Week 6: Sensation and Perception

Learning Objectives: Distinguish between sensation and perception | Define Transduction | Describe the physical properties of waves | Describe how physical properties of light and sound waves correspond to perceptual experience | Describe the concepts from psychophysics: absolute threshold, difference threshold and Weber’s Law | Describe detecting a stimulus using the framework of Signal Detection Theory | Discuss the role of attention in detecting a stimulus | Distinguish between top-down and bottom-up processing. Apply these concepts to new examples | Describe the basic anatomy of the visual system | Describe how we perceive color | Describe how we perceive depth | Explain perceptual constancy and provide examples |

Sensation

detecting physical energy with our sense organs (eyes, ears, skin, nose, tongue)

Perception

the brain’s interpretation of the raw sensory information

• our perceptions do not always match our sensations / physical reality!

Illusion

the way we perceive a stimulus does not match its physical reality → leads to errors in perception

A

Transduction

conversion of an external stimulus into a neural signal through sensory receptors

How does transduction happen?

through sensory receptors

Sensory receptors

specialized cells designed to convert a certain kind of external information into a neural signal

C

Sensory adaptation

sensory neurons adjust their sensitivity based on recent stimulus history

• Eg: perceiving smells; adjusting to dark vs light condition; color after-images; motion effect

After effects

opposing sensory or perceptual distortions that occur after adaptation

Waves

both sound and lights are waves

• Properties important to us: amplitude; wavelength

• 

• Higher frequency = higher amplitude

• Lower frequency = lower amplitude

  • Soundwaves: 

    

  • Lightwaves: 

    • 

    • 

      • Higher frequency = brighter colors = cooler tone colors

      • Lower frequency = dimmer colors = warmer tone colors

C

A

Properties of sound

pitch, loudness, timbre

sound

vibration; mechanical energy that travels through some medium (air, water)

• Sound is derived from tiny vibrations

• Compressed and expanded air molecules create waves

Pitch

frequency of a sound wave (measured in Hz)

• Short wavelength - high frequency = high pitch

• Range of human hearing: 20-20,000 Hz

Loudness

height (amplitude of a sound wave (measured in dB

Timbre

quality or complexity of a sound

Structure of the ear

• Outer: funnels sound to the eardrum

  • Pinna → Ear canal → Eardrum

• Middle: transmits sounds from eardrum to inner ear

  • Ossicles: hammer, anvil, stirrup

• Inner: transduces sound

  • Cochlea: transduction accomplished by movement of hair cells (cilia)

C

Pitch perception and theories

place theory, frequency theory, and volley principle

Place theory

a theory that specific locations on the basilar membrane match tones with specific pitches

• High pitches → 5,000-20,000 Hz

• Base vibrates to high-frequency sounds

• End vibrates to low-frequency sounds

Frequency theory

a theory that neuron firing rate matches pitch

• Sound frequency corresponds to action potential frequency

Volley principle

a theory that clusters of nerve cells can fire together (modification of frequency theory)

• Low pitches → 100-5,000Hz

C

Binaural cues

cues that depend on having 2 ears

Interaural level difference

sound coming from right side is more intense in right ear (because it doesn’t have to pass through your head)

Interaural timing difference

sound from the right side reaches the right ear first (just a little earlier than the left ear)

Conductive deafness

malfunctioning of the eardrum or ossicles

Sensorineural hearing loss

neural signals are not transmitted from cochlea

Noise-induced hearing loss

damage to hair cells due to loud noises

Structure of the eye

sclera

the white part of the eye

pupil

the circular hole where light enters in the eye

iris

the colored portion of the eye that controls pupil size (letting in more or less light)

Cornea

curved, transparent layer covering the iris and pupil that helps focus light

lens

oval shaped disc that bends light

• Accommodation = confining of the lens’ shape to focus on near/far objects

Myopia

nearsightedness → if the eye is too long

Hyperopia

farsightedness → if the eye is too short

Retina

membrane at the back of the eye responsible for converting light into a neural signal

• fovea

• contains a high density of cones

• acuity

• contains two kinds of photoreceptors: rods and cones

• saccades

• optic nerve

• blind spot

Fovea

central portion of the retina, responsible for visual acuity

• Very small → takes up 1% of retinal size

• Highly represented in the brain ! → gives us the acuity

acuity

sharpness of vision

rods

• Responds under low levels of light

• Not color sensitive

• More common outside of fovea

cones

• Sensitive to fine detail

• Primarily located in fovea

• Color sensitive

• Less plentiful than rods

Saccades

small jerky movements of the eye allowing for rapid changes of focus

• Goal: put the fovea on a new location

Optic nerve

bundle of axons that travels from the retina to the brain

Blind spot

area of the retina where the optic nerve exits the eye

what does it mean by the neural pathway of vision is contralateral?

