Chapter 7

Sensation

Sensation

The raw experience of a sensory stimulus, such as a light or sound

Perception: The interpretation of sensory information according to expectations and prior learning

The Senses as Evolved Adaptations

Sensing Tastes and Smells sensitivity to chemicals important for feeding

and reproduction chemical receptors became more

sophisticated Smell vs. Taste receptors evolved

The Senses as Evolved Adaptations

Sensing Light responsiveness to the sun’s energy provides “remote guidance” for sensing things

at a distance eyes allow us to process form, color,

movement and visual acuity

The Senses as Evolved Adaptations

Sensing Sounds sensing sound increases range of sensation

beyond that of smellallows localization and identification

sound can be used as a form of communication

The Senses as Evolved Adaptations

Sensing Touch, Warmth and Pain skin senses allow location of nearby objects touch enables skilled movements pain motivates behavior

Psychophysics

The study of how humans and animals respond to sensory stimuli The mathematical relationship of sensory

intensity to the magnitude of a physical stimulus

Just Noticeable Difference (JND)

The minimal amount of sensory change in a stimulus that can be detected e.g. how much more weight do you need to

perceive a difference in weights?

Just Noticeable Difference

Weber’s Law:

jnd = kI Just Noticeable Difference = Constant x Intensity

The size of the just noticeable difference is equal to some some proportion of the standard

Constant varies depending on sensory modality

Just Noticeable Difference

Fechner: JND is a measure of the “psyche” similar to inches on a ruler

The Absolute ThresholdMinimum amount of stimulation that can be detected

on half the trialsCount up number of “yes” responses (Frequency of

“seeing”)

P(yes)

0

50

100

Stimulus Intensity

Threshold =

50% response point

Psychophysical Methods: How to Measure Thresholds

Method of Limits Start with a low intensity stimulus, gradually

increase until observer reports a sensation (ascending)

Start with a high intensity, gradually decrease until observer no longer reports a sensation (descending)Problems:

• observer may not pay attention on low intensity trials• observer may anticipate stimulus on descending series

Psychophysical Methods: How to Measure Thresholds

Method of Constant Stimuli Present stimuli in a random order

observer cannot predict whether stimulus is above or below threshold

Method of Magnitude Estimation

Stevens: Observers use numbers to describe the

perceived intensity of a stimulus Relationship between stimulus intensity

and magnitude estimates follows a power function

Signal Detection Theory

The detection of a stimulus involves decision processes as well as sensory processes Observers responses will change with

motivatione.g. paid $1 for each detection of stimulus results

in a greater number of detections

Signal Detection MatrixJudgment

Stimulus

Present

Absent

“Yes” “No”

Hit Miss

CorrectRejection

FalseAlarm

Signal Detection MatrixJudgment

Stimulus

Present

Absent

“Yes” “No”

Hit Miss

CorrectRejection

FalseAlarm

Pay $1 for each detection

Hit

Signal Detection MatrixJudgment

Stimulus

Present

Absent

“Yes” “No”

Hit Miss

CorrectRejection

FalseAlarm

Deduct $2 for each False Alarm

Miss

Two-Point Limen

A measure of tactile sensitivity

Sensitivity differs for different body areas Sensitivity

corresponds with Sensory Homunculus

Subliminal Perception

Perception of stimuli below the absolute threshold e.g. very briefly flashing messages no evidence for effectiveness in advertising However, flashed words can “prime”

awareness of other stimuli e.g. “bread” - “butter”

A Five-Stage Modelof Sensory Systems

Each sensory system must have:

1. An adequate stimulus

2. Receptors adapted to the stimulus

3. Nerve pathways

4. Destination points in the brain

5. The psychological experience

Seeing

The Stimulus: The Visible Spectrum The portion of the electromagnetic spectrum

between 400 to 700 nanometers

The Eye

The eye focuses light on the retinaRetina: multilayered structure on the inner

surface of the eye

Transduction

The conversion of energy from one type to another The eye transduces light energy into neural

energy at the retina

Transduction occurs at the photoreceptors: Rods: dim-light receptors Cones: bright-light receptors

The Retina

Photoreceptors receive lightNeural signal sent to Bipolar Cells.Signal then sent to Retinal Ganglion CellsGanglion cells send signal out the eye to

the brain exit point is a “blind spot

The Retina

The Retina

Cones: Located in the center of the retina Often see a single cone connecting to a

single ganglion cell

Rods: Located in the periphery of the retina Often see many rods connecting to a single

ganglion cell

Visual Nerve Pathways

Axons of ganglion cells for the optic nerve pathway

Optic nerve sends signals to the lateral geniculate nucleus (LGN) of the thalamus

Signals are then sent to the primary visual cortex in the occipital lobe primary visual cortex = striate cortex

Conscious vs. Non-conscious Visual Pathways

Retina - LGN - Striate cortex: “conscious visual pathway”

“Non-conscious pathways”: Retina - Superior Colliculus: Responsible for

perception of peripheral movement Retina - Pretectum: Responsible for changing pupil

size when presented with bright light.

