central visual pathway
- retina
- -> ganglion cells form optic nerve; partly crossing optic chiasm - optic nerve forms optic tract
- thalamus –> lateral geniculate nucleus (LGN)
- -> via optic radiation in white matter - visual cortex (area V1)
lateral geniculate nucleus
= structure in thalamus
- input: retinal ganglion cells + visual cortex
- output: visual cortex
- each cerebral hemisphere acts as relay stations
- each LGN cell responds to one eye or the other (never both eyes)
- concentric receptive fields: similar to those of retinal ganglion cells –> respond to same patterns and provide input
- six-layered structure –> visual system splits input from image into different types of information
- magnocellular layers
- parvocellular layers
- koniocellular cells/layer
LGN
magnocellular layer
= bottom two neurone-containing layers of LGN (1,2)
- cells are physically larger than those in top four layers
- -> receives input from M ganglion cells in retina
- -> respond to large, fast-moving objects
LGN
parvocellular layer
= top four neurone-containing layers of LGN (3,4,5,6)
- cells are physically smaller than those in the bottom two layers
- -> receives input from P ganglion cells
- -> responsible for processing details of stationary targets
LGN
koniocallular layer
- neurone located between magnocellular and parvocellular layers
topography of LGN
- left LGN –> receives projections from left part of retina in both eyes
- right LGN –> receives from right side of both retinas
- each layer of LGN receives input from one or other eye.
–> layers 1, 4 and 6 of right LGN receive input from left eye (= contralateral)
–> layers 2, 3 and 5 get input from right eye (= ipsilateral)
=> everything you perceive in right visual field –> left part of the retina –> goes to left LGN (vice versa left visual field)
retinotopy and cortical magnification
lecture
- retinotopy = mapping of visual input from retina to neurones
- -> topographic map: neighbouring points on the retina –> project to neighbouring neurones in LGN + cortex
- cortical magnification = information we perceive at fovea is processed by larger number of neurones in V1
- -> foveal part of retina represented bigger/more detailed in cortex
organisation of the cortex
- neurones are organised in columns and layers –> according to their function
- -> location columns = all of the neurones within a location column have their receptive fields at the same location on the retina (perpendicular to the surface of the cortex)
- -> orientation columns = each column containing cells that respond best to a particular orientation
- -> hypercolumns = one location column with all of its orientation columns
hypercolumns
overview lecture
= basic units that analyses all visual features
- consists of
- -> 2 blobs: 1 in right ocular dominance/1 in left ocular dominance
- -> 2 complete sets of orientation columns
types of V1 neurones
- simple cells
- complex cells
- end-stopped cells
–> can be characteristic of simple and complex cells
=> preference: corners, angles, length
simple cells
- preference: static lines of orientation
- clear ON and OFF receptive fields
- important for contrast perception
- -> sensitive to contrast + position in visual field
complex cells
- preference: orientation + direction of movement
- phase-insensitive –> responsive fields are not static
what/ventral visual pathways
=> vision for perception (input) = object recognition important for: --> depth (monocular) --> forms --> colour
- retina: P ganglion cells
- LGN: parvocellular
- V1: layer 4
- V2
- V4: colour
- -> temporal cortex
6a. inferior temporal (face, hippocampus place)
(6b. V3)
6c. V5
6d. parietal
6e. medial superior (temporal)
where/how/dorsal visual pathway
=> vision for action (output) = spatial attention, motion, orientation in space important for: --> depth --> movement --> forms
- retina: M ganglion cells
- LGN: magnocellular layer
- V1: layer 4
4a. V2
4b. V3
5a. V5: (MT) motion
5c. V3
6a. parietal cortex
6b. vental intraparietal
6c. medial superior (temporal)
visual object agnosia
patient D.F.
- problems matching orientation and placing object in different orientation in visual field
- difficulty: object vision
- no difficulty: performing action
–> indicated double dissociation = one mechanism for judging orientation + another for coordinating vision and action
=> impairment of ventral/what pathway (inferior temporal)
–> brain damage in occitemporal region
optic ataxia
patient R.V.
- unable to reach accurately towards visual targets
–> difficulty: positioning fingers/adjusting orientation of hand when reaching toward object; grasping to reflect size of object
- no difficulty: object recognition or general motor skills
=> impairment of dorsal/where/how pathway (parietal)
–> damage to posterior parietal region
tiling
= effect that working together, the columns cover the entire visual field
double dissociation
- determine whether two functions operate independently from one another
- involves two people
- -> 1 person: damage to area of brain –> causes function Y to be absent + while function X is still present
- -> 1 person: damage to another area of brain –> causes function X to be absent + while Y is present
single dissociation
- determine whether two functions two functions operate independently from one another
- involves only one person
- -> person with lesion to brain structure A –> disrupts function X but not function Y
higher-level visual areas
- fusiform face area (FFA)
- -> located in fusiform gyrus on underside of brain (directly below inferior temporal cortex)
- parahippocampal place area (PPA)
- -> located in inferior temporal cortex
- extra striate body area (EBA)
- -> located in inferior temporal cortex
