CH 6-Vision Flashcards
(46 cards)
1
Q
cornea
A
- transparent outer covering of eye where light first enters
- 15% of outer eye
- works w lens to bend light onto retina
2
Q
sclera
A
opaque, white part of outer eye
-makes up 85% of outer eye
3
Q
pupil
A
- hole in middle of iris that regulates amount of light entering
- uses sphincter muscles to constrict in bright light for better acuity, sharpness and depth of focus, though decreased sensitivity
- uses dilator muscles to let in more light in dim conditions; decreased acuity but increased sensitivity
4
Q
lens
A
- layer of transparent cells just behind pupil that focus light onto retina
- for near objects, ciliary muscles contract, releasing the tension on lens ligaments to hold it in a flat shape
- for far objects, ciliary muscles relax to increase tension on ligaments which flattens lens
- adjustments are reflexive
5
Q
retina
A
- located at back of eye, site of visual transduction
- 5 different cell layers (photoreceptors, horizontal, bipolar, amacrine, retinal ganglion (RGC))
- signals travel through vertical pathway back to front
- horizontal and amacrine cells allow for lateral communication
6
Q
photoreceptors
A
- rods and cones
- first retinal layer, located at the back of the retina
7
Q
horizontal cells
A
- second retinal layer
- responsible for lateral communication
8
Q
bipolar cells
A
- third retinal layer
- connect photoreceptors to RGC
9
Q
amacrine cells
A
- fourth retinal layer
- responsible for lateral communication
10
Q
retinal ganglion cells
A
- fifth and final retinal layer
- axons gather at and exit at optic disk ad the optic nerve, creating a blind spot
11
Q
(visual) temporal integration
A
-add together foveal images from preceding fixations and ensures blinks do not disrupt vision
12
Q
an overview of the pathway from retina to V1 (retinal geniculate striate pathway)
A
retina
optic nerve
lateral geniculate nucleus (LGN)
striate visual cortex (in V1, primary visual cortex)
13
Q
LGN (lateral geniculate nucleus)
A
- 6 layers
- receive input from contralateral visual field (3 layers for the visual field area of each eye)
- fovea has disproportionately large representation in striate cortex (25% of the cortex to be exact) due to high convergence of cones
14
Q
retinotopic organization
A
- top visual field is processed in bottom portion of striate and vice-versa
- 2 stimuli at adjacent areas of the retinal will activate adjacent neurons in LGN or V1
15
Q
M channel
A
magnocellular layers: bottom 2 in LGN
- big cell bodies
- respond to movement
- primarily receive input from rods
- project to top part of striate 4 neurons
16
Q
P channel
A
- parvocellular layers: top 4 in LGN
- small cell bodies
- respond to colour, slow moving/stationary objects, fine detail
- primarily receive input from cones
- project to bottom of striate 4 neurons
17
Q
mach bands
A
- non-existent stripes of brightness and darkness
- run adjacent to edges of real stripes
- enhance contrast at each edge to make edges easier to see
- our perception of an edge is better than the real thing
- contrast enhancement
18
Q
visual edge
A
- perception of contrast btw 2 adjacent areas of visual field
- informative feature (defines position of objects, perception of edge gives perception of contrast, is common across all species)
19
Q
contrast enhancement: horseshoe crab model
A
large photoreceptors called ommatidium, each with their own axon, but all connected through the lateral plexus
-when a single cell fires, it inhibits its neighbours slightly
20
Q
lateral inhibition
A
activity of photoreceptor cells increases with light brightness
- along the edge, cells fire more if they are brightly lit than if they are on the darker side of the edge
- cells on edge with light receive less neighbour inhibition, and more strongly inhibit their darkside neighbours
21
Q
Receptive fields (RGC, LGN and lower layer 4 of striate)
A
hubel and weasel studied cat neurons
- discovered the neurons respond optimally to lines of a certain orientation, and the further from the ideal stimulus, the lower to response
- neurons at all levels have circular receptive fields and are monocular (only respond to info from one eye)
- centre surround cells most common
- neurons responding to fovea have smaller receptive fields than in periphery
22
Q
centre-surround neurons
A
- neurons respond w on or off firing depending on where the light falls within the circular receptive field
- on firing means that they increase their response when light reaches the receptive field
- off firing means the neurons increase their firing rate in response to an absence of light in the receptive field
- seem to respond best to contrast
23
Q
on centre cells
A
- increase firing when light strikes the centre of the receptive field (on firing)
- light in periphery causes off firing inhibition (it doesn’t like it so it won’t respond basically)
- when light in the periphery is turned off, neuron responds with activity bursts
24
Q
off-centre cells
A
- respond to light in centre of receptive field by decreasing their firing
- respond to light in peripheral receptive field by increasing firing
25
receptive fields of simple cortical cells
striate neurons (layer 4 LGN) respond to straight edges
- rectangular receptive fields; have on/off regions (strips) for straight lines, unresponsive to diffuse light
- monocular
- respond to lines of particular orientation, position, respond best to bars of light in dark fields and vice-verse
26
hierarchical model
-complexity of receptive fields at higher levels of processing is from convergence of info obtained in preceding levels (neurons with simple preference converge on cells with more complex preference)
27
receptive fields of complex cortical cells
- rectangular receptive fields
- respond best to straight line stimuli in particular orientations (regardless of position in receptive field, and respond continuously if the edge is moving through the field (do have preference for direction of motion))
- unresponsive to diffuse light
- larger than in simple cells
- no static on/off regions
