Perception

Welcome to the World of Perception!

Have you ever looked at a cloud and thought it looked exactly like a dog? Or wondered why the moon looks huge when it’s near the horizon but tiny when it's high in the sky? That is perception at work! In this chapter, we are going to explore how our brains take in raw information from the world and turn it into something meaningful. Perception is part of the Cognition and behaviour section of your course, which looks at how our "mental hardware" works.

1. Sensation vs. Perception

Before we dive in, we need to know the difference between "sensing" and "perceiving." They are not the same thing!

Sensation: This is the physical process. It’s when your sensory receptors (like your eyes or ears) pick up signals from the environment. Think of it as the raw data being sent to your computer.
Example: Your eyes detect light bouncing off a red, round object.

Perception: This is the psychological process. It’s how your brain organizes and interprets that raw data to make sense of it.
Example: Your brain realizes, "Hey, that red round thing is an apple!"

Quick Review: Sensation is input; Perception is interpretation.

2. Visual Cues and Constancies

How do we know how far away something is? Our brain uses depth cues. Because our eyes are on a flat surface, our brain has to work like a 3D artist to figure out distance.

Monocular Depth Cues (Using one eye)

You can see these even if you close one eye. Artists use these to make paintings look 3D!

• Height in plane: Objects that are higher up in our field of vision look further away.
• Relative size: If two objects are the same size, the one that looks smaller is perceived as being further away.
• Occlusion: If one object covers part of another, the one that is "hidden" is further away.
• Linear perspective: Parallel lines (like train tracks) appear to point toward a single spot in the distance.

Binocular Depth Cues (Using two eyes)

These require both eyes to work together to judge distance accurately.

• Retinal disparity: Our eyes are about 6cm apart, so they see slightly different images. The brain compares these two images. The bigger the difference between the two images, the closer the object is.
• Convergence: When something gets very close to your face, your eye muscles have to work harder to turn your eyes inward. Your brain "feels" this muscle tension and knows the object is close.

Memory Aid: Think of "Bi" in Binocular like a bicycle (two wheels/two eyes). Think of "Mono" in Monocular like a monocle (one lens/one eye).

3. Gibson’s Direct Theory of Perception (Nature)

James Gibson argued that perception is innate (we are born with it). He believed the environment provides us with so much information that we don't need to "guess" or "think" about what we see.

Key Point: This is a nature theory. Perception is "direct" because the light reaching our eyes tells us everything we need to know about depth and distance.

Motion Parallax: This is a key part of Gibson's theory. When we are moving (like in a car), objects close to us seem to zoom past very fast, while objects far away (like mountains) seem to move very slowly. This tells us instantly about distance without any "thinking" required.

Takeaway: Gibson says "What you see is what you get." The world is rich enough that we don't need to make guesses.

4. Visual Illusions

Sometimes our brains get it wrong! Visual illusions happen when our brain's "rules" for perception are tricked. The syllabus requires you to know these categories:

Types of Illusions

• Ambiguity: When an image can be seen in two different ways.
Examples: Rubin’s vase (is it a vase or two faces?) and the Necker cube (which way is the box facing?).
• Misinterpreted depth cues: When we use depth rules (like linear perspective) on a 2D drawing, making us see size wrongly.
Examples: The Ponzo illusion (two lines on tracks) and the Müller-Lyer illusion (lines with "fins" or "arrows").
• Fiction: When the brain "sees" a shape that isn't actually there.
Example: The Kanizsa triangle (your brain creates a white triangle out of gaps).
• Size constancy: We know objects stay the same size even if they look smaller in the distance.
Example: The Ames Room (a trapezoid-shaped room that makes one person look like a giant and another like a dwarf).

5. Gregory’s Constructivist Theory (Nurture)

Richard Gregory had a very different idea from Gibson. He believed perception is a nurture process—something we learn from experience.

Key Point: Gregory said perception is an inference. This means our brain makes a "best guess" about what we see based on our past knowledge. We use visual cues to construct a model of reality.

Analogy: Imagine seeing a blurry shape in the dark. Your brain thinks, "Last time I saw that shape in this hallway, it was my dog," so you "perceive" your dog even though you can't see it clearly. You are filling in the gaps with your past experience.

Takeaway: Gregory says we are "active" perceivers. We don't just see; we interpret and guess.

6. Factors Affecting Perception

Our perceptual set is a "readiness" or "bias" to see things in a certain way. Several things can change how we perceive the world:

• Culture: People from different cultures may see things differently based on their environment.
• Motivation: If you want something, you are more likely to notice it. (e.g., if you are hungry, you might see "food" everywhere).
• Emotion: If you are scared, a dressing gown on a door might look like a ghost.
• Expectation: You are more likely to see what you expect to see.

7. Core Studies in Perception

Don't worry if these names seem hard to remember—focus on what they found!

Gilchrist and Nesberg: Motivation

What they did: They showed pictures of food to two groups: one group had gone 20 hours without food (hungry), and one group had just eaten.
What they found: The hungry group perceived the food pictures as being brighter than they actually were.
Conclusion: Motivation (hunger) changed how they perceived the world.

Bruner and Minturn: Perceptual Set

What they did: They showed participants an ambiguous figure that could look like the letter 'B' or the number '13'. One group saw letters first, the other group saw numbers first.
What they found: People who saw letters first perceived the figure as a 'B'. People who saw numbers first perceived it as a '13'.
Conclusion: Expectation (what you saw just before) creates a perceptual set that changes how you see things.

Quick Summary Checklist

• Can you explain the difference between sensation and perception?
• Do you know 4 monocular and 2 binocular depth cues?
• Can you compare Gibson (Nature/Direct) vs. Gregory (Nurture/Inference)?
• Can you name the 4 types of illusions and an example of each?
• Do you know why motivation and expectation affect what we see?