Sensation and Perception: Foundations of Visual Processing

Sensation and Perception

Introduction to Sensation and Perception

Sensation and perception are fundamental processes in psychology that allow us to experience and interpret the world around us. Sensation refers to the detection of physical energy by sensory organs, while perception involves organizing and interpreting these sensory inputs.

• Sensation: The process by which sensory organs receive stimuli from the environment and convert them into neural signals.

• Perception: The process of organizing and interpreting sensory information to give it meaning.

• Example: Hearing someone’s voice involves air particles pushing against the eardrum (sensation), which is then interpreted as speech (perception).

Transduction and Sensory Receptors

Transduction is the process by which specialized receptors transform physical energy from the outside world into neural impulses.

• Raw Sensations: Sensory organs detect stimuli and send information to the brain through transduction.

• Specialized Receptors: Convert light, sound, and other forms of energy into neural signals.

• Example: Visual receptors in the eye convert light into electrical signals that the brain can interpret as images.

Stimulus Thresholds

Absolute and Difference Thresholds

Thresholds refer to the minimum level of stimulus required for detection or for noticing a change.

• Absolute Threshold: The lowest intensity of a stimulus that can be detected at least 50% of the time. Example: The faintest sound you can hear or the dimmest light you can see.

• Difference Threshold (Just Noticeable Difference, JND): The smallest difference between two stimuli that can be reliably detected. Example: Adding salt to food until you notice a change in taste.

• Weber’s Law: The JND is a constant proportion of the original stimulus. $\Delta I / I = k$ where $\Delta I$ is the change in stimulus intensity, $I$ is the original intensity, and $k$ is a constant.

Signal Detection Theory

Signal Detection Theory explains how stimuli are detected under conditions of uncertainty, considering both sensory and decision processes.

• Sensory Process: Detection of the stimulus by sensory organs.

• Decision Process: Judging whether a stimulus was present or not.

• Types of Errors:

  • Correct rejection: No stimulus present, and none detected.

  • False alarm: Reporting a stimulus when none is present.

Gestalt Principles of Perception

Gestalt Psychology

Gestalt psychology emphasizes that the whole is greater than the sum of its parts, and that we organize sensory information into meaningful wholes.

• Figure-Ground Principle: Objects are seen as figures standing out from the background.

• Proximity: Objects close together are perceived as a group.

• Similarity: Objects that look alike are grouped together.

• Continuity: Lines/objects tend to be seen as continuous.

• Closure: Tendency to fill in gaps to see a whole object.

Vision: Anatomy and Processing

Structure of the Eye

The eye is a complex organ that converts light into neural signals through several specialized structures.

• Sclera: The white, outer surface of the eye.

• Cornea: The clear layer that covers the front portion of the eye and helps focus light.

• Pupil: The opening that regulates the amount of light entering the eye by changing size.

• Iris: The colored muscle that controls the size of the pupil.

• Lens: Focuses light onto the retina by changing shape (accommodation).

• Retina: The inner surface at the back of the eye containing photoreceptors that convert light into neural signals.

Photoreceptors: Rods and Cones

Photoreceptors are specialized cells in the retina that detect light and color.

• Rods: Located in the peripheral retina; sensitive to low light; detect black, white, and gray.

• Cones: Concentrated in the fovea (center of retina); detect color and fine detail.

• Rods vs. Cones:

  • Fovea: All cones, sharp color vision.

  • Periphery: Mostly rods, less color sensitivity.

• Dark Adaptation: Rods become more sensitive in low light; cones adapt faster but are less sensitive.

• Key Numbers: ~120 million rods and ~6-8 million cones in the adult retina.

Color Vision Theories

• Trichromatic Theory: Color vision is determined by three types of cones sensitive to short (blue), medium (green), and long (red) wavelengths.

• Opponent-Process Theory: Color is processed in pairs of opposites:

  • Red-green

  • Yellow-blue

  • White-black

Visual Pathways and the Brain

From Retina to Brain

1. Step 1: Retina to Optic Nerve Photoreceptors send signals to ganglion cells, which form the optic nerve and carry visual information to the brain.

2. Step 2: Optic Chiasm The optic nerve fibers cross at the optic chiasm, so each hemisphere of the brain receives information from both eyes.

3. Step 3: Thalamus (Relay Station) The thalamus (specifically the lateral geniculate nucleus, LGN) relays visual information to the visual cortex.

4. Step 4: Visual Cortex The visual cortex processes and interprets visual information, allowing us to perceive shapes, colors, and movement.

5. Step 5: Two Visual Pathways (Streams)

   • Ventral Stream ("What Pathway"): Travels to the temporal lobe; responsible for object recognition.

   • Dorsal Stream ("Where/How Pathway"): Travels to the parietal lobe; responsible for spatial awareness and guiding actions.

Perceptual Constancy and Depth Perception

Perceptual Constancy

Perceptual constancy allows us to perceive objects as having constant shape, size, and color despite changes in lighting, angle, or distance.

• Size Constancy: Judging an object’s size relative to other objects.

• Shape Constancy: Recognizing an object’s shape under different viewing conditions.

• Color Constancy: Recognizing an object’s color under different lighting conditions.

Depth Perception

Depth perception is the ability to judge distances and spatial relationships, essential for activities like walking and driving.

• Binocular Cues: Use both eyes to perceive depth (e.g., retinal disparity).

• Monocular Cues: Use one eye to perceive depth (e.g., relative size, linear perspective).

Attention and Perception

Selective Attention and Divided Attention

Attention determines what sensory information is processed and perceived.

• Divided Attention: Paying attention to more than one task at once; reduces performance accuracy.

• Selective Attention: Focusing on one task or event; improves focus but decreases awareness of other stimuli.

• Inattentional Blindness: Failure to notice visible objects/events due to focused attention elsewhere.

Tables

Comparison of Rods and Cones

Feature	Rods	Cones
Location	Peripheral retina	Fovea (center of retina)
Light Sensitivity	High (low light)	Low (bright light)
Color Detection	No (black, white, gray)	Yes (color vision)
Detail Detection	Poor	Sharp
Number in Retina	~120 million	~6-8 million

Visual Pathways

Pathway	Destination	Function
Ventral Stream	Temporal lobe	Object recognition ("What")
Dorsal Stream	Parietal lobe	Spatial awareness/action ("Where/How")

Additional info:

• Some explanations and examples have been expanded for clarity and completeness.

• Key terms and processes have been defined and contextualized for exam preparation.