Sensation, Perception, and Visual Illusions Notes

Sensation and Perception

The Difference Between Sensation and Perception

• Sensation:
  • The process where the senses process external cues like light and sound.
  • These cues are detected by specialist receptor cells in the body.
  • The basis of how all animals experience the world.
  • Example: Rods and cones in the human retina, sensitive to light (cones sense color).
  • Receptor cells send messages to neurons (nerve cells), which connect to the brain.
  • For vision, these neurons form the optic nerve.
• Perception:
  • Occurs when the brain uses information from the outside world to build a mental image.
  • Interprets the information that reaches the senses.
  • Not the same as the sensation received.
  • Visual perception adjusts images received by the eyes to make sense of them.
  • The visual system accounts for images hitting the retina reversed and upside down.
  • Rapid and largely effortless, allowing quick reactions to external events.
• Key Point:
  • Sensation is the direct impact of the outside world on senses.
  • Perception is the way information is interpreted to build a coherent picture by the brain's visual cortex.
• Role of the Brain in Perception:
  • Builds a picture of the world using information from the senses and memory.
  • Many receptor cells work together.
  • A single rod cell detects light amount; a network is required for vision.
  • The sensory cortex detects the direction of movement due to changing patterns of light on retinal cells.
  • The shape of objects is detected similarly.
  • Perceptual systems act like a computer, processing simple information to build a complex picture, enabling actions like picking up food or avoiding harm.

Perceptual Constancy

• The ability of the brain's perceptual system to make allowances for changes in the environment.
• Adjustments in visual perception are made for position and lighting conditions in four main ways:
  • Light constancy and color constancy.
    • An object is perceived as having its normal level of lightness/darkness even when lighting conditions change.
    • People still perceive objects as having their usual color, even when seen in darker or unusual lighting conditions.
    • Example: Grass still looks green after sunset.
  • Size constancy and shape constancy.
    • An object is perceived as having a constant size even when it changes distance and projects a different size of image onto the retina.
    • Example: A train pulling away from a station still looks the same size.
    • An object that moves or rotates is perceived as being the same object, even though the image on the retina may have changed radically.
• These constancies require complex computation by the brain.

Visual Cues and Constancies

• Depth and Distance:
  • The mind interprets information from the senses to understand how close or far away objects are.
  • Known as depth perception, essential for survival.
  • Depth cues convert the two-dimensional image that hits the retina into a three-dimensional mental image.
• Monocular Depth Cues:
  • Cues to depth and distance perceived using only one eye.
    • Relative Size: Distant objects appear smaller than similar objects.
      • Example: The apparent size of a car on the road indicates its distance.
    • Occlusion: If one object partially obscures another, it is closer.
    • Linear Perspective: As things move further away, they appear closer together.
      • Used by artists to create realistic paintings with lines drawn towards a vanishing point.
    • Texture Gradient: Distant objects have less detailed surface texture.
      • Objects further away appear to have a smoother and simpler blurry texture.
    • Height in Plane: Closer objects appear lower down compared to a horizon line, while more distant objects appear higher up.
• Binocular Depth Cues:
  • The mind compares visual sensations from the two eyes to build on basic depth cues.

Binocular Depth Cues

• Retinal Disparity:
  • The eyes are at different positions on the head.
  • Differences between the images from the two eyes give a cue to distance.
  • Closer objects have a larger discrepancy between the two eyes.
  • Further objects have a smaller difference between the two eyes.
• Convergence (Eye Convergence):
  • Comes from the muscles that move the eyes.
  • Closer objects require both eyes to rotate inwards slightly to bring it into focus.
  • More distant objects require less rotation.

Illusions

• Taking in information from the senses is not always simple or accurate.
• An illusion is a stimulus that causes a person to see something different from what is actually there, or where there are two or more possible interpretations of the same image.
• Examples of Illusions:
  • Müller-Lyer Illusion: Arrowheads pointing inwards or outwards on a line; lines appear different lengths even though they are the same.
  • Rubin's Vase: Can be interpreted as two faces looking towards each other or a vase.
  • Ames Room: Distorted room that appears ordinary from the front; people at opposite corners appear different sizes.
  • Kanizsa Triangle: Three circles with wedge-shaped sections removed; people tend to see the sides of a triangle appearing faintly.
  • Necker Cube: A two-dimensional shape interpreted as a cube with two possible orientations.

