PSYC-2900 Chapter 6

sensation

process of signal detection

• relies on photoreceptors detecting different stimuli and translating them into neural signals

perception

process of interpreting neural signals

• relies on organising the incoming neural signals and making sense of them to give us a conscious experience of the stimuli

• top-down process that makes sense of sensation info

• group stimuli together

• discriminate between similar stimuli

• detect when stimuli change and are constant

detecting light

photoreceptors are sensitive to certain wavelengths of light in 3 dimensions: hue, brightness, and saturation

hue

a dimension of light that defines the type of perceived colour, determined by the wavelength of light

brightness

a dimension of light determined by the intensity of light

saturation

a dimension of light that defines the intensity of colour, determined by the relative purity of light

sensory transduction

a process in which light is detected by photoreceptors (cones and rods) which convert the light into a neural signal. stimulus alters membrane potential creating a receptor potential

• most photoreceptors lack axons, some of their membrane forms a synapse with dendrites

visual pathway in the eye

light → cornea → lens → retina

• cornea and lens focus light onto the retina

  • ciliary muscle alters shape of lens to focus image

• pupils regulate amount of light that enters

• structures focus the image onto the retina

  • bones & muscles around eye

  • extraocular muscles attached to sclera

accommodation

the ability to focus, allowing clear vision at various distances, by the ciliary muscle changing the shape of the lens to focus the image

retina

posterior structure in the eye, consists of 3 layers: photoreceptive, bipolar, and ganglion cells

photoreceptors

specialised neurons in the retina

• rods (more): detect low light

• cones: detect colour and acuity

  • fovea only contains cones

  • optic disk at the back of the eye has neither

visual pathway in the retina

light signals → photoreceptors → neural signals → bipolar cells → ganglion cells → brain

• horizontal & amacrine cells integrate signals between photoreceptors and pathways

photopigment molecules

molecules in the visual pathway in the retina made of opsin (protein) and retinal (lipid), that transduce light waves into a membrane potential

• human rods have rhodopsin (rod opsin + retinal)

• retinal synthesised from vitamin A

light → rhodopsin molecule → hyperpolarisation of membrane → neurotransmitter release (decreased glutamate)

bipolar cells

within the visual pathway of the retina, have different responses to glutamate depending on type:

• light ON centre cells become hyperpolarised

• light OFF centre cells become depolarised

receptive field

areas in the front of the photoreceptor layer that allows for central and peripheral vision.

• location depends on the location of photoreceptor

vergence movements

a type of eye movement that helps separate environment from target

• keeps both eyes fixed on same target to focus on both retinas

saccadic movements

a type of eye movement that helps separate environment from target

• jerky eye movements used for scanning

pursuit movement

a type of eye movement that helps separate environment from target

• slower, smoother movement by following target/environmental movement

the visual pathway

1. in the eye: light (travels to the back) → photoreceptors → bipolar cells → ganglion cells (signal travels forward) → axons converge into the optic nerve

2. lateral geniculate nucleus (LGN) of thalamus

3. primary visual cortex (V1, striate cortex)

4. visual association cortex (V2, extrastriate cortex) + additional cortical areas

retinal pathways

additional pathways use visual info in other ways:

• pathways to hypothalamus involved in circadian rhythms

• control eye movements: iris constriction, ciliary muscles of lens

lateral geniculate nucleus (LGN)

6 layers of neurons:

• layers 1, 4, 6 receive input from contralateral eye

• layers 2, 3, 5 receive input from ipsilateral (same side) eye

3 layers, each containing different types of cells that process different aspects of visual info (e.g., motion, colour, detail):

• magnocellular layers (2 inner layers) → 4 Cα (striate cortex)

• parvocellular layers (4 outer layers) → 4 Cβ (striate cortex)

• koniocellular sublayers (under other types) → 2nd & 3rd layers of striate cortex

primary visual cortex (V1)

• processes basic features

• receives input directly from LGN + combines info from other areas

• contains neurons that respond to specific features of stimuli (coding)

  • firing rate depends on where stimulus is on receptive field

  • more coding organises further before sending to V2

• organised into modules that process info from diff visual fields

  • receive input from other modules → analysis → output to other modules

cytochrome oxidase (CO) blobs

a part of modules in the 2 & 3 layers of V1, surrounded by interblob regions

• receives input from parvocellular & koniocellular layer (LGN) (colour info)

• project to thin stripes in V2 (colour)

visual association area (V2)

combines input from V1 to build entire visual scene. contains 3 stripes:

• thin stripe (dark): colour

• thick (dark) & pale stripes: orientation, spatial frequency, movement, retinal disparity

and specialised regions:

• receives information from “lower” regions

• passes info to “higher” regions for higher processing

dorsal stream

a pathway from the V2 that processes where objects are located, and speed/direction of movement (action)

• receives from magnocellular pathway → parietal lobe

ventral stream

a pathway from the V2 that processes what an object is and its colours (perception)

