Lecture 7.2 Perceiving Motion

Motion Perception

• Motion perception involves detecting and identifying movement in our environment.

• Event boundaries help us segment observations into discernible events, which are often indicated by changes in movement.

• Example: The transition from talking to taking a sip indicates a new event through distinct movements.

Importance of Movement in Understanding Events

• Movement helps to categorize and parse events in a dynamic world.

• Movement influences social perception; for example, emotional expressions can be derived from the movement of individuals.

• Navigating the environment requires awareness of both personal movement and the movement of surrounding objects.

Examples of Event Perception

• Magicians exploit the perception of event boundaries to misdirect attention during tricks.

• Movement information can evoke narratives and emotions even with simple shapes (e.g., triangles) moving in ways that imply personality or storytelling.

Types of Motion Perception

1. Real Motion: Observing objects moving in real-time across our visual field (e.g., seeing cars on a highway).

2. Apparent Motion: Perception of motion when still images are presented in quick succession, creating the illusion of movement (e.g., movies, flip-books).

   • This involves recognizing motion patterns despite no physical movement in the stimulus.

3. Induced Motion: Occurs when stationary objects appear to move due to the movement of surrounding objects.

   • Example: Sitting still in a car and perceiving that buildings are moving instead.

4. Motion Aftereffects: After observing movement in one direction, viewers may perceive motion in the opposite direction when viewing a neutral scene.

   • This phenomenon is linked to neural fatigue in response to sustained directional motion.

The Mechanisms of Motion Detection

• Tracking Movements: The brain receives signals about the movement of our eyes (motor signals) and from changes in our visual field (image displacement signals).

• Corollary Discharge Theory: Describes how movements perceived in the environment are processed against our own movements:

  • Motor Signals: Brain signals moving the eyes.

  • Corollary Discharge Signals (CDS): Copy of the motor signal that informs other brain areas of the eye movement.

  • Image Displacement Signals (IDS): Signals generated when visual stimuli move across the retina.

  • If only one signal is present, motion is perceived; if both are present, no motion is perceived.

Approaches to Studying Motion

1. Ecological Approach: Focuses on environment changes and disturbances in the optical array without consideration of eye movements.

   • Local disturbances (blocking/unblocking of objects) indicate motion.

   • Global optic flow: perception of motion when all objects in the environment move together.

2. Corollary Discharge Theory: Involves both neural signals from eye movements and those caused by changes in the visual field, integrated in a cognitive structure (the comparator).

3. Single Neuron Responses: Examines specific neurons that respond to movement, notably in the MT/V5 area of the brain, essential for perceiving motion direction.

   • Direction-selective neurons help track movement across the visual field and provide a nuanced understanding of motion.

The Aperture Problem

• The aperture problem highlights challenges in motion perception based on limited visual input (i.e., when only part of a larger object is visible):

  • Misleading interpretations of motion direction can occur.

  • End-stop cells detect edges, helping resolve ambiguities in motion perception and enhancing our understanding of complex moving objects.

  • Successful perception of motion requires pooling information from various neurons that respond to different aspects of an object.

Experiments on Motion Perception

• Coherent Motion Paradigms: Used to study how animals (e.g., monkeys) detect cohesive motion in stimuli and how the medial temporal cortex integrates this information.

• Lesion Studies: Damage to the MT area impacts the ability to determine motion direction and coherence.

• Transcranial Magnetic Stimulation (TMS): Temporarily disrupts cortical areas to observe changes in motion perception.

• Neurosimulation Studies: Activate direction-specific neuron clusters to influence perception; demonstrate wiring differences between perceived motion and actual direction of stimuli.