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.
Component and pattern cells
Component cells do not see real motion and are located in V1, V3a and MT
Pattern cells are tuned for the real perceived motion and are located in MT.
MST (medial superior temporal area) neurons
Respond selectively to particular motion flow fields. These are often cause by self-motion
Biological motion
Recognizing species, sex, mood from movement
The STS is selectively activated by biological compared to non-biological motion
Akinetopsia
Motion blindness.
Caused by lesion in MT, MST, STS
Colour processing brain areas
Wavelength: V1, V2
Colour constancy (V4)
Motion vs colour
Motion wins from colour when both are being activated.
Dorsal pathway vs ventral pathway
Dorsal pathway is Magno dominant
Ventral pathway is Parvo dominant
Depth perception cues
Monocular:
- Perspective
- Size constancy
- Occlusion
- Cast shadows
- Areal perspective
- Texture
- Motion parallax
Binocular:
- Disparity
Disparity
When you fixate on a point the eyes converge. Every other stimulus that falls within the same plane of depth (horopter) will be projected as equal distance
Correspondence problem
There are multiple ways a point can be interpreted by the two eyes. In other words the point can be at different distances. When this problem is solved, binocular fusion is the result, which results in a 3D image.
Strabismus
Misalignment of the eyes, due to congenital eye-muscle disorders or due to cranial oculomotor nerve disorder.
Neurons tuned to specific disparities brain areas
V1, V2, V3/V3a
They can be tuned near, far, zero or inhibitory
V1 disparity neurons other function
These neurons also code for disparity when no depth is perceived
IT neurons
These neurons encode perceived depth and not just contrast disparity.
Depth processing brain areas
Disparity: V1, V2, V3/V3A
Perceived depth: IT
V2 neurons shape detection
These neurons are tuned for the orientation of illusory contours
Lateral Occipital Complex (LOC)
Responds more strongly to shapes than to scrambled objects.
Invariant object coding
- LOC, fusiform and frontal gyri only show activity for identical objects
- Right anterior and posterior fusiform gyri show activity for identical objects of different sizes
- Left anterior and posterior fusiform gyri show activity for objects at different viewpoints
- Broca’s area shows activity for objects within same class
Shape coding brain areas
- Orientation: V1, V2, V4
- Complex shapes: V4, LOC
- Real world shapes: IT
- Invariant object coding
Cortical areas where position is transformed from retinal to other reference frames
VIP, LIP, area 7a
Gain modulation
VIP neurons change their response as function of eye position
Parietal Reach Region (PRR)
Has neurons that respond to hand movement relative to eye position
