PSYC 251 (Perception)

psychology perception

What is perception?

Experiences resulting from stimulation of the senses

The set of processes by which we recognize, organize, and make sense of the sensations we receive from environmental stimuli

What we sense (in our sensory organs) is not the same as what we perceive (in our minds)

Not perceiving what is there...

failure to perceive what is there

Perceiving what is not there...

our brain makes an inference of what it thinks is going on

perceiving what cannot be there...

perceive things that cannot be in the world (perpetual staircase)

Perception

Problem:

- Understand what is going on out there

(outside the brain)

Importance:

- Necessary in order to know how to act to achieve goals

Challenge:

- The inverse problem: how to determine the distal stimulus from the proximal stimulus

Inverse problem of perception

Create a representation (perception) of what is out in the world (the distal stimulus) from what we sense (the proximal stimulus)

Sources of Information for perception

Genes

- Information learned on timescale of evolution

Past experience

- Information learned on timescale of a human life

Internal state

- Information learned on timescale of current episode

Environmental context

- Information learned now

Proximal stimulus

- The stimulus itself

sensory systems

vision, somato-sensation (touch, temp, pain, proprioceptors),

audition

sensory system stages

Distal stimulus

- an object or process out in the world

Proximal stimulus

- The energy or matter that impinges on the sensory receptors

Sensory receptors

- Specialized cells to transduce (convert) external phenomena (light, sound, pressure, etc...) into neural signals

Neural pathway

- From sensory receptors via thalamic nuclei to cerebral cortex

Hierarchy of cortical areas

- Attempt to construct useful representation of distal stimulus

Percept

- Mental representation of the distal stimulus

eye

light comes through the cornea to the retina

Fovea

the central focal point in the retina, around which the eye's cones cluster

Retina

the light-sensitive inner surface of the eye, containing the receptor rods and cones plus layers of neurons (ganglion cells, bipolar cells) that begin the processing of visual information

Blindspot

the point at which the optic nerve leaves the eye, creating a "blind" spot because no receptor cells are located there

Photoreceptors

rods and cones

- 1 type of rods, 3 types of cones

Rods

retinal receptors that detect black, white, and gray; necessary for peripheral and twilight vision, when cones don't respond

Cones

retinal receptor cells that are concentrated near the center of the retina and that function in daylight or in well-lit conditions. The cones detect fine detail and give rise to color sensations.

Retinal receptor density

high density of cones and lower density of rods at fovea

higher density of rods off the fovea

Fovea is packed with cones

Primary visual pathway

Visual fields

- Both visual fields on both retinas

- Partial crossover at optic chiasm

- Left visual field in right V1

- Right visual field in left V1

Pathway

- Ganglion cells

- LGN (thalamus)

- Primary visual cortex

Where does information from the upper left quadrant of the visual field go in V1?

A. Left dorsal V1

B. Left ventral V1

C. Right dorsalV1

D. Right ventral V1

D. Right ventral V1

Visual field topography in V1

flipped left right

flipped up down

auditory system

Responsible for hearing, balance, equilibrium, and communication skills

Ear

Sound = changes in air pressure

Ear drum (tympanum) converts changes in air pressure into mechanical vibrations

Vibrations travel through bones of middle ear (ossicles) to oval window of cochlea

Hair cells in cochlea detect vibrations

hair cells in ear

long tufts of stereocilia on top surface, once basilar membrane vibrates, stereocilia swap back and forth within endolymph - causes opening of ion channels

- caused by tiplings? (little hairs on top of hair cells)

Organization of Basilar Membrane

Location of maximal excitation along the basilar membrane depends on sound frequency

Low frequency --> high part of membrane

Medium frequency --> middle of membrane

High frequency --> base of membrane

Primary auditory pathway

Auditory nerve

Cochlear nuclei (medulla)

Superior olivary nucleus (pons)

Nucleus of lateral lemniscus (pons)

Inferior colliculus (midbrain)

Medial geniculate nucleus (thalamus)

Primary auditory cortex (in the temporal lobes)

somatosensory system

sensory network that monitors the surface of the body and its movements

Somatosensation

Mechanoreception

- Detects pressure, vibration and distortion

Thermoception

- Detects hot and cold

Nocioception

- Detects harmful chemical, mechanical, or thermal stimuli

Proprioception

- Detects mechanical forces on muscles, tendons and joints

Mechanoreceptors for touch

- in the dermis (2 higher in the skin, 2 lower)

Meissner corpuscle (RA1)

Merkel cells (SA1)

Pacinian corpuscle (RA2)

Ruffini endings (SA2)

Primary Somatosensory Pathway

Dorsal root ganglion

Gracile/cuneate nuclei (medulla)

Ventral posterior nuclei (thalamus)

Primary somatosensory cortex

Sensory adaptation

The proximal stimulus is represented on a

relative scale, not an absolute scale.

The influence of context on perception begins very early in the sensory pathways.

