Lecture 6- Occipital Lobe

sensation vs perception

sensation: process of our sense organs receiving information abt the world

• raw information (meaningless)

perception: experience of information, compiled by brain

• unified & meaningful

characteristics of receptive fiels for eyes

some receptor cells detect change & constancy

• rapidly/ slowly adapting receptors

some distinguish self from other

• exteroceptive/ interoceptive

rapidly adapting receptors

detect change

slowly adapting receptors

detect constancy

exteroceptive

• respond to external stimuli

interospective

• respond to internal stimuli

processing steps

1. receptors connect to cortex through intervening neurons

2. some neural relays in each sensory system are in the spinal cord, others in the brainstem, neocortex, defining the hierarchy of motor cortex

3. at each relay point, behaviour related to that sensory system can be evoked

4. messages that sensory systems carry can be gated or modified

5. all sensory info from systems is encoded by AP that travel along PNS, until they enter brain

6. every bundle carries same kind of signal

7. region of processing & neuronal activation rates distinguish types of stimuli from each other in brain

perception

can be embodied or embedded

• embodied: influenced by other bodily states

• embedded: influenced by env and past experiences

• sensory impressions affected by contexts in which they take place

human eye

outside to inside order

conjunctiva > cornea > sclera > iris > lens > fovea > nervus opticus

cornea

clear membrane covering visible part of eye

• helps gather & direct incoming light

iris

color part of eye

• muscle that controls pupil size

pupil

opening in middle of iris

• changes size to allow diff amounts of light to enter eye

lens

transparent structure located behind pupil

• actively focuses or bends light as it enters the eye

what is accomodation in terms of the eye

process by which the lens changes shape to focus incoming light, so that it falls on the retina

retina

thin, light-sensitive membrane located at back o feye

• contains sensory receptors for vision

rods

long, thin, blunt light receptors

• sensitive to light, not colour

• for peripheral & night vision

cones

short, thick, pointed receptors

• detect colour

• for colour vision & acuity

from photoreceptors to optic nerve

light striking retina converted into neural energy by photochemical process

info from rods & cones collected by bipolar cells then ganglion cells

ganglion cell axons bundled together to form optic nerve, transmits information to brain

binocular vision

two overlapping fiels of view create perception of depth

why do we see in 3D? Depth cues

texture gradient

stereopsis

motion parallax

texture gradient

elements tend to appear more closely packed together as the distance from the view increases

stereopsis

ability to perceive 3D bc two eyes receive slightly different view of the world

motion parallax

• provides 3D information when an object is in motion

• points further from the retina will move more slowly than those nearer to the retina

astigmatism

when your cornea/lens has different shape than normal, creating two or more image points on back of eye

eye to occipital lobe

optic nerves from left & right eyes meet at optic chiasm, then partly cross over

left/right visual hemifield

anatomical subdivisions of occipital lobe

areas of visual cortex

V1, V2, V3, V4, V5

V1

segregates pattern vision from motor signals

V2

3D vision/ seeing camouflage/ more complex patterns

V3

shape perception

V4

colour area & shape perception

V5

motion area

what & where pathways

what- ventral

where- dorsal

early phase of visual perception

shapes & objects extracted from visual scene

• features, position, orientation, movement

later phase

shapes and objects are recognised

bottom-up processing

eye-to-brain

top-down processing

brain-to-perception

context & pattern recognition- bottom-up processing

• info from physical stimulus used to help recognise stimulus

  • start with small bits of info & combine them to form perception

context & pattern recognition- top-down processing

information from general context used to help recognise stimulus

• high level general knowledge contributes to interpretation of low-level perceptual units

greebles

when participants are shown figures that sort of resemble humans, their face area is activated

gestalt principles

• proximity

• similarity

• good continuation

• closure

Helmholtz’s theory

how does the perceptual system perceive patterns?

• likelihood principles

• unconscious inference

environmental regularities

characteristics in the environment that occur frequently

physical regularities

oblique effect

light-from-above assumption

oblique effect

horizontals & verticals occur more often, so oblique angles are harder to perceive

light-from-above assumption

we always assume that the light is shining from above (in terms of lighting and shadowing)

semantic regularities

functions common to a scence

scene schema

knowlege of what a scene typically contains

bayesian inference

prior probability: our beliefs about the probability of an outcome

• based on likelihood of that outcome occurring

specialised neurons in occipital lobe

respond only to horizontals & verticals

object recognition

• feature analysis

• template model

feature analysis

detecting features and their combinations

• stimuli are thought of as combinations of elemental features

• we can perceive an object bc we store their features in our LTM

• we compare retinal image and see what object in LTM has most features that match object

template model

comparing whole images to past experiences

• features are simpler

• computationally less intensive

• flexible in terms of real variation

visual processing styles

field-dependence

field- independency

field-dependence

tendency to derive info from context as primary means of learning

field- independency

a tendency to derive focal information as primary means of learning

culture influences processing style

SEA cultures are more context-dependent

blindsight

lacking conscious awareness of visual experience, but being able to respond to those experiences anyways

• caused by damage to V1

visual agnosia

inability to recognise visual objects, which is neither a function of general intellectual loss, nor loss of basic sensory abilities

• apperceptive agnosia

• associative agnosia

• prosopagnosia

• visuospatial agnosia

apperceptive agnosia

unable to recognise objects, draw, or copy a figure

associative agnosia

difficulty understanding meaning of what they are seeing

• can draw or copy, but don’t know the meaning

prosopagnosia

face blindness

• affected FFA

visuospatial agnosia

difficulty with spatial relationship between objectss