VISION Flashcards
1
Q
what is the difference between sensation and perception?
A
- sensation: the effects of a stimulus on the sensory organ e.g. eyes, nose, mouth etc
- perception: the elaboration and interpretation of a sensory stimulus
- in vision sensation is light that comes to eyes & perception is our interpretation of this
2
Q
How does the brain construct our perception of the world through visual experiences?
A
- vision is an inferential constructive process> you don’t see what’s there, you see what’s most likely there based on neural state & assumptions built into visual system
3
Q
what happens in our visual pathway
A
- light enters to eyes > image formed in retina > retina has photoreceptors that capture light energy> transform to neural activity
- Neural signals generated by photoreceptors > sent to the optic nerve.
- those signals get transmitted down visual pathway> first to thalamus then to occipital lobe = process & interpret info
- the brain determines what is where in the world
4
Q
what is the primary visual cortex?
A
- primary visual cortex: basic visual features detected & initial processing of visual info occurs > gateway for flow of visual signals through visual processing hierarchy in brain.
- fundamental aspects of visual stimuli e.g > edges, colours, & orientations, are processed > more complex visual processing occurs in higher visual areas.
5
Q
what is the structure of the eye?
A
- lens: gathers light from world & projects 2d image on retina
- retina: contains receptors that react to light> transmit signals to rest of brain
- optic nerve: where all axons from ganglion cells come & exit eyes > no photoreceptors= blind spot
6
Q
what is the retina?
A
- contains serval layers of neurones & synapses> has photoreceptors (cones & rods) that react to light= transmit signals to other neurones
- after processing> ganglion cells > send signals to brain through optic nerve
- photoreceptor form synapses w/ bipolar cell neurones > form synapse w/ ganglion cell = signals to brain through optic nerve
7
Q
what are photoreceptors?
A
- specialised cells found in retina > sensitive to light
- cones: daylight vision, in fovea (centre of vision), good for high resolution vision & colour
- rods: nighttime, not fovea (everywhere else except the blind spot), low res, no colour (BW), low light conditions, more in peripheral regions of retina
8
Q
what is transduction? (what happens in retina)
A
- transduction in retina = conversion of light energy into electrical signals, which are then transmitted to the brain for further processing and interpretation
- When light strikes photoreceptor cells, > causes a chemical change in the pigments = a change in the cell’s membrane potential > generates electrical signals in the form of nerve impulses
- electrical signals are then transmitted to bipolar cells > & eventually to ganglion cells
- Ganglion cells are the output neurons of the retina = further transmitted through the optic nerve & thalamus > then brain
9
Q
what happens in the optic tract?
A
- The optic tract = bundle of nerve fibers that continues from the optic nerve > the optic nerve splits into two optic tracts, one from each eye
- These tracts carry visual info to diff parts of the brain > thalamus and other visual processing areas
10
Q
what are receptive fields?
A
- specific sensory areas activating neurons > Found in all sensory neurons
- Represent regions in sensory space.
- Example: In vision, neurons in the visual cortex respond to patches of the retina.
- Crucial for processing sensory information
- in ganglion cells & LGN: small & respond to presence or absence of light in visual field
- in primary visual cortex: small but more complex> respond to orientation of lines
11
Q
how are receptive fields identified/mapped?
A
- David Hubel & Torsten Wiesel
- take neuron in brain >stimulate receptor > i.e show light in diff part of visual field = see if there were action potential > high firing rate = that neuron was active
12
Q
what is meant by vision is hierarchical?
A
- brain starts by processing most simple features & works its way up as processing continues
- progressive increase in receptive field size at higher cortical lvl & receptive feed represents aspects of visual field that are progressively more complex
- visual hierarchy > flow of info from basic visual features to more complex & abstract representations
13
Q
what is meant by vision is modular?
A
- functional specialisation of areas of cortex> specific areas of brain deal w/ specific information (e.g. colour, movement)
- specialised modules > arranged hierarchically
14
Q
what is associate cortex?
A
- cortical areas involved in higher-level processing & interpretation of visual info
- processes complex visual info > contributing to tasks e.g. object recognition, contextual understanding, & spatial awareness.