(Ex: flashlight during the day vs during the night)

visual adaptation

The sensitivity of the visual system to a light stimulus depends on the ambient light level

(not just the light, but how the light is impaired from other light in the environment)

Weber's Law

The "just noticeable difference" (JND) is the smallest detectable change (∆I) in a stimulus (I)

The JND is proportional to the magnitude of the stimulus:

(∆I)/(I) = K

K is the Weber fraction

Loudness: K ≈ .05

Brightness: K ≈ .08

Heaviness: K ≈ .02

Slowly adapting mechanoreceptors

continue to respond and send out action potentials even after a long period of continual stimulation

Merkel cells (SA1)

Ruffini endings (SA2)

Rapidly adapting mechanoreceptors

give rise to sensations of touch, movement and vibration fast

Meissner corpuscle (RA1)

Pacinian corpuscle (RA2)

Receptive field

Area of sensory surface to which a neuron responds

Perceptual resolution and acuity are inversely related to sensory receptive field size:

Higher-order neurons have larger receptive fields

Higher-order neurons respond to more complex sensory stimuli

Visual receptive fields

Receptive field of a cone: area on retina

Receptive field size varies with eccentricity

(clearer vision right where you're looking vs where you're not)

Receptive fields of retinal ganglion cells

Photoreceptors contributing to ganglion cell receptive field

- centre-surround structure of ganglion cell receptive field

- on area (centre) fires more, off area (surround) fire less

Horizontal, bipolar and amacrine cells

Retinal ganglion cell

Visual center-surround receptive fields

On cells

- fire best when light in the middle and dark in the surround

Off cells

- fire best when dark in the middle and light in the surround

Auditory receptive fields

- receptive field of a hair cell: frequency of sound

somatosensory receptive fields

Receptive field of a mechanoreceptor: area on skin

Receptive field size and acuity vary with location on body:

fingers and hands have a smaller threshold

calf, legs, etc. have a larger threshold

Somatosensory center-surround receptive fields

Lateral inhibition

lateral inhibition

The pattern of interaction among neurons in the visual system in which activity in one neuron inhibits adjacent neurons' responses.

Topography

Spatial organization (topography) of sensory surface is generally preserved in (projected onto) primary sensory cortex

Cortical magnification

Area of cortex is proportional to density of sensory receptors (and inversely related to receptive field size)

retinotopic map

Topological map that preserves spatial relationships found on Retina

Cortical magnification varies with eccentricity

more photo receptors in middle (fovea) takes up more of the cortex (greater cortical magnification)

tonotopic map

An ordered map of frequencies created by the responding of neurons within structures in the auditory system. There is a tonotopic map of neurons along the length of the cochlea, with neurons at the apex responding best to low frequencies and neurons at the base responding best to high frequencies.

somatotopic maps

Cortical or subcortical arrangements of sensory pathways that reflect the organization of the body

somatosensory homunculus

Broad areas of primary somatosensory cortex devoted to particular body regions

- larger regions due to higher nerve endings in the area

Gustotopic map

An alternative to a topographic map in which different taste qualities are mapped to specific areas of the cortex, despite having no spatial differentiation in the periphery. This may also be referred to as a chemotopic map.

Plasticity

Changes in neural organization

Occurs from the molecular to systems level

Synaptic plasticity

- Changes in the strength of synapses

Cortical reorganization

- Changes in topographic maps

Reorganization of retinotopic map

Lesion of the visual field(in both eyes!) leads to reorganization in primary visual cortex

Ex: can reorganize due to damage

Reorganization of somatotopic maps and phantom limbs

Example: After amputation of arm, pursing of lips causes perceived sensation in missing arm

Ex: Cells now receive info from adjacent cells

- Still experience sensation in the arm thats not there.

- can experience itch because cells are still in cortex

- patients could relieve itch from phantom limb by scratching face because neurons reorganized from arm to receive input from the face

hierarchical organization

Moving from lower-order sensory neurons (those closer to sensory receptors)to higher-order sensory neurons(those farther from sensory receptors):

- Receptive fields get larger

- Sensory features get more complex (and abstract)

- Sensory features get more specific

- Processing proceeds in serial (sequentially),in parallel (simultaneously), and is recurrent (loops)

- Multi-sensory integration increases

Cortical hierarchy

EX:

Visual

Primary -> Secondary -> Tertiary

- Primary would contain 1st order neurons

- Tertiary would contain 2nd order neurons

Similar order for somatosensory category, auditory category. etc.

Hierarchy in visual system

First info is received in V1

- serial, parallel and recurrent processing in the visual system

Modularity

- Primary visual cortex = striate cortex = V1

- Secondary visual cortex = extrastriate cortex = V2, V3, V4, V5/MT

- Tertiary visual cortex = visual association cortex = MST, LIP, etc...

- Multimodal association cortex = VIP, etc...

