Vision Flashcards
1
Q
LIGHT SOURCES
A
- sun/stars/heated objects/bioluminescence
- sunlight filtered via atmosphere; reflected from surfaces
- light = electromagnetic energy w/wave properties (ie. light ray radiation) & charged particles (photons/quanta)
- difs in intensity/wavelengths
2
Q
PSYCH PROCESS REGULATION IN DAY/NIGHT
A
- light levels detected via eye sent to SCN (suprachiasmatic nucleus) in hypothalamus
- special ganglion cell class in retina containing photopigment melanopsin (sensitive to short wavelengths (blue light); keeps circadian clock in SCN accurately timed w/natural daily light cycles
- pineal gland = unpaired midline structure near epithalamus; produces melatonin (from serotonin) during darkness
3
Q
HUMANS/ANIMALS’ IMAGE-FORMING EYES
A
- eyes evolved via gradual improvement sequence for detecting directions/forming image
- advanced eye types evolved several times in animal kingdom
- fossil records date back to Cambrian explosion (540mya)
- faster movement/navigation in animals required better vision
4
Q
VISION
A
- detecting/interpreting electromagnetic radiation patterns observer is exposed to
- image-forming eye/visual pathways in observer’s brain
- moving eyes to analyse/acquire visual info
- execution of visually-guided behs/visual memory recall
5
Q
SEEING
A
- conscious/unconscious visual perception detected in beh responses/described via language (in humans)
- requires light presence in visible range of wavelength spectrum (can’t see if too dark)
6
Q
HOW DO WE SEE?
A
- visual field = object/person seen against background in area of space
- retinal projection = inverted 2D image distorted by eye curvature
- perceived image = 3D/large/upright/stable/non-distorted/colourful
- projections onto:
1. retina = 1d visual angle = 0.288mm (ie. thumb nail when extending arm = 1.5d)
2. fovea = 0.6mm
3. horizontal retina = 32mm
7
Q
FIRST PROCESSING STEPS IN RETINA
A
- photoreceptors/bipolar cells = graded potentials
- ganglion cells = long axons form optic nerve; action potentials
8
Q
VISUAL PATHWAYS
A
- projections from retina to other brain areas
GENICULATE-STRAITE PATH
retina -> LGN (lateral geniculate nucleus) of thalamus -> VI (primary visual cortex) -> higher visual cortex areas (90% retinal projections) - VI required for conscious visual experiences
EXTRAGENICULATE PATH
retina -> SC (superior colliculus) -> several projections to areas of higher visual cortex/pulvinar nucleus of thalamus (eye movement control/visual attention; 10% retinal projections)
9
Q
VISUAL FUNCTIONS IN BLIND HUMANS/PRIMATES
A
STOERIG (1999)
- VI damage causes cortical blindness (conscious vision loss); patients performed visually-guided behs (ie. grasping/pointing to object location/avoiding obstacles) correctly STATSIG
- this is blindsight
10
Q
LIGHT PROJECTIONS ON RETINA
A
- vision starts w/formation/processing of images in eye
- rod/cone cells form 2D array in retina
- human retina = ca100m rods/4m cones/1m ganglion cells
- 108MP (megapixel) modern camera = poor technical imitation of retina; has much larger sensor area/more sophisticated processing circuits
11
Q
FIRST STEPS IN IMAGING PROCESSING
A
- Lens to focus image
- Aperture to control light entering (iris)
- Pixels to register image (photoreceptors)
- Filtering media (glass body/macula/pigment)
- Filter to protect lens (cornea)
- Lens cover for when not used (eye-lid)
- Cleaning mechanism (tears)
- Processing algorithms (retinal interneurons)
12
Q
DIM-LIGHT VISION (RODS)
A
- doesn’t use central fovea
- acuity = proportional to receptor cell density
- vision acuity = highest in fovea; decreases towards retina periphery
- eye movements position fovea in visual field positions where most important = collect high-acuity info
- at night high acuity sacrificed for sensitivity; more advantageous to have no rods in fovea
13
Q
COPING W/CHANGING LIGHT LEVELS
A
- duplex retina in vertebrate eye
- cones specialised for day vision (1-100m times brighter in sunlight > moonlight)
- rods specialised for night vision
- both detect light in similar way (opsins/metabotropic transduction) BUT rods = ^ sensitive
- opsin = light-sensitive protein (G-protein coupled receptor molecule) in photoreceptors’ membrane; bound to chromophore retinal (for transduction)
- 3 functional cone classes: S-/M-/L-
- cone opsins differ in wavelength; specific affinity to absorb light (S/M/L opsins); only one type p/cone
- 1 functional rod class = same opsin (RHI (rhodopsin))
14
Q
EYE MOVEMENT
A
- saccades (jumps)/fixations (stops)
- 2-3 saccades p/second
YARBUS (1914-1986) - developed first methods to accurately measure eye movements/viewing beh
- direct fovea to collect info about visual scene
15
Q
CONTROLLING EYE MOVEMENT
A
- field of view defined by position/orientation of eye ball/head/body
- can move eyes/head separately; many animals cannot (ie. insects/birds); move head/body to see
16
Q
STABILISING GAZE FOR BETTER VISION
A
- movement described as combo of 3 translation/rotation directions each
