Sensation and perception overview

The senses (sensory modalities)

Vision

Audition (hearing)

Olfaction (smell)

Gustation (taste)

Touch (pressure, temperature [warmth and cold], pain)

ProprioceptionVestibular sense (body’s position in space)Kinesthesia (movement and position of parts of the

body in relation to one another)

Sensation and Perception

Sensation and perception are two stages in the process whereby we construct our internal representation of the environment.

Our goal as biological organisms is to respond to, interact with, and adapt to our environments.

Our goal is to extract meaning from the objects that exist and events that occur around us.

Sensation: physical energy is converted into neural signals

Perception: signals are selected, organized, and interpreted

Sensation to attention to perception

1) Transduction: sensory receptors translate physical stimulation (light, sound waves, heat, molecules) into neural signals

2) Neural pathways transmit this information to the brain

3) Rudimentary neural/mental codes, representing features in the environment, are constructed in the brain/mind

4) Features (properties and parts) are combined (bound) to form objects (wholes); may require attention

a) In the case of object recognition, a feature is any property of an object, such as it’s shape (as determined by it’s edges or boundaries), color, size, and movement

i. The letter A has three features: /, \, and –b) In the case of categorization, some features of the bird category are feathers,

beaks, lays eggs, builds nests, etc.

Illusory conjunctions

Sensation to attention to perception cont.

5) Objects are matched with prior knowledge and pattern categorization and recognition occurs

6) Some objects are selected for further processing (another job for attention)

7) Names of objects are accessed

8) Meanings of objects are accessed

9) Objects are organized into scenes

10) The scene emerges into subjective awareness (i. e., consciousness); total time = 100 ms

Sensation to attention to perception cont.

Low-level vs. high level processes Sensory processes are also known as low-level or peripheral processes Perceptual processes are also known as high-level or central processes

As information processing progresses, the representations/codes become more abstract and less modality-specific. For example, if the goal of sensation and perception is to extract

meaning from our world, does it really matter whether you learned something by reading it (a visual stimulus) or by hearing it (an auditory stimulus)?

The information processing progression described in steps 1-10 is mostly a bottom-up processing view of sensation and perception In bottom-up processing, the conscious percept is determined primarily

by the information received via sensory receptors

One can also identify top-down processing influences on sensation and perception In top-down processing, memory and general knowledge take part in

determining the conscious percept See next slide

Sensation to attention to perception cont.

To function effectively, our minds need both quantitative information and qualitative information Quantitative information

The intensity of a stimulus is coded in the brain by the frequency with which neurons fire – the higher the intensity, the greater the frequency of firing

For example, if someone lightly touches your back, the frequency of firing would be relatively low

Qualitative information The difference between two different stimuli is coded in the brain by

different patterns of neurons firing Coarse coding: a single neuron fires when your back is touched

anywhere in a relatively large patch of skin Then how do you know precisely where you’re being touched? Because each particular spot is defined in terms of a unique set of

several overlapping patches Thus, a touch on any particular spot is coded as a unique set (or

pattern) of neurons firing

The cycle of perception:Sensation and perception are

active processes.

Psychophysics

PsychophysicsPsychophysics The study of the correspondence between physical reality

and psychological reality There is no one-to-one correspondence between physical

reality and psychological reality. 3 examples

There are physical energies that exist in our environment that fall outside the normal range of human sensation, but that other animals can detect (i.e., represent psychologically)

Some bees can detect ultraviolet radiation Some snakes can detect infrared radiation Elephants can detect very low frequency sounds Dogs can detect very high frequency sounds

Absolute thresholds Difference thresholds

Psychophysics cont.

Within the normal range of human sensation, sometimes the intensity of a stimulus is so faint that we do not report that we perceive it, even though the physical energy from that stimulus (e.g., light waves bouncing off an object) is striking our sensory apparatus (e.g., our retina).

Absolute Threshold The smallest amount of stimulation that can be accurately

detected 50% of the time is the absolute threshold. “Limen” is the German word for theshold. Subliminal means “below the limen”. Subliminal perception: Does stimulation that falls below the

absolute threshold influence behavior?

Psychophysics cont. Within the normal range of human sensation, sometimes a change in the

intensity of a stimulus occurs, but we do not report that the stimulus has changed.

Difference Thresholds The lowest level of stimulation required to sense a change in stimulation

Weber’s Law In order to perceive a change in stimulation, that change

must be greater as the magnitude of the stimulation becomes greater

The ratio of the change in stimulation (delta I) to the magnitude of the stimulation (I) is a constant (k)

delta I/I = kFor judging the weight of objects, k = 1/50

Thus, if an object weighs 50 grams, an increase of 1 gram will be noticeable.

However, if an object weighs 100 grams, the weight will need to increase by 2 grams in order for you to report that you’ve noticed a difference.

Psychophysics cont.

Signal-Detection Theory Reporting that a stimulus is

present is jointly determined by the signal and the subject’s response criterion.

