How We Experience the World
This guide explores the journey from raw sensory input to meaningful understanding. Explore the core processes of Sensation, Attention, and Perception and how they work together.
The Cognitive Pipeline
1. Sensation
Detecting physical energy.
2. Attention
Filtering & focusing.
3. Perception
Interpreting & organizing.
Click on a step above to learn more about it.
Key Cross-Cutting Themes
Active Systems
Sensory systems don't just passively receive information; they actively preprocess it through adaptation, edge detection, and cognitive interaction.
Limited Capacity
Our cognitive resources are finite. Attention and memory limits require us to constantly select and prioritize what we process.
Constructive Perception
What we perceive is a "best guess" constructed by the brain, combining raw sensory data with our prior knowledge, biases, and expectations.
Sensation: The Raw Input
Sensation is the fundamental process of detecting physical stimuli from our environment and converting it into neural signals. Here, we explore the limits and properties of our sensory organs.
Core Principles of Sensation
Psychophysics & Thresholds
Absolute Threshold: The minimum intensity of a stimulus that can be detected 50% of the time. This isn't fixed; it changes with fatigue, expectation, and focus.
Difference Threshold (JND): The smallest detectable difference between two stimuli. Weber's Law states this is a constant proportion of the original stimulus intensity.
Functional Limitations
Range: Each sense operates within a specific range of stimuli (e.g., humans can't hear ultrasonic sounds).
Adaptation: Our senses become less responsive to constant stimuli over time, allowing us to focus on changes in the environment.
Focus on Vision
The Human Eye
The eye is our organ for vision, transducing light into neural signals. Its key components—the cornea, lens, and retina—work together to focus light and capture visual information. The fovea provides high-acuity vision, while the periphery is sensitive to motion and low light.
Color and Adaptation
Trichromacy Theory: We perceive color through the differential activation of three types of cone cells sensitive to short, medium, and long wavelengths of light.
Dark/Light Adaptation: The eye adjusts its sensitivity to varying light levels. Dark adaptation (becoming sensitive to dim light) is a slow process involving the regeneration of rhodopsin in rod cells.
Attention: The Gatekeeper
With a flood of sensory information available, attention is the cognitive process that selects which information to focus on and process further, while filtering out distractions.
Theories of Selective Attention
How do we select what to pay attention to? Psychologists have proposed several models to explain this filtering process. Click on a theory to learn more.
1. Broadbent’s Filter Model (Early Selection)
2. Treisman’s Attenuation Model
3. Johnston & Heinz’s Multimode Theory
Facets of Attention
Divided Attention & Automaticity
We can sometimes attend to multiple tasks at once, but performance usually suffers. Divided attention is easiest when at least one task is highly practiced and has become automatic, requiring few cognitive resources (e.g., walking while talking).
Inattentional Blindness
A striking failure of attention where we fail to notice a fully visible, but unexpected, object because our attention was engaged on another task or object. The famous "invisible gorilla" experiment is a classic demonstration of this phenomenon.
Perception: Creating Meaning
Perception is the active process of organizing, interpreting, and giving meaning to sensory information. It is here that raw data is transformed into our conscious experience of the world.
Gestalt Principles of Perceptual Organization
The brain automatically and innately organizes sensory information into meaningful wholes. The Gestalt psychologists identified several principles that guide this process. Click on a demonstration to learn about the principle.
Proximity
Similarity
Continuity
Closure
Figure-Ground
Select a principle to see its definition.
Depth Perception
We perceive a 3D world despite the 2D image on our retinas by using a variety of depth cues.
Monocular Cues (One Eye)
- Linear Perspective: Parallel lines appear to converge in the distance.
- Occlusion: An object that blocks another is perceived as closer.
- Relative Size: Smaller objects are perceived as farther away.
- Motion Parallax: Closer objects appear to move faster than distant objects.
Binocular Cues (Two Eyes)
- Binocular Disparity: Each eye receives a slightly different image. The brain uses this difference (disparity) to calculate depth.
- Convergence: The extent to which our eyes turn inward when looking at an object provides a cue to its distance.
Perceptual Constancy & Illusions
Our perception of objects remains stable even as the sensory information we receive changes. This is called perceptual constancy.
- Size Constancy: We perceive an object as being the same size regardless of its distance.
- Shape Constancy: We perceive an object as having a constant shape, even when viewed from different angles.
- Brightness Constancy: We perceive an object as having constant brightness even when illumination changes.
Sometimes, the strategies our brain uses to maintain constancy can be tricked, leading to visual illusions — systematic errors in perception that can reveal the underlying assumptions of our perceptual system.
Müller-Lyer Illusion
Which horizontal line is longer? Both are the same length. The fins trick our depth cues.