Explanations for Visual Illusions

• Illusions show that some aspects of perception are fairly automatic.
• Illusions do not have a single explanation; they rely on different factors affecting perception.
• Causes of Illusions:
  • Ambiguity: Two or more ways a two-dimensional shape can be perceived.
    • Examples: Necker cube and Rubin's vase.
  • Fiction: Perceiving something that is not actually there.
    • Example: Kanizsa triangle.
    • Gestalt approach: Perceiving objects as wholes and connecting objects that belong together.
  • Misinterpreted Depth Cues: Cues that guide us to depth and distance can mislead perception.
    • Example: Ponzo illusion, where linear perspective tricks the mind into thinking images closer to the vanishing point are larger.
  • Size Constancy: When the context makes an object look closer or further away than it is, size constancy causes it to appear larger or smaller.
    • Example: Ames Room illusion.

Theories of Perception

• Perception involves building a coherent mental representation of the world that is accurate and makes sense.
• Researchers have tried to explain how perception works; these ideas are known as theories of perception.
• Two main theories focus on visual perception.
• Gibson's Direct Theory:
  • Perception is a matter of piecing sensory information together.
  • Expectations and knowledge do not play a major role.
  • The environment provides affordances, helping its meaning to be understood (e.g., depth cues).
  • Bottom-up processing: Initiated by sensations from the world rather than cognitive processes.
  • Illusions are exceptions to normal perception.
  • Perception is largely innate.
  • Gibson believed that sensation is perception and if we can see something we don't need to perceive it.
  • Gibson's view- environmental affordances such as cues to depth are good evidence for direct processing. He belived that there is a single correct interpretation of sensations coming from the environment- it doesn't depend on who is looking at it.
  • Gibson thought that all animals are able to perceive the world in similar ways.
  • Motion parallax: The way the visual world changes when a person or animal moves; closer objects appear to move more.
• Gibson's Theory Evidence
  • Newborn animals and human babies are able to perceive depth, supporting the bottom-up theory.
• Gregory's Constructivist Theory:
  • Perceptions are based partly on information from the senses but depend on expectations and experience.
  • Perception depends on making inferences based on past experiences.
  • Top-down processing: Begins with thoughts and memories rather than sensation.
  • A person's schemas can influence and distort what they remember and what they precieve.
• Gregory's Theory Evidence
  • Illusions suggest that the mind is trying to make sense of partial or ambiguous information, using schema knowledge and expectations.
  • The hollow face illusion also supports top-down processing.

Factors Affecting Perception

• Processes involved in perception do not always produce an accurate impression of the world.
• Individual differences affect how people perceive the world, influenced by life experience.
• Hallucinations: Perceiving things in the absence of real sensations.
• Dream: Everyday example of a hallucination.
• Perceptual Set:
  • A group of assumptions and emotions that affect perception, creating bias.
  • Affected by motivation, emotion, and expectations.
    • Motivation: People are more likely to perceive what they want to perceive.
      • Example: A football supporter is more likely to perceive their favourite team's play as skillful.
      • Emotion: Fears and worries can affect perception.
        • Example: A fearful child thinking that a shadow looks like a monster.
      • Expectations: People are more likely to see what they expect to see.
        • Example: Ambiguous figure illusions.
  • Factors can change, meaning the same scene or object could be perceived differently by the same person on different occasions.
• **Perceptual Set Research Evidence **
  • Gilchrist and Nesberg (1952): People judged the brightness of food colors differently based on hunger levels.
  • Bruner and Minturn (1955): Participants shown an ambiguous figure perceived it as a letter or number based on the surrounding context.
    • Bruner and Minturn's findings have real-life application as they help to explain why we sometimes make mistakes even when the stimulus is right in front of us.
• Culture:
  • A factor in a person's perceptual set.
  • Affects expectations through cultural knowledge and beliefs.
  • Affects how people choose to represent the world in art.
  • Hudson (1960) study in Zambia found that villagers couldn't easily perceive occlusion, suggesting depth perception may depend on culture.