Class 8

Sensation:

the detection of physical stimuli (stimulation of a sense organ)

Transduction:

sensory systems translate/convert the physical properties of stimuli into patterns of nerve impulses the brain can interpret

Perception:

brain’s active processing, organization, and interpretation of sensory information

Brain actively constructs XXX from sensory information

perceptual experience

Bottom-up processing:

perception based on the physical features of the stimulus

Top-down processing:

the interpretation of sensory information based on knowledge, expectations, and past experiences

Context affects perception: what we expect to see (XXX) influences what we perceive (XXX)

1. higher level

2. lower level

McGurk Effect

• Multisensory illusion: Vision in the perception of speech

• Visual information from lip configuration changes the sound that is heard

• Incongruent lip configuration and auditory information – brain tries to make sense of the conflict

  • Changes your perception of what you hear

Visual perception:

The eye receives light and the brain processes the images that result

Light:

electromagnetic radiation (energy) that travels around the world in waves of varying lengths (wavelengths) and intensity (amplitude)

• Visible light: part of electromagnetic spectrum that humans can see

• Length of a light wave determines its hue (what humans perceive as color)

• Amplitude or intensity of a light wave determines what we perceive as the brightness of light

• Blue light – shorter wavelength

• Red light – longer wavelength

Steps for light entering the eye (4)

1. Light reflected from an object first passes through the cornea (thick, transparent membrane covering the eye)

2. Light is refracted (bent) as it passes through the cornea so it can be focused on the retina (photosensitive, thin membrane of back of eyeball)

3. From cornea, light passes through the pupil, a small opening in the iris

4. Iris expands and contracts, changing the size of the pupil to let more or less light in

What do prisms do?

split white light into its component colors because different wavelengths bend by different amounts

XXX does most of focusing of light onto retina

Cornea

Lens vs. cornea on being adjustable

Lens acts to ‘fine tune’ the focus

• lens is adjustable

• cornea is not

Changing lens curvature allows…

focusing of light from different distances. Muscles change shape of lens

Lens alters its shape by XXX to focus on distant objects or XXX to focus on closer objects to create an even sharper focus of light on the retina

flattening or thickening

Accommodation:

process by which the eye maintains a clear image on the retina

Near sighted (myopia):

lens focuses either in the middle of the eye or in front of the retina.

Far sighted (hyperopia):

lens focuses at some point past the retina.

Sunlight exposure may reduce XXX in children

myopia

• Randomized control trial in Taiwan

• Encouraged school children to go outdoors for up to 11 hours weekly

• Effectively reduced myopia progression in nonmyopic and myopic children

Retina:

Light-sensitive membrane lining the back of the eye ball (contains photoreceptor cells)

Image is usually formed in the XXX

Retina

Before reaching the retina, light needs to go through gel-like substance known as XXX (fills 80% of volume of eye)

vitreous humour

Due to the angle at which light hits the eye, images that form on the retina are

inverted and left/right reversed. After they are transduced into neural signals and these signals reach the brain, the image are perceived in their correct orientation

Photoreceptors:

Neurons dedicated to capturing light

How many photoreceptors does the retina contain?

Over 125 million

What do photoreceptors do?

Initiate process of transduction — converting light into electrical signals to be sent to the brain

What are the 2 principal types of photoreceptors in the human eye?

Rods and cones

6 characteristics of rods

1. ~120 million in retina

2. Contain rhodopsin

3. Do not signal color

4. Sensitive in low light (scotopic) conditions (can respond to a single photon)

5. Allow night vision and vision in low light

6. Absent from fovea, most in periphery

5 characteristics of cones

1. ~6 million in retina

2. Contain “red-sensitive”, “green-sensitive” or “blue-sensitive” opsin depending upon their type

3. Subserve daylight (photopic) vision and color perception

4. Specialized for day vision, color vision, and capturing fine detail

5. Concentrated in fovea (small region near center of retina – clearest images and sharpest focus), reduce in number in periphery

During daylight, rods are permanently XXX– low sensitivity

bleached

In the dark, XXX replenishes and XXX become more sensitive

rhodopsin; rods

• Takes about 20 -30 minutes to reach full scotopic sensitivity

Fovea:

Small region near center of retina – area of retina where vision is clearest and there are no rods

Where are cones most concentrated?

the fovea

• Perceive images that fall on the retina more clearly (clearest and sharpest focus)

Images are less sharply focused the farther from the XXX they get (mostly XXX in the periphery)

fovea; rods

Explain the blind spot (5 main points)

• Bipolar cells collect electrical signals from

rods and cones and transmit them to

outermost layer of retina where neurons

called retinal ganglion cells (RGCs) organize

signals and send them to the brain

• Bundled RGC axons form the optic nerve

• Optic nerve carries neural signals through a

hole (out the back of the retina) to thalamus

and then primary visual cortex

• Area where optic nerve leaves retina (optic

disc) has no rods or cones. It is ‘blind’

• Brain fills in the missing information.

