories; they can account for perceived motion from fixed retinal images in various conditions.

• Experiments: Tested bilateral responses and retention of perceived motion to further justify the theories' capabilities and limitations.