Feature detectors and tuning curves (visual)

Example: Orientation feature detectors in V1

- neurons fire when they see the stimulus in vertical position

- tuning curve --> shows how neurons fire more when stimulus is in a certain orientation then others (vertical vs slanted vs horizontal)

Example: Build an orientation feature detector from center-surround neurons

- wiring up a neruon so its receptive field is more complex

Cortical columns

Organization of orientation feature detectors in V1

- For each location in visual field, for each eye: detectors for all orientations

- Organized by eye (ocular dominance columns) and by orientation (orientation columns)

More complex feature detectors (visual)

Example: Oriented lines of a specific length

Example: Building shapes from oriented lines

- feed neurons different shapes, can wire up neurons that can respond to complex patterns

Hierarchy in visual system (specific areas)

MT/V5 = Motion

V4 = Colour

Modularity

- Primary auditory cortex = A1 = Core

- Secondary auditory cortex = A2 = Belt

- Tertiary auditory cortex = auditory association cortex = Parabelt (PB), etc...

- Multimodel association cortex = T2/T3, PP, etc...

Feature detectors and tuning curves (auditory)

Example: Directional feature detectors in superior colliculus of ferret

Numbered areas: Directional tuning curves for individual neurons in superior colliculus

Auditory directional feature detectors

Sound arriving at ears is out of phase when distance from sound source to ears differs. Size of this difference, interaural time delay (ITD), determines horizontal location of sound source.

(look at slides :(()

Hierarchy in somatosensory system

Modularity

- Primary somatosensory cortex = S1= BA 1, 2, & 3

- Secondary somatosensory cortex = S2 = PV

- Tertiary somatosensory cortex = somatosensory association cortex = BA 5, MIP, AIP, etc...

- Multimodal association cortex = VIP, etc...

Feature detectors and tuning curves (somatosensory)

Example: Orientation feature detectors in S2

- responds more to specific orientations versus others

Build an orientation feature detector from simpler detectors

More complex feature detectors (somatosensory)

Somatosensory motion detectors in S1

- Lower traces show motion of probe on finger

Motion-sensitive neurons:

Respond to any motion in receptive field

U D P R

(thumb)

Orientation-sensitive

neurons:

Respond to motion along a particular axis

UR

(middle finger)

Direction-sensitive neurons:

Respond to motion in a particular direction

R

(Back of hand)

What and where streams

Higher-order sensory processing is generally

divided into "what" and "where" streams

where stream

- Dorsal pathway: occipital lobe into parietal lobe

- Emphasis on location and motion

- Processing for action

What stream

- Ventral pathway: occipital lobe into temporal lobe

- Emphasis on shape and color

- Processing for object recognition

Visual What and Where stream pathway

retina --> LGN --> V1

--> V2 --> V4 (Ventral (temporal) pathway)

--> MT (Dorsal (parietal) pathway)

fusiform face area (visual what stream example)

a region in the temporal lobe of the brain that helps us recognize the people we know

- have face sensitive cells

- inferior temporal cortex

The where stream (visual function example)...

Guides movements

Intraparietal sulcus (IP)

- Anterior (AIP)

Represents space for hand

movements

- Medial (MIP) Represents space for arm

movements

- Lateral (LIP)Represents space for eye

movements

- Ventral (VIP)

Represents space for facial movements

Auditory 'what' and 'where' streams

Where - intraparietal sulcus

What - temporal lobe

Somatosensory What and where streams

Where - intraparietal sulcus

What - temporal lobe

bottom-up processing

- Stimulus driven

- Feedforward connections

- Depends on proximal stimulus and genetic "hard-wiring"

of sensory systems

** Perception depends on both

top-down processing

- Driven by goals and expectations

- Feedback connections

- Depends on past experience, internal state, environmental

context

** Perception depends on both

Bottom-up and top-down processing example

bottom-up

--> takes lines, and shapes, and colours

top-down

--> includes higher-level interpretive processes

likelihood principle

We perceive the world in a way that is "most likely" based on our past experiences

(top-down processing)

interactive activation model

a theory proposing that both feature knowledge and word knowledge combine to provide information about the identity of letters in a word

** Model of letter & word perception

** Integrates bottom-up and top-down processes

interactive activation model - bottom-up processing

FEATURE DETECTORS

FEATURES EXCITE OR INHIBIT LETTERS

LETTERS COMPETE WITH OTHER LETTERS & EXCITE OR INHIBIT WORDS (Lateral Inhibition)

interactive activation model - top-down processing

WORDS COMPETE WITH OTHER WORDS & EXCITE LETTERS

INTERACTIVE ACTIVATION

Explanation of Word Superiority Effect

Both bottom-up and top-down processes are necessary to explain perception.

Tinnitus

Perception of sound in absence of auditory stimulation

Potential cause(s): Damage to either cochlea or structures along auditory pathway or somatosensory structures or limbic system or reorganization of tonotopic map