Task 4: Columns and pathways Flashcards
Central visual pathway
Retina -> Optic chasm -> Lateral geniculate body -> Area V1 primary visual cortex (striate cortex) in occipital cortex
Compared to the actual visual field, the retinal image is
inverted and reversed
What happens to the “image” in the optic chiasm?
In the optic chiasm:
1- axons from the temporal halves of each retina continue into the optic tract on the same side
2- axons from the nasal halves cross to the optic tracts on the contralateral side
Axons in the optic tract can terminate in
mostly = lateral geniculate nucleus
some axons = superior colliculus
The left primary visual cortex gets input from both eyes but
only from the right visual field
Lateral geniculate nucleus
- in the thalamus
- each LGN cell respond to one eye or another, but never to both eyes
- concentric receptive fields
- six layered structure
The six layers in the LGN are separated by
koniocellular cells
The layers of the LGN are called
Magnocellular and Parvocellular layers
Magnocellular layers
- bottom two layers
- input from M ganglion cells => large cell bodies
- fast-moving and large objects
Parvocellular layers
- top four layers
- input from P ganglion cells => small cell bodies
- stationary targets
- details : colour, texture, pattern, depth
Topographical mapping of the LGN
Contralateral layers: 1, 4 and 6
Ipsilateral layers: 2, 3 and 5
Right visual field = different layers of left LGN
V1 or Striate cortex =
Primary visual cortex
Features of V1
- retinotopy
- cortical magnification
Cortical magnification
visual acuity declines in orderly fashion with the distance from fovea
Retinotopy
orderly mapping = where things are in space (layer 3 in striate cortex = position 3 in visual field)
Properties of receptive fields in the striate cortex
- RF of V1 have an elongated shape
- selective responsiveness to orientation (better response to horizontal and vertical lines), spatial frequency and ocular dominance
Types of cortical V1 Neurons
Simple cells
Complex cells
* end-stopped cells
Simple cells
- clearly defined excitatory and inhibitory regions
- sensitive to orientation and contrast
- phase sensitive = position in the receptive field (response only to centre)
- edge detector = light on one side of RF and darkness on the other
- stripe detector = line of light surrounded by darkness
- ocular preference
Complex cells
- no clearly defined excitatory and inhibitory regions
- no specific on/off centre
- phase insensitive (position in the RF is irrelevant)
- sensitive to movement
- ocular preference
End-stopped cells
- subclass of simple and complex cells
- the cell first increases its firing rate as bar length increases to fill up the receptive field, and then decreases
- sensitive to corners, angles and lengths
Hypercolumn
- analyses all visual features in a small part of visual field
- two sets of column (orientation and location)
- covers every possible orientation
- ocular dominance slabs
Streams for vision
Ventral Pathway = what
Dorsal Pathway = where/how/action
Ventral pathway
- what = object identity and recognition
- parvocellular system
- P ganglion cells to inferior temporal lobe
- high-resolution sensitivity
- sensitive to form, patter, colour
- damage = visual agnosia
Dorsal pathway
- where/how = action
- magnocellular system
- M ganglion cells -> V1 -> parietal lobe
- determine object location
- information how to direct action toward stimulus
- damage = optic ataxia
Both Ventral and Dorsal pathways are not
entirely separated = have connections between them
In neuropsychology, effects of brain damage can be understood by determining
double or single dissociation
Double dissociation
determine whether two functions operate independently from one another
- involves two people with lesions to different brain areas
Other higher-level visual areas
FFA - fusiform face area = faces
PPA - parahippocampal place area = houses and scenes
EBA - extra striate body area = body shapes
LOC - lateral occipital complex = scrambled images
VWFA - visual world form area = alphabetic strings
Visual agnosia
- unable to recognise objects, faces, pictures
- disabled perceptual experience
- damage in occipitotemporal region
Optic ataxia
- unable to reach objects accurately
- no difficulty recognising
- damage to posterior parietal region