Dark Adaptation

An increase in visual sensitivity as a result of time spent in the dark Sensitivity appears to plateau at 10 minutes,

but then starts to increase again at 15 minutesRod-Cone Break

Dark Adaptation

Color Vision: Trichromatic Theory (Young-Helmholtz)

Color vision results from the activity of three cone pigments, each maximally sensitive to on of three wavelengths Trichromatic Theory explains additive color

mixing - the mixing of colored lights to create other colorsDichromatism: color blindness resulting from

missing one of three color receptors

Color Vision: Opponent Process Theory (Hering)

Colors are sensed by “opponent pairs” Red-Green Blue-Yellow White-Black

Can be used to explain negative afterimages

Color-Opponent Cells

Ganglion cells are connected to photoreceptors such that they respond in an opponent process fashion to color e.g. inhibited by green and excited by red

Hearing

The Stimulus: Sound Waves a wave of compressed air resulting from

vibration

Sound Waves

Waves of air that can vary in amplitude and frequency

Sound as a Wave

Amplitude (intensity): related to psychological dimension of loudness

Frequency: related to psychological dimension of pitch

Complexity: related to psychological dimension of timbre

Amplitude

Determined by size of wave Measured in decibels (dB)

Frequency Determined by number of waves per second Measured in Hertz (Hz)

The Ear

Three Parts: The Outer Ear The Middle Ear The Inner Ear

The Ear

The Outer Ear

Consists of Pinna Auditory Canal Tympanic Membrane (Eardrum)

Main Function: Gather sounds to send to middle and inner

ear

The Middle Ear

The Middle Ear

Ossicles: Transfer and amplify sound to inner ear Malleus (Hammer) Incus (Anvil) Stapes (Stirrup)

Oval Window To inner ear

Inner Ear (Cochlea)

Sound vibrations enter at oval windowTravel through fluid, vibrating basilar

membrane

Organ of Corti

Where sound is transduced into a neural signal

Sound is transduced by Hair CellsCilia on hair cells contact tectorial

membraneAs basilar membrane vibrates, hair cells

are pulled and neural signal is generated

Flowchart of the Ear and Other Things

Airborne Vibrations

Mechanical Vibrations (Eardrum & ossicles amplify)

Pressure Waves (Cochlear Fluid)

Bending (Cilia)

Electrical Charges (Hair cells)

Ripples (Basilar Membrane)

Neuro-transmitter (Auditory Nerve Fibers)

Brain

Place Theory: How we perceive pitch

Sound waves generate vibration in cochlear fluid and basilar membrane - travelling wave

Frequency of sound is encoded by the stimulation of specific place on basilar membrane High frequencies cause vibrations at thin part

of basilar membrane near oval window Low frequencies cause vibrations at thicker

part

Place Theory: How we perceive pitch

Loudness Perception

Increased amplitude of sound wave leads to greater displacement of basilar membrane

Increase displacement of basilar membrane leads to increased activity of hair cells

Increased activity of hair cells leads to greater number of EPSPs

Conductive Hearing Loss

Hearing loss due to reduced functioning of outer or middle ear e.g. damage to ear drum or damage to

ossicles otitis media: middle ear infection

reduces movement of ossicles

Sensorineural Hearing Loss

Hearing loss due to damage to the cochlea

Central Hearing Loss

Hearing loss due to damage to brain areas e.g. Wernicke’s aphasia - an inability to attach

meaning to language

Central Auditory Processes

Taste

The Stimulus: Chemicals in solution Four basic tastes:

sweetsaltsourbitter

Taste is also a product of what we smell

How we Taste

Taste receptors are found in taste buds on the tongue

Membranes of receptor cells bathed in solution of chemicals in saliva

How we Taste

Receptor cells generate action potentials in taste nerves

Smell

The Stimulus: Airborne chemicals (olfactants)

How we Smell (just terrible)

Olfactants are dissolved in olfactory mucosa at top of nasal passageway

EPSPs are generated in olfactory neuronsSignals sent to the olfactory bulb then to

brain

How we Smell

Touch

The Stimulus: Mechanical Pressure

The receptor: Receptors found in skin

Touch Receptors

Free Nerve Endings Process touch, temperature and pain

Pacinian Corpuscles: Process “deep pressure”

Meissner Corpuscles and Organ of Ruffini Process gradual changes in skin pressure

Pain

The Stimulus: Typically, damaging stimuli - mechanical, heat,

chemical

Pain can be influenced by non-sensory factors e.g. rubbing a hurt area

Phantom Limb Pain Pain associated in a “limb” even though it has

been amputated