Optic nerve carries neural impulses to area …

V1 (primary visual cortex) in the occipital cortex of the brain

Neurons I V1 are sensitive to…

edge orientation – selectively respond to bars and edges in specific orientations in space.

“What” and “where” pathways

Visual info travels in separate parallel processing streams/pathways from visual cortex (occipital lobe) to different parts of the brain for further processing

• Ventral “what” stream – specialized for the perception and recognition of objects (e.g. colors and shapes)

• Dorsal “where” stream: Specialized for spatial perception. Determining where an object is and relating it to other objects in a scene.

“What” and “where” pathways – how do we know?

Brain injuries within either pathway and fMRI

Damage to lateral occipital cortex in ventral stream –

1. couldn't identify objects by XXX

2. Could identify by XXX.

3. Visual representation of objects, but not XXX for objects, was damaged.

4. Could still guide her actions by sight to an object, but could not XXX the object

1. sight

2. touch

3. memory

4. recognize

Damage to parietal lobe (dorsal stream)

1. difficulty using XXX to guide reaching and grasping

2. Can still XXX objects

1. vision

2. identify

Normally, 2 streams work together during visual perception to integrate perception for identification (XXX) and perception for action (XXX)

• ventral

• dorsal

Is color a property of an object?

No.

• An object appears a particular color because of the wavelengths of light it reflects and the way the brain interprets it

• Color is always a product of our visual system (brain) – perception of color is created by the brain

things lose their color at XXX

night - rods cannot create color vision

S-, M-, and L-cones in the retina are particularly sensitive (peak sensitivity/respond maximally) to …

short (S), medium (M), or long (L) wavelengths – which correspond to the perception of blue, green, and red

Young-Helmholtz trichromatic theory of color vision

• Colors are created through the additive mixture of blue, green, and red

• Color matching used to test theory that humans have trichromatic vision – can match any colour using just three lights

• If all 3 cones are equally activated, pure white is perceived

Because cone types respond to overlap, it is more efficient to compare differences in XXX

cone responses

Hering’s (1878/1964) opponent process theory of color vision

Thought each of the 3 cone types responds to 2 different wavelengths

• Blue or yellow. Red or green. Black or white

Explain: Ganglion cells in the retina and neurons in the thalamus and visual cortex respond in an opponent-process by altering their firing rates

• One type of ganglion cell receives excitatory input from L-cones (red) but inhibited by M-cones (green)

• Another type of ganglion cell is excited by input from S-cones (blue) but inhibited by both L- and M-cone activity (yellow)

Explain yellow and green American flag negative afterimage illusion

Staring at image of flag causes neurons responding at peak efficiency to black, green and yellow to ‘tire’ (sensory adaptation). Look at white and it switches to opponent color which had been inhibited. Afterimage appears in opponent colors

What effect is the waterfall illusion using?

motion after-effect

Explain the waterfall illusion

• Specialized neurons in secondary visual area of brain respond to orientation of movement

  • Some neurons respond to upward movements, others to downward, etc.

• Direction-specific neurons adapt to the motion, become fatigued and less sensitive. Stimulus removed, motion detectors that respond to other directions are more active.

  • See it moving in other direction = motion-after effect

• Adaptation in visual neurons that respond selectively to moving contours

If one set of motion detector cells is fatigued through adaptation to motion in one direction, then wen the motion ceases, the relative greater firing in the opposing less fatigued cells is interpreted as motion in the opposite direction

Perception of color depends on

relative activity in opposing sets of cones

Perception of motion depends on

relative activity in opposing sets of motion detector cells

Apparent motion:

Perception of movement as a result of alternating signals appearing in rapid succession in different locations

Misspellings follow certain basic patterns and small connecting words are correctly spelled.

Perceive whole words from alphabetic cues. Organization and context are important for perception.

Perception is a XXX process

constructive

3 evidences that perception is a constructive process

• Eyes are completely functional, damage to the visual cortex will impair vision

• Information that the retina projects to the brain results in 2-D representation of edges and colors that the brain automatically transforms into a 3-D world of objects and background

• Visual system uses organizing principles to determine the meaning of visual input

Figure-ground relationships

Basic organizing principle: distinguish between figure and ground

• Simply visual world by automatically dividing visual scenes into objects and background

Figure vs. ground

• Figure = image that holds our attention

• Ground = remaining portion

Reversible figure illusion

• Assignment of figure and ground is ambiguous

• Identify figure, brain assigns rest to the background.