- larger/faster head movements render vision blurry when eyes can’t compensate
17
Q
DIFFERENT EYE MOVEMENTS
A
SACCADES
- eye moves v quickly to new position between periods of gaze stabilisation (fixations) to scan scene across entire view field
SMOOTH PURSUIT MOVEMENTS
- slower; keeps moving stimulus on fovea
OPTOKYNETIC NYSTAGMUS
- brings eye back from peripheral to more central position after following large-scale moving stimulus (while head static)
VESTIBULO-OCULAR MOVEMENTS
- compensate for head movement by moving eye same distance but in opposite direction to maintain constant field of view
18
Q
ATYPICAL EYE MOVEMENT: DYSLEXIA
A
PRADO et al (2007)
- difficulties in reading words/sentences/text
- longer/more durations of fixations during reading; shorter saccades
- shorter visual attention span impacts eye movement patterns
19
Q
ATYPICAL EYE MOVEMENT: SCHIZOPHRENIA
A
BENSON et al (2012)
- difficulties tracking objects w/smooth-pursuit eye movements
- rapid/jerky eye movements
- complex analysis of eye movements using mathematical modelling = possibly future avenue for developing diagnostic tool
20
Q
BRAIN CIRCUIT FOR SACCADIC EYE MOVEMENTS
A
- saccadic eye movements directed via midbrain/cortex
- conscious control of eye movements comes from FEF (cortical frontal eye fields)
- automatic control of eye movements comes from superior colliculus
- both use vision input BUT also auditory/somatosensory systems
21
Q
COMPLEX VERTEBRATE RETINA STRUCTURE
A
- functional classes of retina cells:
1. 4 photoreceptor classes (3 cones/rods)
2. 50-70 horizontal/bipolar/amacrine cell classes
3. 20-30 ganglion cell classes - first visual processing stages:
1. Edge detection in visual scenes.
2. Edge enhancement in patterns.
3. Filtering of spatial/wavelength/movement/directional info
22
Q
LATERAL INHIBITION IN RETINA
A
- photoreceptors in retina inhibit neighbours via bipolar/horizontal cells
- edges enhanced for better detection/object discrimination/foreground/background in visual scenes
- if light falling on retinal neuron group = uniform, their reciprocal inhibitions cancel each other out w/o effects
- when edge (dark/light illumination) created, cells on both sides strongly influence each other; changes signals so much stronger contrast coded than physically existing
- more distant cells unaffected so edge perception = enhanced
23
Q
IDENTIFYING SPATIAL RELATIONSHIPS/OBJECT PROPERTIES
A
- w/o context cues we perceive physical reflectance of surfaces carrying little info
- edges/shadows provide context info about object spatial structure/spatial relationships between objects
24
Q
SEGRAGATED ROD/CONE-CONNECTED PATHWAYS IN RETINA
A
HORIZONTAL CONNECTIONS
- horizontal cells
- amacrine cells
VERTICAL CONNECTIONS
- fovea = 1 cone: 1 bipolar
- periphery = many cones: 1 bipolar; many bipolars: 1 ganglion/rods BUT connect to rod bipolar cells/other ganglion cell classes
- cones/rods converging on bipolar cell form its receptive field; similarly ganglion cell field formed via all converging bipolar cells
25
IMPORTANT FILTERS IN RETINA TO DETECT/ENHANCE EDGES
- objects can be dark against bright background/bright against dark background
26
ON/OFF CENTRE CELLS FILTER VISUAL INFO
- ON/OFF-centre bipolar cell = non-spiking
- ON/OFF-centre ganglion = spiking
OFF/ON SURROUND
- formed by lateral inhibition from neighbouring cells (ie. cones) that surround cell representing receptive field centre
27
ON/OFF CENTRE CELLS x LIGHT/DARK RATIOS
- respond to light/dark ratios
- at rest, ganglion cell fires action potentials (spikes) at spontaneous rate
- ON-centre bipolar depolarisation -> ON-centre ganglion cell increases spike rate
- OFF-centre bipolar cell hyperpolarises -> OFF-centre ganglion decreases spike rate
28
GANGLION CELLS x UNIFORM ILLUMINATION
- don't respond
- when light spot covers whole ON-centre, ganglion responds w/highest spike rate
- when light ring covers all surround BUT not ON-centre, ganglion responds w/lowest/no spike rate
- when whole receptive field equally stimulated, ganglion rests/fires w/spontaneous frequency
29
INVERTED RESPONSES IN OFF-CENTRE GANGLIONS
- also respond to light/dark ratios BUT not uniform illumination
- inverted responses
- if spot illuminates centre of OFF-centre ganglion, spike rate reduces
- if spot illuminates ON-surround of OFF-centre ganglion cell, spikes increase
- if whole receptive field illuminated, ganglion rests/fires spontaneously
30
RETINA AS COMPLEX NEUROPILE W/MANY CIRCUITS
- blind spot = exit of ganglion cell axons/optic nerve in back of eye
- first visual path stage = photoreceptors/retinal neurons
- retina performs first filtering/decomposition of scene; codes local/global contrasts; detects/enhances features ie. edges; lateral inhibition/vertical & horizontal connectivity from circuits needed for tasks
- early processing circuits (ie. centre-surround receptive fields) help separate coding of visual features in parallel paths/streams in visual system
