Sensation & Sensation & PerceptionPerception

Chapter 5Chapter 5

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Sensation and Perception

Perception –

interpretation, organization of internal or external events (involves awareness and activity)

Sensation –

immediate, basic experience generated by a stimulus

Sensation & Perception Processes

Transduction – Transformation of one form of energy into another

Receptors – Specialized neurons that are activated by stimulation and transduce (convert) it into a nerve impulse

Transduction

ThresholdsAbsolute threshold –

Amount of stimulation necessary for a stimulus to be detected

Difference threshold – Smallest amount by which a stimulus can be changed and the difference be detected (also called just noticeable difference – JND)

Weber’s law – Size of a JND proportional to intensity of stimulus

Signal Detection TheorySignal detection theory –

Perceptual judgment as combination of sensation and decision-making processes

Stimulus eventStimulus event

Neural activityNeural activity

Comparison with Comparison with personal standardpersonal standard

Action (or no action)Action (or no action)

Signal-Detection Theory• A psychophysical theory that divides the

detection of a sensory signal into a sensory process and a decision process.

Stimulus is

Present

Stimulus is

AbsentResponse: “Present”

Hit False Alarm

Response: “Absent”

Miss Correct Rejection

The senses all operate in much the same way, but each extracts different

information and sends it to its own specialized processing

region in the brain

How Are the Senses Alike? How Are the Senses Alike? And How Are They Different?And How Are They Different?

Sensory Adaptation

Sensory adaptation – Loss of responsiveness in receptor cells after stimulation has remained unchanged for a while

An Eye on the World• Cornea

– Protects eye and bends light toward lens.

• Lens– Focuses on objects by

changing shape.• Iris

– Controls amount of light that gets into eye.

• Pupil

The Structures of the Retina

An Eye on the World

• Retina– Neural tissue lining the back of the

eyeball’s interior

• Two Photoreceptors: Rods: respond to dim light/movement (125 million)

Cones: involved in color vision/detail (7 million)

Why the Visual System is not a Camera

• Much visual processing is done in the brain.– Some cortical cells respond to lines

orientations (e.g. horizontal).– Other cells in the cortex respond to other

shapes (e.g., bulls-eyes, spirals, faces).

• Feature-detectors

Hubel & Wiesel’s Experiment

Constructing the Visual World

• Form perception.

• Gestault Principles– “whole is more than the sum of the parts”

Form Perception

• Gestalt principles describe the brain’s organization of sensory building blocks into meaningful units and patterns.

Figure – Part of a pattern that commands attention

Ground – Part of a pattern that does not command attention; the background

Examples “Gestault”

More “Gestault”• Proximity

– Seeing 3 pair of lines in A.

• Similarity– Seeing columns of

orange and red dots in B.

• Continuity– Seeing lines that

connect 1 to 2 and 3 to 4 in C.

• Closure– Seeing a horse in D.

Law of Prägnanz

Depth and Distance Perception

How can you tell what is closer?

Depth and Distance Perception

• Binocular Cues: – Convergence– Retinal Disparity

• Monocular Cues: – Relative size, overlap, texture gradient,

linear perspective, atmospheric perspective, shadow/shading, motion parralax

Visual Constancies

• The accurate perception of objects as stable or unchanged despite changes in the sensory patterns they produce.– Shape constancy– Size constancy– Color constancy

Shape Constancy

• Even though these images cast shadows of different shapes, we still see the quarter as round

Bottom-Up and Top-Down Processing

Bottom-up processing – Analysis that emphasizes characteristics of the stimulus, rather than internal concepts

Top-down processing – Emphasizes perceiver's expectations, memories, and other cognitive factors

Context Effects

• The same physical stimulus can be interpreted differently

• We use other cues in the situation to resolve ambiguities

• Is this the letter B or the number 13?

Visual Illusions

• Illusions are valuable in understanding perception because they are systematic errors.

• In the Muller-Lyer illusion (above)

Visual Illusions

The Ponzo Illusion

• Linear perspective provides context

• Side lines seem to converge

• Top line seems farther away– But the retinal

images of the red lines are equal!

How Sound Waves Become Auditory Sensations

Cochlea –Where sound waves are transduced

Taste Buds• Photograph of

tongue surface (top), magnified 75 times.

• 10,000 taste buds line the tongue and mouth.

• Children have more taste buds than adults.

Four Tastes

• Four basic tastes– Salty, sour, bitter and sweet.

• Different people have different tastes based on:– Genetics– Culture– Learning– Food attractiveness

Smell: The Sense of Scents

• Airborne chemical molecules enter the nose and circulate through the nasal cavity..

• Receptors on the roof of the nasal cavity detect these molecules.

The Skin Senses

• Touch

• Temperature

• Pain

Perceptual Powers: Origins and Influences

• James J. Gibson : Direct Perception– Invariants and affordances

• Inborn abilities and perceptual lessons

Psychological and Cultural Influences on Perception

• “needs”, beliefs and emotions can influence perceptions of sensory information.

• Expectations based on our previous experiences influence how we perceive the world.

– Perceptual Set• Readiness to detect a particular stimulus in a given context• We all are influenced by our culture/experience.

Perceptual Set

• What you see in the center figures depends on the order in which you look at the figures:– If you scan from the left, see an man’s face– If you scan from the right, see a woman’s figure

Extrasensory Perception

• Extrasensory Perception (ESP):– The ability to perceive something without

ordinary sensory information– This has not been scientifically demonstrated

(issues with reliability/validity)

• Three types of ESP:– Telepathy – Mind-to-mind communication– Clairvoyance – Perception of remote events– Precognition – Ability to see future events