Separates subject’s sensitivity from subject’s propensity to guess.

Hits – false alarms.

Yes No

On 89% 11%

Off 41% 59%

Yes No

On Hit Miss

Off False alarm

Correct rejection

Yea sayer

Nay sayer

Yes No

On 65% 35%

Off 8% 92%

Response

Sti

mul

us

Sensory modalities

Taste (Gustation)

Photograph of tongue surface (top), magnified 75 times500 to 10,000 taste buds line the tongue and mouth Taste receptors (~ 50) are

down inside each “bud”

Children have more taste buds than adults Older adults have less intense

taste sensations than younger adults

Taste preferences begin in the womb

Supertasters have more taste buds, but are often more picky eaters

Smell (Olfaction)

Touch (Haptic sense)Skin is the largest organ in the body (18 sq ft)Pressure, temperature, and painDifferent receptors in the skin transduce different types of physical information that, in the end, is routed to the somatosensory cortex (by way of the thalamus)

Sensitivity to Touch

The Heat Grill

The brain interprets simultaneous firing of both receptor types in interesting ways. When a person grasps

two braided water pipes – one with cold water running through it and one with warm water – the sensation is “very hot” or “wet”.

PainPain in the skin When cells are damaged, they release substance P, that

stimulates free nerve endings

Two types of pain sense neuronal axons Myelinated L-fibers transmit all types of information

very quickly to the spinal cord Adaptive because this type of pain causes us to recoil quickly

Unmyelinated S-fibers transmit pain information relatively slowly to the spinal cord

Adaptive because this type of pain forces us to allow injured tissue to recuperate

Gate control theory L-fibers inhibit neural transmission in S-fibers This is why rubbing the skin near an injury can lessen

the pain.

Hearing (Audition)Sound Sound waves are vibrations of molecules in the airFrequency (perceived as pitch)

What is white noise?Amplitude (determines loudness)

Measured in decibels (dB), which is a logarithmic scale

An increase of 10 dB corresponds to a change in sound power of 10 times the comparison stimulus

Complexity (determines timbre or tonal quality)Determines the difference between middle C played

by a trumpet vs. a flute

The Human Ear

Auditory transduction

When sound waves hit the fluid of the inner ear, the fluid causes the basilar membrane to move up and down, activating electrical potentials in the hair cells (the receptor cells for hearing).

Temporal coding The entire basilar

membrane vibrates slowly for low frequencies and more rapidly for high frequencies

Place theory

Auditory Localization

Sounds from different directions are not identical as they arrive at left and right ears Loudness Timing Phase

The brain calculates a sound’s location by using these differences

Visual sensation

The Electromagnetic Spectrum

Structures of the Human Eye

The Retina

Receptive Fields I-- a place or spot in your visual environment for which an individual neuron is “responsible”

-- allows for the detection of edges

Receptive Fields II-- the cell fires more when light illuminates the center and less when the center is dark

-- the cell fires more when the periphery is dark and less when the periphery is illuminated

Hermann Grid

Color Vision

Trichromatic theory: T. Young (1802) & H. von Helmholtz (1852) both

proposed that the eye detects 3 primary colors: red, blue, & green

There are three types of cones, each of which is most maximally sensitive to a different wavelength

All other colors can be derived by combining these three

Color Vision cont.

Opponent process theory Color opponent

cells in the LGN are activated by one color and inhibited by the other: black-white, blue-yellow, red-green

Color “blindness” Afterimages

Proprioceptive senses

Vestibular sense: position of the body in spaceVestibular sacs and semicircular canals are located

in the inner ear, above the cochleaVestibular sacs sense gravity and the position of

the head in spaceSemicircular canals sense acceleration as the head

moves

Kinesthesia: movement and position of muscles and limbs relative to each otherReceptors located in joints, tendons, and muscles.

Extrasensory Perception

Extrasensory Perception (ESP): The ability to perceive something without ordinary sensory

information

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

57% of Americans believe in ESP CBS News (2002)

But ESP has not been scientifically demonstrated

Parapsychology

J. B. Rhine conducted many experiments on ESP using stimuli such as these Guess the symbol on the card In a 25-card deck, with 5 different cards, participants correctly identified 7.1 (5 correct

guesses would be expected by chance)Rhine believed that his evidence supported the existence of ESP, but his findings were flawed

Experimental procedures were faulty Cards had been handled by the participants and were marked in various subtle ways

Experimental findings could not be replicated

Perception

Neurons in the primary sensory areas of the cerebral cortex have topographic organizational structures.Neurons in the primary auditory cortex have a

tonotopic organization.Different parts of the primary auditory cortex are

differentially sensitive to different frequencies, just like the basilar membrane.

Neurons in the somatosensory cortex have a somatotopic organization.

Connected areas of the body are represented next to each other in the cortex (see next slide).

