w4 w gemini Flashcards
List the range of image properties to which V1 cells show selectivity.
colour
What is a hyper-column?
A hyper-column is a region of V1 that contains neurons covering the full range of RF types for a single spatial location.
Briefly describe the stimulus selectivity of simple cells in V1.
Simple cell: optimum response to an appropriately oriented stimulus
Briefly describe the stimulus selectivity of complex cells in V1.
Complex cell: optimum response to an appropriately oriented stimulus
Describe how simple cell responses could be modelled using convolution.
A simple cell RF can be well described by a Gabor function. Convolving the image with a Gabor mask will simulate the response of all simple cells selective for the same parameters across all hyper-columns. Repeating the convolution with Gabor masks with different parameters (e.g. orientation
Describe how complex cell responses could be modelled using convolution.
A complex cell can be modelled by combining the outputs of two or more simple cells. For example
Gabors functions are the components of natural images under the “sparsity” constraint. What is the sparsity constraint?
The sparsity constraint requires that the minimum number of components are present in each image.
How is the sparsity constraint relevant to efficient coding in the context of Gabor functions?
By using Gabors as the components by which an image is represented
Briefly describe what is meant by the classical receptive field (cRF).
Classical Receptive Field (cRF) = the region of visual space / the stimulus properties that can elicit a response from a neuron.
Briefly describe what is meant by the non-classical receptive field (ncRF).
Non-classical Receptive Field (ncRF) = the region of visual space / the stimulus properties that can modulate the response from a neuron
What is an “association field”? Describe the association field for a V1 cell with an orientation preference.
An association field is the pattern of long-range lateral connections received by an orientation selective neuron in V1. It defines the ncRF of such a neuron. A V1 neuron with a cRF selective for a particular orientation will receive lateral excitation from neighbouring V1 cells with similar orientation preferences that are aligned so that they are collinear or co-circular with it. It will receive lateral inhibition from other neighbouring V1 cells with similar orientation preferences.
How do lateral connections in V1 give rise to contour integration?
Contour integration is generated principally by lateral excitation between cells with nearly co-linear/co-circular orientation preferences. These cells enhance each others response
How do lateral connections in V1 give rise to pop-out?
Pop-out is generated principally by lateral inhibition between cells with similar preferences. These cells suppress each other’s response making cells responding to different image features relatively more active
How do lateral connections in V1 give rise to texture segmentation?
Texture segmentation is generated principally by lateral inhibition between cells with similar preferences. Hence
Briefly describe the difference between bottom-up and top-down influences on grouping.
Top-down influences come from prior knowledge and experience. They cause image elements to be grouped because of prior expectations about what elements belong to the same object. Bottom-up influences come from image properties. They cause image elements to be grouped because they have similar properties.
For image (a) (two rows of alternating black and white circles)
identify the Gestalt Law that gives rise to the observed grouping.
For image (b) (black circles grouped within ovals)
identify the Gestalt Law that gives rise to the observed grouping.
For image (c) (closely spaced black circles forming columns)
identify the Gestalt Law that gives rise to the observed grouping.
For image (d) (connected pairs of black circles)
identify the Gestalt Law that gives rise to the observed grouping.
For image (e) (incomplete squares)
identify the Gestalt Law that gives rise to the observed grouping.
For image (f) (a line of oriented bars)
identify the Gestalt Law that gives rise to the observed grouping.
Explain how lateral connections in V1 give rise to the Gestalt bias of similarity.
Lateral inhibitory connections cause mutual suppression of neurons representing similar image elements. At borders between dissimilar elements there is less inhibition
Explain how lateral connections in V1 give rise to the Gestalt bias of continuity.
Lateral excitatory connections cause mutual enhancement of neurons representing co-linearly orientated image elements. Hence
Explain what is meant by border ownership.
Border ownership refers to the fact that the boundary between two regions in an image is perceived as part of one region (the foreground) and not the other region (the background). This means that foreground objects have a defined shape (delineated by the border)
What is the role of V2 in border ownership?
V2 contains cells that encode border-ownership.
How could V2 cells compute border-ownership?
One mechanism by which V2 cells could compute border-ownership is via lateral connections within V2. Imagine that at each location there are multiple V2 neurons selective to different orientations. At each location and orientation there are a pair of neurons that prefer the foreground object to be on opposite sides of the border. These neurons compete with each other.
Describe the excitatory connections in a V2 border-ownership model.
Excitatory connections link neurons encoding segments consistent with a probable object.
Describe the inhibitory connections in a V2 border-ownership model.
Inhibitory connections link neurons encoding segments inconsistent with a probable object.
What is the sparsity constraint in the context of natural images?
The sparsity constraint requires that the minimum number of components are present in each image.
How is the sparsity constraint relevant to efficient coding?
