1
Q
path of image processing from the eyeball to the brain
A
- eye (vertical path): photoreceptors, bipolar cells, RGCs
- LGN
- Striate cortex (V1)
2
Q
What’s a grating orientation task?
A
- a way to measure visual acuity
- uses gratings; composed of cycles and illustrate contrast
3
Q
what does 20/20 vision mean?
A
distance at which a person can identify the letters/distance at which a normal person (young adult) can see them
*general population: 15 feet
4
Q
acuity
A
- sharpness of vision, smallest visual angle of a cycle of grating that can be perceived
5
Q
where is visual acuity better?
A
in the fovea
6
Q
visual crowding
A
- deleterious effect of clutter on peripheral objec recognition
- stimuli that can be seen in isolation in peripheral vision become hard to discern when other stimuli are nearby
- we need to move our eyes to look at the object with foveal receptive fields
7
Q
sine wave grating
A
- black and white stripes
- will appear grey if you’re far away enough because the visual system “samples” the grating discretely; PERCEPTION DEPENDS ON WHERE THE CONES FALL
8
Q
vertical meridian asymmetry
A
your peripheral acuity is better horizontally than vertically
9
Q
why sine gratings?
A
- patterns of stripes with fuzzy boundaries are common in nature (ex: trees)
- the edge of any object produces a single stripe
- the visual system breaks down images into a vast number of components (spatial frequency)
10
Q
spatial frequency
A
- number of times a pattern repeats (a cycle) in a given unit of space (degree of visual angle)
- measure as a number of cycles per degree of visual angle
11
Q
visbility of a pattern is a function of…
A
spatial frequency and contrast
12
Q
can contrast sensitivity vary?
A
yes (age, lighting conditions, etc.)
13
Q
phase (of a grating)
A
position of the grating within a receptive field
14
Q
how do retinal ganglion cells respond to sine waves pattern?
A
- depends on spatial frequency and phase
- think of ON-center RGCs and the different phases
15
Q
how many LGNs do we have and how many layers?
A
2 LGNs, 6 layers each
16
Q
types of cells in LGN
A
magnocellular, parvocellular, kiniocellular
17
Q
magnocellular cells
A
- large (magno = large in latin)
- bottom 2 layers in LGN
- receive input from M ganglion cells
- respond best to large, fast-moving objects; “where” objects are
18
Q
parvocellular cells
A
- small (parvo= small in latin)
- top 4 layers in LGN
- receive input from P ganglion cells
- respond best to fine, spatial details of stationary objects; involved in the “what” of an object; recognition
19
Q
kiniocellular
A
- tiny (kinio = tiny in greek)
- between magno and parvocellular sections
- not clear what these cells do; may be involved in the color processing
20
Q
how does the LGN receive input?
A
- from RGCs
- each layer of the LGN receives input from one eye OR the other
- review diagram
21
Q
receptive fields found in LGN
A
concentric; respond well to spots and gratings
22
Q
names for striate cortex
A
primary visual cortex
area 17
V1
23
Q
types of receptive fields in striate cortex
A
elongated “stripes”; rectangular shaped
24
Q
how many cells in striate cortex?
A
200 million
25
how many layers in striate cortex and which one is important?
- 6 layers
- fibers from LGN mainly project to layer 4C
26
2 important features in striate cortex
1- topographical mapping
2- cortical magnification
27
topographical mapping
orderly mapping of the world (regions in striate cortex corresponds to regions in the visual field)
28
cortical magnification
- dramatic scaling of information from different parts of the visual field
- the central part of the visual field is over-represented in the cortex
- images in the periphery have much lower resolution than images at fixation
29
cells in striate cortex respond best to (bars/spots) of light
bars
30
orientation selectivity
- tendency of neurons in striate cortex to respond most to bars of certain orientations (orietnation preference)
- more cells responsive to horizontal or vertical orientations than to oblique
31
how are circular receptive fields in the LGN transformed into elongated receptive fields in striate cortex?
- concentric LGN cells that feed into a cortical cell are all in a row and feed into an elongated linear arrangement in cortex
- lateral inhibition within the cortex might also be involved
32
types of receptive fields that cortical cells respond well to
- bars: elongated bright areas surrounded by dark area on either side, and vice versa
- edges: luminance tends to vary smoothly within objects and abruptly between objects
- moving lines: some celles respond best to lines in motion, no matter what the direction
- certain motion directions: some cells prefer leftward or rightward mvt
- gratings: striate cortex cells respond to gratings of a certain frequency and orientation
33
do striate cells respond to one or both eye?
- both
- by the time info gets to the primary visual cortex, inputs from both eyes have been combined
34
ocular dominance
- cortical neurons tend to have a preferred eye
- tendency to respond more vigorously to input from one eye or the other
35
types of cortical neurons
simple and complex cells
36
simple cells
- have clearly defined excitatory/inhibitory regions; stimulus can only fall on on-center
- "phase sensitive"
- some cells prefer bars of light, other bars of dark
- edge detector and stripe detector
37
complex cells
- do not have clearly defined excitatory/inhibitory regions; stimulus can fall on both on and off centers
- "phase insensitive"
38
what is arranged in columns?
- neurons with similar orientation preferences; extend vertically through the cortex (distance of 0.5mm)
- neurons that share the same eye preference
- other stimulus dimensions (ex: processing of color, motion, spatial structure)
39
end stopping
- property of simple and complex cells that means that the neuron fires when the stimulus is just the right length
40
hypercolumn
- a 1mm block of striate cortex containing "all the machinery necessary to look after everything the visual cortex is responsible for, in a certain small part of the visual world"
- each hypocollumn contains at least 2 sets of ocular dominance columns (one for each eye) and many orientation columns, each covering every possible orientation (0-180 degrees)
41
relation between cortical magnification and hypercolumns
not all hypercolumns see the world at the same level of detail