The topographic nature of primary perceptual processing

Somatopic organization in cortex

Visual Perception

The organization of features into objects and scenes, the location of those objects in space, and the interpretation (beginning with classification) of those objects.

Retinotopic maps in the brain

Macaque monkeys were injected with a glucose-linked radioactive substance

After viewing an image, the monkeys were sacrificed and their striate cortex was flat mounted

Then the radiation was measured Those cells firing the most will

use the most glucose and will show the most radiation

Hierarchical processing of features

Simple cells: respond to lines of a particular orientationComplex cells: may respond to lines that terminate or to corners; may respond to lines anywhere in their receptive fieldHypercomplex cells: respond to a combination of features

Visual Pathways

Form perception (or perceptual organization)

We must be able to segregate the stimulation the visual system receives into separate objects. “Parsing” our visual world.

Gestalt maximThe whole is greater than the sum of it’s parts.

Figure-groundPeople inherently distinguish between figure (the

object they are viewing) and ground (background)Ambiguous or reversible figures

Ambiguous (reversible) figures

Ambiguous (reversible) figures

Ambiguous (reversible) figures

Ambiguous (reversible) figures

Similarity

Objects that are of a similar color, size, or shape are usually perceived as part of a pattern

Proximity

When objects are close to one another, we tend to perceive them together rather than separately

Good continuation

Items that continue a pattern or direction tend to be grouped together as part of a pattern

ClosureWe are inclined to over-look incompleteness in sensory information and to perceive a whole object even where none really exists.

Subjective Contour

Geon TheoryGeons (geometric icons) are simple 3D component shapesAn “alphabet” of 36 are stored in memoryGeons are combined to identify essential contours of objects

Face perception

In human beings, facial expression and the perception of faces are an important mode of communicationA specific part of the brain is dedicated to face perception The fusiform gyrus in the right hemisphere

Located on the ventral portion of the right temporal lobe Damage to the fusiform gyrus results in prosopagnosia – an

inability to recognize faces

Thatcher illusionSame race effect in face recognition Face memory is better for faces from one’s own race

And this is more true of whites, than blacks. There is also more activation in the fusiform gyrus when

perceiving a same race face than when perceiving a different race face.

Depth Perception

How far away is a particular object?

Binocular cues Information integrated from both eyes

Monocular cues Information from a single eyeArtists working in two-dimensional media rely on

monocular depth cues to represent a 3-D world

Binocular CuesOculomotor cuesOur eyes converge (turn inward towards the nose)

to focus on objects that are closer and diverge to focus on objects that are farther away

Receptors in the oculomotor muscles send this information to the brain

Retinal disparityThe difference between images captured in the two

retinasThis difference diminishes as depth increasesBinocular cells in visual cortex are retinal disparity

detectors, firing to specific retinal disparities

Monocular CuesRelative sizeWhen looking at two objects known to be of similar

size, the object that looks smaller (because it projects a smaller image on the retina) will be perceived as farther away

Texture gradientAt close distances, patterns appear coarse and less

closely packed, while at farther distances they appear finer and more densely packed

Linear perspectiveParallel lines converge at a distance

Monocular Cues cont.

InterpositionWhen one object blocks our view of another object,

the obstructed object is perceived as more distant

ElevationAs objects in our plane of vision that are below the

horizon get closer to the horizon (and, if above the horizon, they get farther from the horizon), they are perceived as more distant

Motion parallaxWhen a person is moving, nearby objects speed by

whereas far objects move by more slowly

Perceptual ConstancyThe perception of objects stays constant despite changes in sensory stimulation. Size constancy Even though two objects of the same size but at

different depths (distances) create differently sized retinal images, we still perceive them as the same size.

Color constancy The color of an object remains constant despite

changes in illumination.

Shape constancy Despite the fact that the same object produces a

different retinal image nearly every time we encounter it, we still recognize it as that object.

Size perception (and size constancy) depends on depth perception

Ames Box (Ames Room) When one child stands in

a near corner and another (of similar height) stands in a far corner, the room creates the illusion that they are both equidistant from the viewer

Therefore perceptual processing does not adjust for distance and we interpret the small retinal image as coming from a smaller person and the larger retinal image as coming from a large person.

The Ponzo Illusion

The lines project the same size retinal image because they are the same size. However, linear perspective cues tell us that the top line is farther away. However, in order for something farther away to project the same size retinal image, it must be larger. So, we perceive the top line as larger.

The moon illusion

The moon produces retinal images of the same size no matter where it is in the sky

However, we perceive it as closer when it is not on the horizon

In order for it to produce the same size retinal image and be closer, it must be smaller

So, we perceive it as smaller.

The Muller-Lyer Illusion

Again, the lines project the same size retinal image because they are the same size. However, linear perspective cues tell us that the right-hand line is farther away. Therefore, in order for something that is farther away to project the same-size retinal image, it must be larger! So we perceive it as larger.

Shape Constancy

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

Two systems for motion perception