Lectures 2, 3, & 4 Flashcards
Layers of the LGN
Layers 1 and 2: Magnocellular (Y-type)
Layers 3-6: Parvocellular (X-type)
IC layers: Koniocellular (input)
Layers 1,4,6: contralateral eye
Layers 2,3,5: ipsilateral eye
Retinotopic mapping in visual cortex
This means that the visual field is represented in the brain in specific areas
P and M cells from LGN projection of the visual cortex
P cell layers project to layers 2, 3 and 4C in V1
M cell layers project to layers 4B and 4C
Ocular Dominance Columns
There is strong seperation in layer 4 of the visual cortex regarding columns in which one eye is more dominant.
The other layers have weaker seperation.
Pathological dominance of one eye (Amblyopia)
- Squinting results in suppression of the input in one eye
- If this happens during critical period in development, input from that eye occupies less space in V1, the OD columns of that eye become smaller.
- After critical period this reduced representation remains and amblyopia remains.
Receptive field tuning in V1
In V1 the neurons are tuned to orientation and direction of motion.
Orthogonal organization components of primary visual cortex
- Orientation columns
- Ocular dominance columns
- CO blobs
These combined are called hypercolumns
How elongated RF’s in V1 are constructed
They are constructed from LGN inputs that have their circular RF along a line in visual space.
This creates a Gabor filter which is a sinusoid combined with a Gaussian envelope.
Explanation for white/gold or black/blue dress
Colour is not strictly the wavelength but more so how the brain interprets the incoming wavelength.
Colour constancy
Perceived colours are not absolute wavelengths but are computed by the brain depending on the wavelengths of the surround. This happens at V4 where the receptive field is large enough to integrate numerous colour opponent signals and discount the illuminant
Achromatopsia
Also called cortical colour blindness. This happens when there is a lesion in V4 or V8
Apparent motion
When a stimulus goes off in one place and on in another. In this sense the neuron is a coincidence detector because it sort of sees this as one single thing moving.
Reichardt detector model for direction selectivity
Neuron in brain receives input from two cells that have spatially seperate RF’s. One of the cells has a delayed input. Only when the stimulus moves in the right direction the cell receives simultaneous input from both cells and will fire.
Direction of motion selectivity
V1, V3, MT
Plotted in a polar tuning curve
Aperture problem
Detecting motion through an aperture is ambiguous, many motion vectors can yield the same motion through an aperture.
V1 cells suffer from this problem.
Component cells have solved this problem because they encode the plaid motion direction.
