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Chapter 8: Perceiving Motion

1. Functions of Movement Perception

  • Kinetic Depth Effect:

    • Movement of 2D information can create the perception of a 3D object.

    • Example of structure-from-motion.

2. Heider & Simmel (1944)

  • When an object moves while the observer remains stationary, the image moves on the observer’s retina.

  • If the observer follows the object with their eyes:

    • Movement is tracked, keeping the image stationary on the retina.

3. Three Situations Leading to Movement Perception

  1. Stationary Observer:

    • Object moves, image shifts across the retina.

  2. Tracking with Eyes:

    • Object moves while eyes follow, creating stationary image on the retina.

  3. Observer Movement:

    • Observer moves through a stationary environment, while the image of the environment moves across the retina.

4. Mechanisms Explaining Movement Perception

  • Ecological Approach (Gibson):

    • Focus on required environmental information.

    • Optic Array: Structure formed by surfaces and textures changes as the observer moves.

5. Behavioral Approach to Movement Perception

  • Local Disturbance:

    • Movement relative to the background results in covering and uncovering.

  • Global Optic Flow:

    • Overall movement pattern in the environment.

6. Physiological Approach to Movement Perception

  • Corollary Discharge Theory:

    • Movement perception relies on three signals:

      • Image Movement Signal (IMS): Movement of an image stimulates retina receptors.

      • Motor Signal (MS): Signal sent to eye muscles indicating eye movement.

      • Corollary Discharge Signal (CDS): Copy of the motor signal indicating muscle operation.

    • Movement perceived when there is a mismatch between CDS and IMS.

7. Computation of Visual Motion

  • Building a Motion Detector:

    • Involves changes in position over time, using adjacent receptors.

  • Reichardt Detector:

    • Example motion detector circuit processing light and dark shades.

8. Apparent Motion

  • Phi Phenomenon:

    • Illusion of smooth motion by sequentially displaying objects in different positions.

    • Requires changes at approximately 16 Hz (~62 ms per frame).

9. Flicker

  • Movie Frames:

    • Traditionally 24 frames per second, now often 48 or 72 Hz to prevent flicker.

  • Critical Flicker Fusion Threshold:

    • Typically between 40-50 Hz.

10. Motion Perception and Image Processing

  • Television frame rates:

    • Europe: 25 Hz; US: 30 Hz; HD: 60 Hz for 720p; 30 Hz interlaced for 1080i.

  • Discussion of frame interpolation and human perception of motion.

11. Correspondence Problem in Motion Visualisation

  • Challenges in identifying corresponding features in successive frames, known as the correspondence problem.

  • Aperture Problem: Ambiguity in perceived direction of motion when viewed through a receptive field.

12. Detection of Global Motion

  • Lesions in the magnocellular layers of the LGN impair large moving object perception.

  • Medial Temporal Lobe (MT): Key role in motion perception with most neurons selectivity for direction.

13. Experimental Evidence on Motion Detection

  • Study on monkeys with lesioned MT areas showed increased difficulty in identifying motion direction, demonstrating MT's role in motion perception.

14. Biological Motion

  • Biological Motion: Movement of living organisms, represented effectively through point-light walker stimuli.

  • Processed in the superior temporal sulcus (STS) and inferior temporal sulcus (ITS), part of motion and object perception networks.