lecture recording on 11 March 2025 at 14.17.06 PM

Light and Color Perception

• Different Types of Light Sources

  • Light sources can significantly affect the color perceived from surfaces.

  • Example: Illuminance A acts as a pure white light, illuminating the surface evenly across wavelengths, making it appear mostly red with some blue.

  • Illuminance B is described as cold or blue light, with short wavelengths predominating.

• Reflectance Properties

  • Surfaces reflect varying wavelengths: certain surfaces reflect longer wavelengths more, leading to specific color perceptions.

  • For example, a surface under white light appears more red due to the reflectance properties.

• Color Constancy

  • The brain attempts to deduce the illuminant type by averaging the color information across the visual field.

  • It discounts the illumination to maintain consistent perception of the surface color, a phenomenon known as color constancy.

Optical Illusions

• Example of Perceived Color

  • An example involves a gray square perceived as blue in a yellow context due to the visual system subtracting yellow from its gray to infer a blue appearance.

• The gray square's perception changes based on the illuminant context.

• The Dress Illusion

  • Some perceive a dress as blue and black, while others see it as white and gold, attributed to assumptions regarding the illuminant's color.

  • Different viewers interpret the dress based on their perception of the illumination.

  • Adjusting the context can alter individual perceptions.

Motion Perception

• Nature of Motion Perception

  • Motion is both visual and temporal, as it requires changes in position over time.

  • Perception of motion is essential for interacting with the environment, as it aids actions like pouring water.

• Layering of Perception

  • Visual exposure typically involves successive static images, yet the brain constructs the experience of fluid motion.

  • The neuropsychological disorder akenatopsia results from damage to specific brain regions, a loss of the perception of fluid motion, causing a stroboscopic effect.

• Behavior of Movement

  • Patients may experience trailing or shadowing effects due to their inability to smoothly perceive motion.

  • They often face challenges in interacting with their environment, making basic activities dangerous.

• Apparent Motion

  • Apparent motion is the impression of smooth movement from successive positions of an object.

  • Movies and animations rely on this principle, illustrating that perceived movement occurs even when intermediate frames are not visibly connected.

Neural Circuits in Motion Detection

• Neural Circuitry

  • A simple neural circuit model can describe motion detection based on neuron activation, with delay neurons providing temporal sensitivity.

  • The introduction of delay allows motion direction to be identified based on the timing of signal arrival from activated neurons.

  • Different circuits are required for separate movement directions, indicating complexity in motion processing.

• Aperture and Correspondence Problems

  • The aperture problem describes difficulty discerning motion direction through a small visual field.

  • The correspondence problem relates to identifying matching features across different time frames, critical for accurate motion perception.

  • Combined circuits allow the visual system to infer the coherent direction of motion across multiple receptive fields.

• Active Visual Processing

  • Eye movements are essential for active visual processing, enabling us to scan and interpret our environment clearly.

  • Different types of eye movements (e.g., saccades and micro-saccades) prevent visual fading and improve the clarity of visual information.

• Saccadic Suppression

  • Saccadic suppression occurs during voluntary eye movements, where the visual system can suppress movements to maintain stable perception.

  • During involuntary movements, the visual system processes changes, contrasting with the suppression experienced during voluntary shifts.