• receives signals from all layers → temporal lobe

vertical occipital fasciculus

a white matter tract that connects the ventral and dorsal visual streams, allowing for exchange of info

perceiving light and dark

ON & OFF bipolar cells → (light/dark signals) → ganglion cells

ganglion cells

3 types:

• ON cells

• OFF cells

• ON/OFF cells

  • briefly excited when light is turned on/off

trichromatic theory

a theory of colour perception that states each of the 3 types of cones are sensitive to a single hue

• supported by physiology in primates: 3 types of cones that have diff peak absorption wavelengths

• can’t explain colour perception on its own

opponent-colour system theory

a theory of colour perception that states colours are presented as opponents

• explains why you can’t see reddish-green or blueish-yellow

trichromatic coding

changes in colour vision due to cone abnormalities

• some X-linked, leading to higher rate in XY

protanopia

first-colour defect in trichromatic coding

• red cones have green cone opsin

• see shades of yellow and blue

• red & green look yellowish

deuteranopia

second-colour defect in trichromatic coding

• green cones filled with red cone opsin

• confuse red and green

tritanopia

a rare genetic condition in trichromatic coding in which the individual lacks blue cones

monochromatic vision

a very rare genetic conditions in which the individual completely lacks cones

retinal ganglion cells

specialised cells supporting the opponent-colour theory that respond to opposing pairs of primary colours. 2 types of colour sensitive cells:

• yellow-blue

• red-green

• when centre on, surround off, firing rate increases

• when surround on, centre off, firing rate decreases

other cells are black-and-white detectors

red colour

stimulates its corresponding cone and excites red-green ganglion cells

green colour

stimulates its corresponding cone, inhibiting red-green ganglion cells

blue colour

stimulates its corresponding cone, inhibiting yellow-blue ganglion cells

yellow colour

an intermediate wavelength received by red and green cones

• excites & inhibits red-green ganglion cells (no change)

• excites yellow-blue ganglion cells

rebound effect

a change of firing rate shown in ganglion cells that are excited or inhibited for a long time

• (opposite-coloured apple) big green stimulus inhibits red-green ganglion cells, change to neutral stimulus, cells become excited & fire faster than normal

cortical processing of colour

ganglion cells end in different layers of LGN

• parvocellular layer receives wavelength info from cones & receive red and green cone info

• koniocellular layer receives wavelength info from cones & info from blue cones

• magnocellular layer cells colourblind, detect differences between light & dark movement

neurons from LGN → areas of V1 → areas of V2

perceiving form

V1 → V2 → ventral stream pathway

• neurons in V1 sensitive to particular spatial frequency (important for perceiving size & detail)

inferior temporal cortex

has associations with visual pattern & object recognition/identification

fusiform face area (FFA)

in the V2. associated with face perception & other areas of expertise

• expansion associated with age & performance

• pattern of activity altered in ASD and William’s syndrome

extrastriate body area (EBA)

overlaps with the fusiform face in the V2, active when perceiving silhouettes, stick figures, body parts (not faces or objects)

parahippocampal place area (PPA)

in the V2, active during perception of scenes and backgrounds

visual agnosias

a condition that results from damage to part of V2, leading to deficits in visual recognition, and discrimination in areas of expertise & details

prosopagnosia

the inability to perceive faces

• can occur at birth

• can be associated with differences in nearby structures (EBA) or connectivity

monocular vision

vision requiring 1 eye, allows for location perception using perspective & relative retinal size,

• loss of detail through effects of atmospheric haze & appearance of movements

binocular vision

vision requiring both eyes, allows for location perception using vivid depth perception (stereopsis)

• consists of most V1 neurons, project to posterior parietal cortex (dorsal stream)

• respond to retinal disparity, difference in retinal image that reveals change in depth

flat vision

impaired depth perception caused by damage to the parieto-occipital areas involved in processing retinal disparity

parietal lobe

receives auditory, somatosensory, and vestibular info

• involved in spatial & somatosensory perception

• damage can impair perception, memory of location, & influence movements of eyes & limbs

intraparietal sulcus (IPS)

a groove on the parietal lobe containing 5 important dorsal stream areas:

• LIP & VIP: control saccadic eye movements

• VIP & MIP: visual control of reaching & pointing

• AIP: visual control of grasping & manipulation

• CIP: depth perception

perceiving orientation & movement

• feature detectors → higher visual areas perceiving movement (medial temporal (MT) (← superior colliculus (visual reflexes)) & V5)

  • processing motion is faster than form & colour

feature detectors

neurons in the V1 that fire at most rapid rates to certain orientations of stimuli

medial superior temporal area

receives input from V5, responds to pattern of movement

• dorsolateral region involved in processing optic flow (how retinal image changes as it moves)

centre of expansion

the process by which the centre of field changes size instead of changing position

akinetopsia

damage to the bilateral V5, impairing perception of motion

form from motion

perception of movement that helps perceive 3D objects

• does not involve V5

• suggested association with right superior temporal gyrus