contralateral: processed on the opposite side

• ie right visual field is processed in the left primary visual cortex

C

B

A

Color perception

• When light hits and object, some is absorbed, some is reflected

• Hue

hue

the color of light

• corresponds to wavelength

Trichromatic theory

theory that olor vision is based on three primary colors – blue, green, and red

• We have three types of cones – one likes red, one likes green, one like blue wavelengths

• Issue with this theory: doesn’t explain why we see opposing colors in afterimage

Opponent process theory

theory that we perceive colors in terms of three pairs of opponent colors

• Red-green

• Blue-yellow

• Black-white

• Visual system uses principles of both Trichromatic Theory and Opponent Process Theory at different stages of color processing

• Can explain color-blindness

Color-blindness

the inability to see some or all colors

• Due to loss of one or more types of cones

fill in the blanks with “2D” or “3D”: the image reflected in the retina is in _____, but our perception of the word is in _____.

1. 2D

2. 3D

Depth perception

the ability to judge distance and spatial relations

• depends on binocular and monocular depth cues

Binocular depth cues

cues that involve two eyes

• Binocular disparity

• Convergence

Binocular disparity

each eye sees slightly different images

Convergence (in relation to depth perception)

eyes converge to see near objects

Monocular depth cues

cues that involve one eye

• Relative size

• Texture gradient

• Interposition

• Linear perspective

• Height in place

• Light and shadow

• Motion parallax

Relative size

a monocular depth cue where distant objects look smaller than closer objects

Texture gradient

a monocular depth cue where texture is more clear on closer objects

Interposition

a monocular depth cue where closer objects appear in front of distant objects

Linear perspective

a monocular depth cue where parallel lines converge with distance

Height in place

a monocular depth cue where distant objects appear higher than closer objects

Light and shadow

a monocular depth cue where shadows can tell us about form

Motion parallax

a monocular depth cue where closer objects pass more quickly than distant objects

Perceptual constancy

perceive stimuli as consistent across varied conditions

• shape, size, and color constancy

Shape constancy

a type of perceptual constancy where perceived shape is constant, even though shape of the image (on retina) varies

Psychophysics

the study of how our sensations (psychological events) correspond to physical events in the world

• Absolute threshold

• Just noticeable difference (JND)

Absolute threshold

the lowest level of a stimulus needed for the nervous system to detect a change 50% of the time

Just noticeable difference (JND)

the smallest change in the intensity of a stimulus that we can detect

• Weber’s Law

Weber’s Law

the stronger the stimulus, the bigger the change needed to detect it

• related to JND (just noticeable difference)

B

Signal detection theory

how stimuli are detected under different conditions

• signal

• noise

• signal-to-noise ratio

• 

signal

what you are trying to detect

noise

similar stimuli that might compete with the signal and interfere with your ability to detect the signal

Signal-to-noise ratio

difficulty of detecting the signal depends on the strength of the signal in relation to the strength of the noise

C

Attention

• Allows us to focus on some sensory information, and de-emphasize other information, making it easier to detect what we’re interested in

• We know attention is somewhat like a filter due to effects that show streams of information popping through the filter

Dichotic listening

play different information through each ear of headphones. Information reported only from the attended ear.

Cocktail party effect

when important information pops out in a conversation that you are not attending (eg: your name)

Inattentional blindness

when unattended stimuli are ignored as if they weren’t there

Change blindness

a version of inattentional blindness that occurs when you fail to detect obvious changes in your environment

Bottom-up processing

constructing a representation from parts and basic features

• Start with the raw data and construct meaning from that

Top-down processing

processing influenced by previous experience and knowledge

• Start with meaning and use it to understand a stimulus

which type of processing is sensation: top-down or bottom-up?

bottom-up

which type of processing is perception: top-down or bottom-up?

includes top-down, which can change what we report about sensations

Perceptual sets

set formed when our expectations influence our perceptions

B

Gestalt principles of perceptual grouping

rules that govern how we perceive objects as wholes within their overall context

1. Proximity

2. Similarity

3. Continuity

4. Closure

5. Symmetry

6. Figure-ground separation

7. Common region

8. Connectedness 

9. Common fate

10. Synchrony

Figure-ground separation

your visual system automatically tries to identify the figure

• Figures are integrated units of perceptions

• Lots of cues converge to give us the figure

• Gestalt psychologists identified grouping principles we use to identify figures

Common fate

objects moving at the same direction at the same speed are perceived as a group

• Geese flying in a “V”

• People doing the wave at a stadium

synchrony

stimuli that are perceived to occur at the same time are perceived as part of the same event

• Keys are dropped and a sound occurs the instant they hit the floor

D

B

When you watch sporting events on TV, people sitting in the bleachers at the very top of the stadium often appear to be the same size as people sitting in the seats much closer to the cameras, even though they project much smaller images onto your retina. This is an example of size

a. constancy.

b. adjustment.

c. adaptation.

a. constancy.

As you’re reading this sentence, your eyes are constantly moving so that the image from each letter and word is always focused on which part of the eye?

a. pupil

b. fovea

c. iris

b. fovea