- While primary visual cortex (located in the occipital lobe) processes basic visual features e.g. edges, colors, & orientations > association cortex takes on more complex tasks related to visual perception & cognition.
15
Q
how do we know about vision beyond visual cortex?
A
- single cell recordings are much rarer > recordings in animals also rarer cos can’t report what they see
- we know from patient studies, fMRI, neuropsychological cases etc
16
Q
what is v4?
A
- V4 (Striate Cortex): Specialised for colour processing & form perception > Posterior part of the occipital lobe along ventral pathway.
- we now from patients studies w/ damage to v4 = Achromotopsia > inability to see colour after cortical damage
- colour is also processed in other parts of visual pathway > (inc retina, v2, LGN..)
17
Q
what is v5?
A
- V5 (Middle Temporal Area or MT): Specialised for motion processing & visual motion perception > Middle temporal gyrus, extending into superior temporal sulcus.
- Akinetopsia: motion blindness> inability to see objects in motion after corticall called damage= damage in v5
18
Q
what ventral pathways?
A
- further specialised processing in dorsal & ventral pathways
- ventral = (What Pathway)
- Function: Processes info for object recognition & identification
- Characteristics: Analyses shape, colour, & texture > perception of “what” aspects of visual info
- recognition of objects, faces, and scenes
- Location: Extends from primary visual cortex (v1) to inferotemporal cortex.
19
Q
what are Dorsal pathways?
A
- Function: Processes spatial information & guides actions.
- Characteristics: Analyses motion, depth, & spatial relationships.
- Location: Extends from primary visual cortex to posterior parietal cortex.
- uses info abt size, shape, location of an object to perform an action e.g. grasp
- where/how pathway
20
Q
what happens when there is damage to ventral visual pathway?
A
- visual agnosia = disorder of visual object recognition
- person can ‘see’ - perceive colour, movement + understand there’s object infront of them but don’t know WHAT the object is
- no problems with words > can say words or w/ memory can identify object from verbal description
21
Q
what is visual agnosia>
A
- disorder of visual object recognition
- Lissauer (1890) divided visual agnosia into 2 types:
- Apperceptive (form) agnosia: inability to perceive shapes & objects> shape discrimination > i.e. recognising, matching, copying (can’t copy shapes) visual stimuli
- Associative agnosia: perception is ok but can’t associate w/ meaning of object < can copy shapes etc but don’t know what it is
22
Q
what happened to patient DF?
A
- suffered brain damage > ventral pathway> Apperceptive (form) agnosia: inability to perceive shapes & objects
- Severe perceptual & object recognition problems: e.g. called a cup an ashtray; a fork a knife; couldn’t identify any drawings of common objects
- walked normal despite visual agnosia > was able to do appropriate grasp for tools e.g. ask for knife or cup was able to grasp
- done posting experiment: letter in hand & slot to. ‘ post letter’ > 2 conditions perception & action =. perception condition> couldn’t tell orientation of slot but in action condition was posting letter correctly = dorsal system working fine even though ventral damaged
23
Q
what happens when there is damage to dorsal pathway?
A
- optic ataxia: difficulty reaching & pointing > unable to scale grip size to match object size
- unable to grasp object at appropriate places
- no deficit in object recognition or in action pantomime (mimic actions)
24
Q
how do we process faces?
A
- Face Detection: brain is sensitive to facial features from an early age. e.g. Newborns preferentially look at faces & show a preference for face-like stimuli.
- specific brain areas, inc > fusiform face area (FFA) & occipital face area (OFA) > involved in early face detection.
- Face Perception: brain processes facial components, such as eyes, nose, and mouth, holistically rather than as individual parts.
- easier to recognise people we know in lots of diff distortions e.g. blurred pics etc
- fusiform face area found to have more activity in recognition of faces than objects
25
what happens when we lose inability to recognise faces?
- prosopagnosia: inability to recognise faces
- can describe faces but not recognise ppl the knew before
- nothing wrong with their intellect or vision
- acquired prosopagnosia: after brain damage > lose ability to recognise faces
26
what is the optic chiasm?
- Acts as a crossing point for some nerve fibers, ensuring information from both eyes is processed together in the brain
- fibers from each eye cross to the opposite side > image from left visual field is directed to right visual cortex
