L19 - Visual Cortex 3 Flashcards
How does Oregon Green (Ca2+ indicator dye) work?
It gets trapped in cells and fluoresces during Ca2+ influx - indicating cellular activity
Pinwheel centres are highly ordered: neurons selective to different orientations are clearly segregated even in the very centre! Thus, pinwheel centres truly represent singularities in the cortical map. This highly ordered arrangement at the level of single cells suggests great precision in the development of cortical circuits underlying orientation selectivity
Pinwheel centres are highly ordered: neurons selective to different orientations are clearly segregated even in the very centre! Thus, pinwheel centres truly represent singularities in the cortical map. This highly ordered arrangement at the level of single cells suggests great precision in the development of cortical circuits underlying orientation selectivity
Orientation pin-wheel centres (singularities) tend to fall over?
Centres of ocular dominance columns
!Hypercolumns
A module of about 1 mm x 1 mm x 2 mm covers a pair of ocular dominance columns (one for each eye) and all the orientations. It acts as a small window on a small part of the visual world and processes the visual input in terms of elementary features
Why do we have columnar systems?
1) Ensure all possible combination of stimulus features are represented within any given region of the visual field
2) To avoid the occurrence of functional blind spots for stimulus attributes across a sensory map
Similar columnar systems have been demonstrated in other sensory systems - e.g. columns tuned to slowly or rapidly adapting mechanoreceptors from the same area of skin
Where in the visual pathway the koniocellular pathway found?
In the early parts of the visual pathway: retina and LGN - not PVC
There are two streams: Dorsal (Where) and Ventral (What) - which brain region does each go to after the LGN?
Dorsal - Thick stripes -> MT -> Posterior parietal cortex, input mainly from magno- cells
Ventral - Blobs -> Thin stripes -> V4 ->IT input mainly from parvo-cells
Classical view of properties of blobs and inter-blobs - but what did exp show
Blobs - colour selective, poor orientation selectivity
Inter-blobs (between) - achromatic, good orientation selectivity
Exp showed that orientation tuning is similar in cytochrome oxidase blobs and interblobs
What do horizontal connections tell us about orientation selective cells?
Cells tuned to the same orientation are connected horizontally
What is the relationship between different functional domains of the primary visual cortex?
In recordings of single cell responses in separate electrode penetrations, Hubel and Wiesel found that penetrations perpendicular to the cortical surface contained cells which contained similar orientation AND ocular dominance. Cytochromse oxidase blobs and orientation pin-wheel centres (singularities) , where orientation domains converge, tend to fall over centres of ocular dominance columns
!Horizontal connections are formed primarily by
Axon collaterals of pyramidal neurons in layer 2/3 of the PVC. They extend for mm parallel to the cortical surface and form patch terminations
