Sample Q 2018a Flashcards
What are the major differences between parvocellular and magnocellular
divisions of the primate LGN?
Magnocellular Pathway: • Large receptive fields • High contrast sensitivity • Poor isoluminant response • Low spatial resolution • Good motion sensitivity (high temporal resolution) • Fast conduction velocity • X-like (linear) or Y-like (non-linear) • Transient response • Input from Parasol cells • Projects to layers 4Cα & lower 6 in area V1
Parvocellular Pathway: • Small receptive fields • Lower contrast sensitivity • Chromatic selectivity • High spatial resolution • Poor motion sensitivity • Slow conduction velocity • X-like (linear) • Sustained response • Inputs from Midget cells • Projects to layers 4Cβ, 4A & upper 6 in V1
Which cortical layer/s do the parvocellular cells project to?
- Mainly goes to layer 4Cβ
- Some to layer 6 and 4A
Which cortical layer/s do the magnocellular cells project to?
- Mainly goes to layer 4Cα
- Some to layer 6
Which cortical layer/s do the koniocellular cells project to?
- Mainly goes to supergranular layers 2 & 3 (within blob areas)
What is the defining feature of a simple cell in the primary visual cortex?
Simple cells:
- found in layer 4, 6
- responds to bars of light in a certain orientation
- still respond to simple stimuli, but not very well
- distinct ON and OFF subregions
- small receptive field
- linear spatial summation (when you have a spot of light, the central surround gets added up and you don’t get a response)
What is the defining feature of a complex cell in the primary visual cortex?
Complex cell:
- found outside layer 4
- receive input from multiple simple cells of the same orientation
- overlapping ON and OFF subregions
- large receptive field
- non-linear properties (also won’t respond to diffuse light, adding up central surround will get a response in some situations)
What is the defining feature of a hypercomplex cell in the primary visual
cortex?
Hypercomplex cell:
- respond well for short bars, but have end inhibiting areas
- poor responses for bars that aren’t the optical length
- receive input from multiple complex cells
What is the basic model that Hubel and Wiesel proposed to explain cortical
simple cells’ orientation selectivity?
Simple cells get orientation selectivity:
- convergent excitatory connections from a number of lateral geniculate cells
- circular receptive fields are arranged in a row in visual space
- when there’s a long bar aligned across the cells, stimulating all the receptive fields, then it will stimulate all the geniculate cells at the same time= simple cells get vigorous input
- if the light bar had an orientation perpendicular to the orientation that it moves across, this will only simulate 1 or 2 cells at a time, which isn’t sufficient for the simple cells to fire
What are some of the criticisms of their model (Hubel and Wiesel orientation selectivity model)?
ORIENTATION SELECTIVITY- may also arise before the level of the striate cortex
- removal of inhibition on a striate cell abolished the orientation selectivity, this wouldn’t happen if it was a result of excitatory convergence
HIERARCHY SCHEME- only partially true
- complex cells can respond to stimuli that simple cells don’t respond to
- both complex and hypercomplex cells can be directly excited from the LGN without going through simple cells
What are orientation and ocular dominance columns?
ORIENTATION COLUMNS:
- cells with similar orientation selectivity were arranged perpendicular to the surface of the brain
- cells measured obliquely across the cortex tend to differ more
OCULAR DOMINANCE COLUMNS:
- cells arranged into alternating left/ right eye input dominant columns perpendicular to the surface
What are some of the methods used to demonstrate them (orientation and ocular dominance columns)?
RADIOACTIVE AMINO ACIDS (ocular dominance columns):
- inject into the eye, where they travel to the cortex
- image the ocular dominance columns in V1 (layer 4) and the lateral geniculate nucleus (LGN) a week after injection
2-DEOXYGLYCOSE (2-DG) (orientation columns):
- glucose analog that is taken up into active regions of the visual cortex, but isn’t metabolised
- images of orientation columns in the visual cortex are found in infragranular layers 5 & 6
CYTOCHROME OXIDASE (orientation columns):
- mitochondrial enzyme that correlates with cellular activity
- cytochrome oxidase staining reveals ‘blobs’ in V1 and stripes (both thick and thin) in V2
- blobs are typically at the centres of ocular dominance columns, rarely at boarders
What kind of visual stimulation would you do to show up ocular dominance
columns with optical imaging?
- Radioactive amino acid
What kind of visual stimulation would you do to show up orientation columns
with optical imaging?
- 2- deoxyglucose (2-DG)
- Cytochrome oxidase
What are pin-wheel centres in optical imaging maps?
- point where all the different orientation domains converge
- tend to fall in the centre of ocular dominance columns where the iso-orientation lines (lines of cells with the same orientation selectivity) converge
What are the functions of the two major cortical streams?
DORSAL STREAM- where
- gated by area MT
- informs goal directed actions by outputting to the posterior parietal cortex
VENTRAL STREAM- what
- gated by area V4
- conscious visual perception that we use to identify objects by outputting to the inferior-temporal cortex (area IT)
Which cortical regions do the two streams project to?
DORSAL STREAM:
Magnocellular > 4Cα in V1 > thick stripes in V2 > V3 > MT > parietal
VENTRAL STREAM:
Parvocellular > 4Cβ, 4A in V1 > thin stripes in V2 > V4 > inferotemporal