- many are binocular, but show ocular dominance
- fire more robustly when the preferred stim is presented in both eyes simultaneously, though at slightly different retinal positions (so these cells likely play a role in depth perception)
28
columnar organization
as we move lower, simple receptive fields become complex, and we move from on/off centre cells to simple cells to complex cells
-cells in a column have receptive fields in the same general area of visual field and prefer straight lines in the same orientation (moving horizontally through a layer, receptive fields respond to different locations in the visual field and to lines of slightly different orientations; successive cells alternate wrt eye dominance)
29
ocular dominance
V1 divided into functionally independent columns responsible for analyzing input from a particular area of the visual field
-each cortical column can then be subdivided for ocular dominance, and those halves further divided for preference of lines at a particular location
30
achromatic hues
black-lack of white light
white-intense mixture of all wavelengths in equal proportion
gray-equal proportioned mixture of wavelengths at lower intensities
31
chromatic hues
depend on wavelengths of light reflected into the eye
- rare for object to absorb only one wavelength
- most objects absorb a few and reflect the rest; the mixture reflected influences our perception of colour
32
component process (trichromatic theory)
- 3 diff colour receptors, each w diff spectral sensitivity
| - any colour in spectrum visible by mixing 3 different wavelengths of light in different proportions
33
opponent processing
-complementary colours cannot co-exist in the same colour
-component processing cannot explain why the afterimage of red is green or vice-versa
2-classes of cells encode colour, 3rd class encodes brightness; each one encodes complementary perceptions (ex. a cell may signal red by firing in one direction (ex. depolarization) and green by firing the opposite (hyperpolarization))
34
component or opponent?
BOTH exist w/in our visual system
- microspectrophotometry confirms existence of 3 different types of cones responding maximally to diff wavelengths
- all other levels of the geniculate striate system have opponent processes
35
colour constancy
objects are still perceived as the same colour, despite major changes in light reflected when placed in diff lighting conditions, so long as the objects are in a natural scene and s-, m-, and l- wavelengths are all still reflected in some proportion
36
land's retinex theory
colour is determined by reflectance
-cortical neurons calculate contrast through dual opponent colour cells (circular receptive field, one firing when centre illuminated w one wavelength and surround illuminated w its complement (off firing in reversed illumination conditions))
37
scotoma
damage in one hemisphere in V1, often caused by caused by combat wounds
- causes area of blindness in contralateral visual field of both eyes
- perimetry used to determine area of blindness; patient stares at a fixation point with one eye open, point moves and they respond when they see the dot
- generally causes damage in periphery
- patients often report seeing whole objects event though only part is in the visual field (completion!)
38
blindsight
seeing without conscious awareness
- patients with complete lesions of V1 report total blindness, but can still perform visually guided tasks such as grabbing for moving objects or indicating direction of motion
- because their eyes are fully functional still, some signals bypass V1 and go to the prestriate cortex, mediating spared visual function
39
secondary visual cortex
- input mostly comes from V1
- 24 subsections, each one retinotopic, responding to different stimuli with hypercomplex cells (respond to stim of certain shape, size, length and movement; receptive fields larger than earlier in the hierarchy)
- important areas are peristriate cortex (band surrounding the striate) and inferotemporal cortex (in the inferior temporal lobe)
40
association visual cortex
outside of primary v areas
- performs higher visual function, gives meaning
- developed area that increases in size as the complexity of the animal increases
- recognizes objects rather than features
41
dorsal stream
"where" pathway (perception of where objects are) OR, controls behavioural interactions with objects
V1, dorsal prestriate, posterior parietal cortex
-in ppc, neurons respond robustly to spatial stimuli (location, movement), damage to this area results in difficulty reaching for an object
-fed by M pathway input
42
ventral stream
"What" pathway OR, mediate conscious perception of objects
V1, ventral prestriate cortex, inferotemporal cortex
-damage to inferotemporal cortex results in inability to identify objects
-fed by P pathway
-responsible for perception of shape and colour (clusters of neurons respond to classes of objects (ex. faces, letters, animals etc))
43
achromotopsia
inability to see colours, only shades of grey
| -caused by damage in V4, which is crucial for processing colour and shape info
44
prosopagnosia
- failure to recognize faces, or specific objects that belong to complex classes (ex. birds, cows, doors)
- with face deficiencies, they can usually recognize that a face is a face, but can't recognize individual faces or read facial expression; results from damage to inferior prestriate cortex and adjoining portions of inferotemporal cortex
45
fusiform face area
- area dedicated to processing human faces, though may just be a product of being raised surrounded by faces
- increases activity when viewing familiar faces and activity is generally selective for human faces
- stimulation can make facial features morph, TMS inhibits facial recognition
46
akinetopsia
inability to perceive fluid motion
- caused by uni or bilateral damage to MT (V5, in dorsal stream)
- can be produced w TMS
- patients see the world in snapshots, which makes pouring drinks and crossing the street challenging, but are still able to identify shapes, judge distance and see colour