Gestalt psychology

Perceive meaningful ‘wholes’ out of inherently meaningless and fragmented sensory impressions

• “whole is greater than the sum of its parts”

Gestalt laws of perceptual organization/grouping main idea + 4 principles

How separate stimuli come to be perceived as parts of larger wholes; Group and segregate features to create whole objects organized in meaningful ways

1. Proximity

2. Similarity

3. Closure

4. Good continuation

Kanizsa Triangle (Illusory contours) (which of the four does this illusion capitalize on: proximity, similarity, closure, or good continuation)

closure

Top-down illusions

• Not produced by the mechanics of the eye, but by expectations of your brain

• the interpretation of sensory information based on knowledge, expectations, and past experiences

Depth perception

• Retina receives. Brain transforms

• Judge the relative distance of objects by various depth cues

  • e.g. relative size – objects closer are larger, occlusion/interposition – one object partly blocks another

Linear perspective (depth cue)

Parallel lines moving away from you (e.g. railway tracks) converge (angle toward each other) as they stretch out into the distance

Ponzo Illusion

Parallel lines moving away, farther rectangle percieved as bigger when they are the same size

• Linear perspective (depth cue)

Müller-Lyer illusion

Arrows up or down on line

Perception

The process by which we take in, organize, interpret, and make sense of information about the outside world

• The process by which the brain organizes and interprets sensory signals

Infants’ knowledge about support events violation detected at each stage

• contact/no contact

• amount of contact

• type of contact

• proportional distribution

Looking and preferential reaching

• size constancy (4 months)

• depth cues (5-7 months)

3-day old infants prefer to look at faces and patterns depicting faces than at non-face patterns, but not if…

not if faces are upside down or the facial features are scrambled

Do 2-day old infants are able to distinguish their mother’s face from that of a stranger?

Yes. They prefer to gaze at it

Face Pareidolia

Non-face objects look like faces

Hollow Face / Charlie Chaplin Illusion

See a 3D face in the hollow face

Flashed Face Distortion Effect

Not face specific, but depends on increased visual blur with peripheral vision and sufficient presentation time for adaptation

Face recognition ability varies A LOT or A LITTLE across individuals

A lot. Substantially.

If faces and objects are processed by a single common mechanism, then one would expect

object and face recognition abilities to always correlate

Good face perception with poor object recognition

object agnosia (Mr. CK)

Poor face perception with good object recognition

prosopagnosia (Edward)

CK: Object agnosia: Perception of objects is very XXX- focuses on separate parts without appreciation for the whole

piecemeal

• Given contextual clues can infer what many objects in the environment are from their separate components (e.g. piece of paper and yellow pencil on a desk)

• If objects are not specified in context, identifies them incorrectly (e.g. identifies a pen placed in a holder fixed on a marble stand as a “trophy you must have won for your research”), didn’t know it was a pen until he touched it

Mr. CK Asked for coffee – didn’t drink it

when asked why, he couldn’t distinguish the coffee cup from other containers on the desk

The Arcimbaldo Effect

Control subjects immediately perceive these arrangements as a face and are also aware of the objects that comprise it

• CK sees the face within Arcimboldo paintings but does not detect the non-face parts

• Doesn’t notice they are made up of objects

• Can’t identify the objects that comprise it

Why can he see the “animal man” and not the “fruit man”?

“The animals all have faces, and it is the faces that I see. Although they aren’t people faces, they look like faces and that may be enough.”

Object agnosia:

Normal face perception (see them as a whole), see objects in pieces – difficulty with perception and recognition Use colour, shapes, and memory to guess at what the objects are

Thatcher illusion

Upright – two faces are dramatically different

• Process upright faces holistically – encoding all the features in a configuration so can easily detect when any feature is changed (Dahl, et al., 2010)

• Process upside down faces in pieces. Features of the face are processed separately - less sensitive to relations among them

Inverted faces are harder to recognize than (2)

• upright faces

• inverted objects

Face inversion effect theory

inverted faces can’t engage face-processing mechanisms and instead rely on processes used to recognize objects. Upright faces are recognized holistically, but inverted faces are recognized by part-based, featuredependent processes that are also used to recognize objects

• If recognizing inverted faces uses the same processes as normal recognition of upright faces, CK should be no more impaired at inverted faces than controls

• If inverted faces are recognized by mechanisms used to recognise objects, CK would be much more impaired at inverted faces than controls.

CK: Normal recognition for upright faces, severely impaired on inverted faces

Faces for CK vs. Non-faces for CK

LOK at slide 47 in brain like a camera

Prosopagnosia word root

Greek ‘prospori’ (face) + ‘agnosia’ (ignorance)

Chuck Close

Chuck Close is a painter who is suffering from prosopagnosia

Edward: Prosopagnosia has difficulties discriminating the (3) from a face

has difficulties discriminating the identity, emotion and gender from a face

Greebles

Greebles are designed to place similar demands on recognition system as faces do

• Greebles have 4 features that are configured in a uniform matter. Subjects must rely on the shape of the features and/or the precise spatial relations of those features (similar to faces)

Introduce 20 greebles and their individual names and family names to Edward

Edward performs as well as controls on greeble learning

• Edward and control subjects relied on ordinary object recognition mechanisms to recognize the greebles.

Facial recognition distribution approach

-Face recognition is normally distributed

-Developmental prosopagnosics represent the lower tail

-Predicts the existence of super-recognizers at the upper-end of the distribution?

Facial Recognition traditional view:

- Dichotomous

- Face recognition is either ‘normal’ or ‘impaired’

Cambridge Face Memory Test

• the average score on this test was around 80% for adult participants

• a score of 60% or below may indicate face blindness