By using a minimal number of components to represent an image
What is the classical receptive field (cRF) in simpler terms?
The region of visual space that a neuron directly responds to.
What is the non-classical receptive field (ncRF) in simpler terms?
The region of visual space that can influence a neuron’s response
What is the “association field” for a V1 cell?
The pattern of lateral connections it receives from other neurons with similar orientation preferences.
Explain contour integration in the context of lateral connections in V1.
Lateral excitation between neurons responding to collinear or co-circular orientations makes contours more visible.
Explain pop-out in the context of lateral connections in V1.
Lateral inhibition between neurons with similar preferences makes dissimilar items stand out.
Explain texture segmentation in the context of lateral connections in V1.
Lateral inhibition suppresses responses within uniform texture regions
Give an example of a top-down influence on grouping.
Grouping elements to form a familiar object based on prior knowledge.
Give an example of a bottom-up influence on grouping.
Grouping similar elements together based on their visual properties.
Name the Gestalt Law illustrated by grouping elements that are close together.
Proximity
Name the Gestalt Law illustrated by grouping elements that are similar in appearance.
Similarity
Name the Gestalt Law illustrated by perceiving complete shapes even when parts are missing.
Closure
Name the Gestalt Law illustrated by grouping elements that form smooth
continuous lines or curves.
Name the Gestalt Law illustrated by grouping elements that move together.
Common Fate
Name the Gestalt Law illustrated by grouping elements that form symmetrical arrangements.
Symmetry
Name the Gestalt Law illustrated by grouping elements enclosed within the same region.
Common Region
Name the Gestalt Law illustrated by grouping elements that are connected by other elements.
Connectivity
In the context of border ownership
which region “owns” the boundary?
Why does the background appear shapeless in terms of border ownership?
Because the border is “owned” by the foreground object
What is the significance of border ownership for object segmentation?
It helps us perceive objects as distinct entities by assigning boundaries to them.
What evidence suggests that V2 cells are involved in border ownership?
V2 neurons respond selectively based on which side of a contour the figure appears on.
How does the V2 border-ownership model use lateral connections?
Excitatory connections link neurons for consistent object boundaries
What does the V2 border-ownership model achieve?
It simulates how border ownership can be computed through local interactions
What is the ‘energy model’ in the context of Gabor filters and complex cells?
It takes the square root of the sum of the squared outputs of a quadrature pair of Gabor filters to achieve phase invariance.
What are “wavelet transforms” in the context of multiscale Gabors?
Convolving a signal with a family of similar masks sensitive to different frequencies.
How can Gabor functions be seen as image components?
Images can be represented as a superposition of Gabor functions or elementary features.
What is the mathematical representation of an image as a combination of Gabor components?
Ay ≈ x (where A is the matrix of Gabor filters
How is the concept of representing images with Gabor components used in image compression?
By representing the image with a smaller set of Gabor filter activations
How is the concept of representing images with Gabor components used in image denoising?
By reconstructing a noisy image using Gabor components
How is the concept of representing images with Gabor components used in image inpainting?
By reconstructing missing parts of an image using a sparse subset of Gabor components learned from non-corrupted parts.
What is retinotopic organization in V1?
The spatial arrangement of neurons in V1 that preserves the spatial relationships of the ganglion cells in the retina.
What is cortical magnification in the context of retinotopic maps?
The disproportionate representation of the central visual field (fovea) in V1 compared to the periphery.
What is the purpose of lateral excitation in contour integration?
To enhance the responses of neurons aligned with a contour
What is the purpose of lateral inhibition in texture segmentation?
To suppress responses within uniform texture regions
What are the two main pathways in the cortical visual system?
The “What” pathway (ventral stream) and the “Where” pathway (dorsal stream).
What is the function of the “What” pathway?
Object recognition and identification (V1 to inferotemporal cortex).
What is the function of the “Where” pathway?
Spatial processing and motion analysis (V1 to parietal cortex).
What are the key characteristics of simple cells in V1 receptive fields?
They respond best to oriented stimuli at a specific location with a specific contrast polarity.
What are the key characteristics of complex cells in V1 receptive fields?
They respond to oriented stimuli regardless of the exact position or contrast polarity within their receptive field.
What are the key characteristics of hyper-complex cells in V1 receptive fields?
They are sensitive to orientation and also to the length of the stimulus
What is the role of lateral connections in implementing Gestalt principles in V1?
Lateral excitation supports continuity
Why are Gestalt Laws considered heuristics rather than strict laws?
Because they are rules of thumb that are often obeyed but not always.
What is the “common fate” Gestalt Law?
Elements that move together are perceived as grouped.
How does the concept of “common region” influence perceptual grouping?
Elements located within the same closed region tend to be grouped together.
How does “connectivity” influence perceptual grouping?
Elements that are connected to each other are perceived as a group.
