Visual Motion

Types of Motion

• Real motion: Objects physically moving through our visual field.
• Apparent motion: Static images presented in quick succession, creating the illusion of movement.
  • Examples: Animations, flip books, and movies.
• Induced motion: The movement of one part of an image causing the perception of movement in another part.
  • Example: Clouds moving across the moon, making the moon seem to move.
• Motion after-effect: After viewing a moving stimulus for a long time, a stationary scene appears to move in the opposite direction.
  • Also known as the waterfall illusion.
  • Occurs due to adaptation to a specific motion.

Biological Motion

• We are very sensitive to the familiar motion of people or animals, even when only represented by points of light.
  • This is evident in "point-light walkers" where simple light dots create the perception of human movement.

Reichardt Motion Detectors

• Motion: Defined as the change in position over time.
• Motion detection: Occurs when a moving object activates one receptive field (RF) followed by an adjacent RF.
  • Larger objects activate two RFs simultaneously.
• Reichardt motion detector: A model for motion detection that captures the output from the second RF and a delayed output from the first RF.
  • Requires mirrored circuits to detect motion in both directions.
  • Multiple detectors are joined together to detect motion over larger areas.
• Adaptation: This model can explain the motion after-effect.

Detection Tuning

• Motion detectors are not perfectly selective for a specific direction.
  • They have a preferred direction and respond less strongly to other directions depending on their distance to the preferred direction.

Aperture Problem

• Aperture problem: The ambiguity in determining the direction of motion of an object when viewed through a restricted window (aperture).
• Neural motion detectors: Function as apertures.

Global Motion Detectors

• Combine multiple local detectors with different preferred directions and spatial locations.
• Each local detector can respond to various directions of motion with a preferred one.
• Some neurons in the MT (middle temporal lobe) receive input from multiple motion-sensitive neurons in different RFs in V1 (primary visual cortex) and are sensitive to the global motion of a stimulus.

Types of Motion

• Real motion: An object physically moving through our visual field.

• Apparent motion: Static images presented in rapid succession, creating the illusion of movement. Examples include flip books, animations, and movies.

• Induced motion: The perception of motion in a stationary object due to the movement of another nearby object. Example: the moon appearing to move through the clouds.

• Motion after-effect: Stationary objects appear to move in the opposite direction after prolonged exposure to a moving stimulus. Also known as the waterfall illusion. Occurs due to adaptation of our visual system to the motion.

Biological Motion

• Humans are highly sensitive to the familiar motion of people and animals.
• This sensitivity extends to recognizing motion represented by just a few points of light, as demonstrated by "point-light walkers."

Reichardt Motion Detectors

• Motion: Defined as a change in position over time.
• Detectors have receptive fields (RFs) that respond to stimuli.
• Motion Detection:
  • A moving object activates one RF followed by an adjacent one.
  • Larger objects activate two RFs simultaneously.
  • To detect motion, detectors require input from both RFs, with a delayed signal from the first RF.
  • Requires mirrored circuits for detecting motion in different directions.
  • Requires several simple detectors connected to detect motion over a larger area.
• Motion After-effect: Adaptation of the motion detectors can explain the motion after-effect.

Detection Tuning

• Motion detectors are not perfectly selective for a specific direction of motion.
• They respond most strongly to their preferred direction, and less strongly to other directions depending on their closeness to the preferred direction.

Aperture Problem

• Aperture problem: Occurs when a moving object is viewed through a limited window, making it difficult to determine the true direction of motion.
• The RF of neural motion detectors act as apertures.

Global Motion Detectors

• Integration of multiple local detectors with various preferred directions and locations.
• Each local detector can respond to many possible directions of motion with a preferred direction.
• Neurons in the middle temporal (MT) area receive input from multiple V1 neurons responsible for motion sensitivity and respond to global motion.

Sources of Motion

• Motion of our environment.
• Motion of our eyes.

Self-Motion

• Our movement in the environment causes optic flow, which is the perceived motion of objects in the visual field.
• The focus of expansion (FOE) of optic flow tells us the direction we are moving in.

Eye Movements

• Fixation involves small, involuntary movements of the eye: micro-saccades, drift, and tremor.
• Smooth pursuit is voluntary, tracking eye movement.
• Nystagmus is an involuntary eye movement, often associated with inner ear problems.
• Saccades are voluntary, rapid eye movements used to shift gaze.
• Vergence involves turning the eyes inward or outward to focus on objects at different distances.

