lecture 4 Flashcards
where do ganglion cell fibres leave the retina?
along the optic nerve
optic chiasm
cross over point of some of the optic nerves
what do optic nerves become beyond the optic chiasm?
the optic tract
what does the optic tract feed into?
the Lateral geniculate nucleus
how is information separated once it reaches the optic tract
by visual field rather than by eye
where is information from the right visual field represented?
the left hemisphere
where is information for the left visual field represented
the right hemisphere
what type of structure is the lateral geniculate nucleus?
bilateral structure
where does each LGN receive it input from
from the left and right eye, but keeps these inputs separate
what type of receptive fields does the LGN have? what is this ideal for? what does it not detect
centre surround antagonism receptive fields
ideal for detecting spots of light and edges
not able to detect orientation of bars/ edges
what is V1
primary visual cortex
where does V1 receive input from?
the LGN
feature of the LGN
striate cortex
activity of V1 cells when no stimulus present
baseline
what did Hubel and Weisel find caused a big response in V1 cells? what does this show?
when the edge of slide moved across the receptive field
V1 likes lines, not spots
what do V1 cells show a preference for?
lines of a certain orientation
how is V1 organised?
retinotopic mapping
cortical magnification
orientation columns
retinotopic mapping
objects close together in the visual field are measured by neighbouring parts of V1
cortical magnification
amount of cortex devoted to representing each area of the visual field is distorted
the fovea is represented by a large area of the cortex
orientation columns
orientational preferences of V1 arranged in an ordered way
how was the organisation of orientation columns recorded?
1) recording from an electrode penetrating the cortex perpendicular to the surface
2) recording from an electrode penetrating the cortex at an angle to the surface
relation of perpendicular surface cells?
an orientation column: all cells have the same orientation preference
relation of surface cells organisation at an angle
cell’s orientation preferences vary systematically
monocular
respond to input from left or right eye, but not both
is the LGN monocular or binocular?
binocular
how many cells in V1 are binocular?
80%
binocular
respond to inputs from both eyes
ocular dominance:
most binocular cells respond better to one eye that the other
how are cells with the same ocular dominance organised in V1?
in columns
names of different cell types in V1
simple cells
complex cells
hypercomplex cells
simple cell receptive fields
respond to oriented bars and edges
phase sensitive
elongated excitatory and inhibitory regions
phase sensitive
response depends on position of bar within the receptive field
orientation tuned neurons
neurons that respond best to their preferred orientation but also respond to other similar orientations
complex cells
respond to oriented lines
no discrete on off regions
phase insensitive
what do complex cells respond best to?
oriented bars and edges
a particular direction of movement
another name for hypercomplex cells
end-stopped cells
what do hypercomplex cells respond to?
bars of:
- a particular orientation
- moving in a particular direction
- particular length
how many visual areas beyond V1?
30+
discoveries about visual areas beyond V1
areas seem to be specialised
areas are interconnected
V3 function:
form
V4 function
colour
V5 function
motion
importance of ‘what’ stream
recognising and discriminating objects
where does the ‘WHAT’ stream travel?
ventrally to the inferotemporal cortex
where does the ‘WHERE’ stream travel?
dorsally to the posterior parietal cortex
what is the ‘WHERE’ stream important for?
determining where an object is and how to act upon it
visual form agnosia
damage to the ventral pathway
cannot identify objects despite knowing their features
monkey lesion study
Underleider and Mishkin, 1982
task 1: object discrimination
rectangle and prism in site
food is always under the prism
task 2: landmark discrimination
food always located close to the cylinder out of 2 possible locations
result of the monkey lesion study
1) lesion to inferotemporal cortex causes problems for object discrimination but not landmark discrimination task
2) lesion to posterior parietal cortex causes problems for landmark discrimination but not object discrimination
where is the damage in optic ataxia
dorsal pathway
what symptoms seen in optic ataxia?
cannot grasp objects but can recognise and describe them
are the what and where streams separate?
not totally, there are many connections between them
which way does the signal flow in what and where streams?
upwards and backwards
why can V1 cells be though of as feature detectors?
they respond to certain features of an image
what happens as we move higher up the visual system
receptive fields get more complex, and the features they respond to more specific
what is in area IT of visual system
cells that respond to faces