- we don't notice consciously most of seeing BUT paradoxically we can sometimes become aware (ie. visual illusions)
31
P/M-GANGLION CELLS PROJECT TO DIFFERENT LAYERS IN LGN
P-GANGLION CELLS
- project to parvocellular layer in LGN
- small RFs; slower conduciton speed; high acuity; poor response to transient stimuli; colour sensitive
M-GANGLION CELLS
- project to magnocellular layer in LGN
- large RFs; higher conduction speed; motion sensitive; low acuity; no colour discrimination
32
P/M-PATHWAYS FROM RETINA TO VI
- p-pathway = parvocellular (small soma)
- m-pathway = magnocellular (large soma)
- neurons of both project to dif VI layers:
- m-cells project -> layer 4ca
- p-cells project -> layer 4c & 2/3 interblob
33
SPATIAL LAYOUT OF RETINAL GANGLION CELL PROJECTIONS
- preserved
| - retinal ganglion cells project retinotopically to each LGN layer; right/left eye projections also segregated in LGN
34
NEURON RESPONSES IN VI ORIENTATION COLUMNS
- when recording from neurons of particular orientation column, some neurons respond to orientation columns only in small visual field part corresponding to their receptive field
- dif to retinal ganglion cells; neurons fire at maximal spike rate when bar stimulus shows their preferred orientation
- other VI cortical cells respond w/maximal spike rate to preferred motion direction of bars/patterns
35
SIMPLE/COMPLEX CELL FUNCTIONS
- analysis of contours/boundaries/objects
- shape/positional invariance
- contour enhancement for object identification
- VI fundamentally important for conscious vision/perception
36
VI COLUMNAR STRUCTURE
- along w/6 horizontal layers, neurons in VI further segregated into functionally distinct hypercolumns
- hypercolumn (1mm^2) composed of:
1. 1 left & right eye ocular dominance column
2. several orientation columns (rainbow colours) containing simple/complex cells that respond to shape orientations (ie. a bar)
3. blobs (drawn via cylinders) = structures in layers II/III of VI involved in colour vision
- retinotopic organisation; spatial mapping arising from projection of image onto retina preserved in VI
37
2 VISUAL STREAMS IN PRIMATE/HUMAN BRAIN CORTEX
PARIETAL CORTEX
- dorsal stream/pathway = interacting w/world via V5/MT
INFERIOR TEMPORAL CORTEX
- ventral stream/pathway = making sense of world via V4
38
V4
TANAKA et al (1991)
- responds to more complex stimuli than V1/V2
- strong responses in V4 (red/orange strongest)
- strong responses in anterior area of inferior temporal cortex
39
OBJECT RECOGNITION
DI CARLO et al (2012)
- discrimination (<200ms)
- object recognition also when object position/size/viewpoint/visual context changes
- categorisation
- ventral cortical stream = critical for object recognition
- V1-V4 = occipital lobes
- IT = inferior temporal cortex (temporal lobes)
- colour-portion dedicated to central 10deg of visual field response latencies
40
EYE-HAND CO-ORDINATIONQ
- guiding hand movements requires 2 processes:
1. Deciding which objects to interact w/.
2. Interacting w/objects skillfully.
- require dif info type from both dorsal/ventral streams
41
SUMMARY I
- retina = 1st visual processing stages (edge detection in visual scenes/edge enhancement in patterns/spatial filtering/wavelength/movement/directional info)
- lateral inhibition in retinal cells responsible for edge enhancement (Matchband effect)
- edges/shadows provide context info about spatial structure of objects/spatial relationships between objects
- cones/rods converging on bipolar cell form its receptive field; cones/rods/bipolars converging to ganglion cell form ganglion's receptive field; fields are often larger in periphery (blurrier vision via low acuity)/smaller in fovea (helps achieve highest acuity)
- some bipolar/ganglion cell classes have centre-surround receptive field; can be ON-centre/OFF-surround (lateral inhibition from surround receptors) or OFF-centre/ON-surround (lateral inhibition from photoreceptors in centre of centre-surround receptive field)
42
SUMMARY II
- ganglion cells respond to light/dark rations (ie. small light dot) but not to uniform illumination
- P/M ganglion cells project to dif layers in LGN/V1; have dif properties (receptive field sizes/conduction speed/acuity/presence/lack of colour sensitivity)
- P/M ganglion cells project retinotopically to segregated in hypercolumns which combine orientation/ocular dominance columns for each part of visual field; all neurons in orientation column share same preference for particular orientation of bar stimulus in their receptive field; within hypercolumns orientation columns found together if they receive input from either left/right eye forming left/right eye pair ocular dominance columns in each hypercolumn
- simple cortical cells (aka bar/edge detectors) = respond best to edge/bar of particular width/orientation/location in visual field
- complex cortical cells = respond best -> bad/particular size/orientation anywhere in particular visual field area