Anatomy

• Six muscles control eye movement, organized as three pairs:
  • Inferior/Superior Rectus: control up and down movements.
  • Medial/Lateral Rectus: control left and right movements.
  • Inferior/Superior Oblique: control rotation of the eyeball.

Visual Motion and Eye Movements

• A copy of the motor command for eye movement (corollary discharge) is used to predict the sensory consequences of that movement.
• This prediction helps our brain interpret the retinal image.
• If our eyes are moving and the retinal image doesn't move, we know the object must be stationary.

Saccadic Suppression

• During a saccade, our visual system suppresses information from the retinal image to avoid blurring.
• The corollary discharge signal remaps the receptive field of frontal eye field (FEF) neurons to their future target before the movement starts.

Visual Motion Types

• First-order motion is defined by changes in luminance.
• Second-order motion is defined by changes in contrast.

Visual Motion Sensitivity

• Sensitivity to visual motion develops over time in infants.
• Infants have reflexive eye movements at birth.
• Adult-like sensitivity to motion does not mature until 3-4 years.

Akinetopsia

• Akinetopsia is a rare neurological disorder characterized by the inability to perceive motion.
• Caused by impairment in the medial temporal (MT) area of the brain.

Motor Command & Eye Movement

• When we make an eye movement, a copy of the motor command (called a corollary discharge) is used to predict the sensory consequences of the movement.
• This prediction is used to interpret the retinal image.
• If our eyes are moving, a stationary retinal image indicates that the object is moving.

Eye Muscles

• Six muscles control eye movement in three pairs:
  • Inferior/Superior rectus (Up & Down)
  • Medial/lateral rectus (Left & Right)
  • Inferior/Superior oblique (rotation of eyeball)

Types of Eye Movements

• Fixation: Eyes are almost still; consists of micro-saccades, drift, and tremor.
• Smooth pursuit: Voluntary tracking of a moving object.
• Nystagmus: Reflexive eye movements.
• Saccades: Voluntary, rapid eye movements.
• Vergence: Turning eyes inwards or outwards.

Global Motion Detectors

• Global motion detectors are formed by a combination of multiple local motion detectors.
• The local detectors have different preferred directions and spatial locations.
• Each local detector responds to many possible directions of motion, but has a preferred direction.
• Neurons in the MT area receive input from multiple motion-sensitive neurons in different receptive fields (RFs) in V1 (visual cortex).
• The MT neurons are sensitive to the global motion of a stimulus.

Visual Motion Development

• Sensitivity to visual motion emerges gradually during infancy.
• Basic reflexive eye movements are present at birth.
• Adults reach full motion sensitivity around the ages of 3-4 years.

Akinetopsia

• A rare neurological disorder marked by the inability to perceive motion.
• Occurs due to damage to the medial temporal (MT) area of the brain.

Motor Command & Perception

• When we move our eyes (or any body part), a copy of the motor command (called corollary discharge) is used to anticipate the sensory outcome of the movement.
• This prediction helps interpret retinal images and determine if an object is static or moving.
• Stationary retinal images during eye movement indicate object movement.

Eye Muscles

• Six muscles control eye movement, organized in three pairs:
  • Inferior/Superior rectus (up & down)
  • Medial/Lateral rectus (left & right)
  • Inferior/Superior oblique (rotational movement)

Eye Movements

• Different eye movements serve distinct functions:
  • Fixation: Maintaining a relatively still gaze, with micro-saccades, drift, and tremor.
  • Smooth Pursuit: Voluntary, continuous tracking of a moving object.
  • Nystagmus: Involuntary, rapid eye movements, often reflexive.
  • Saccades: Voluntary, rapid eye shifts between points of interest.
  • Vergence: Converging or diverging the eyes to adjust focus at different distances.

Global Motion Detection

• Global motion perception emerges from integrating information from local motion detectors.
• Multiple local detectors with varying preferred directions and spatial positions contribute.
• Each local detector can respond to several motion directions but has a favoured one.
• Neurons in the MT area receive input from various motion-sensitive neurons in V1, enabling them to identify the global motion of a stimulus.