W3 - Line and Texture Flashcards
What is the role of the Primary Visual Cortex (V1) in spatial vision?
V1 is the first cortical area where detailed spatial processing occurs, handling the perception and organization of spatial relationships between objects, including oriented edges and bars.
What is the concept of “Oriented Edges and Bars” in V1?
In V1, cells detect the orientation, length, and thickness of edges and bars, which are processed by simple, complex, and hyper-complex cells that receive input from the LGN.
What is “Inhibition over space,” and how does it influence perception?
Inhibition over space affects the perception of textures and illusions by suppressing visual information across certain regions of space, impacting how we perceive surface details and depth.
How does “Inhibition over time” explain tilt and size aftereffects?
Inhibition over time explains phenomena such as tilt and size aftereffects, where previous visual stimuli influence the perception of subsequent stimuli, altering how we perceive orientations or sizes temporarily.
What are Simple Cells in V1, and how do they function?
Simple cells in V1 are orientation-selective, position-sensitive and size-selective. They respond to edges and bars of a specific orientation (e.g., vertical, horizontal) and position, built from the output of multiple concentric LGN receptive fields with structured ON/OFF regions.
What are Complex Cells in V1, and how do they differ from Simple Cells?
More abstract. Complex cells are orientation selective but less position-sensitive than simple cells, responding to edges and lines of specific sizes, regardless of exact position. They do not have well-defined ON/OFF receptive fields and have a higher resting output.
How do Hyper-Complex Cells (End-Stopped Cells) contribute to spatial vision?
Hyper-Complex cells in V1 & V2 are sensitive to the length of lines and help detect corners, curves, and ends of objects. They inhibit further stimulation if the line extends beyond their receptive field.
Who suggested the classical model - oriented edges and bars
Hubel and Wiesel 1950-60s
What is Orientation Selectivity in V1?
Orientation selectivity refers to the preference of simple cells in V1 for specific orientations of bars (vertical, horizontal, etc.), which are fundamental for detecting object shapes and edges.
What is meant by “Position Sensitivity” in V1 cells?
Position sensitivity refers to cells in V1 that respond only when a stimulus is at a specific location within their receptive field, aiding in the localization of edges and object boundaries in space.
What is “Size Sensitivity” or Spatial Frequency in V1, and why is it important?
Size sensitivity (spatial frequency) refers to how different cells in V1 respond to bars of different thicknesses. High spatial frequency detects fine details (sharp textures), while low spatial frequency detects coarse shapes (larger features).
Why is texture important for visual perception?
Texture helps differentiate objects from their background by providing visual cues that aid in object identification. A lack of texture can lead to misperceptions, such as misjudging the surface type (e.g., road vs. pavement).
How is texture defined in visual perception?
Texture is defined by statistical differences in the visual properties of patterns. Textures are identified based on these differences rather than specific or absolute values. Similar statistical properties in textures make them appear the same.
What is Julesz’ (1984) Texton Theory?
Julesz’ Texton Theory proposes that textures are made up of basic elements, called “textons” (e.g., elongated blobs, line endings, and line crossings)- texture atoms, which are the simplest defining properties of textures. Textures can be segmented based on the density and arrangement of these textons.
How do Textons help in texture segmentation?
Textons like elongated blobs, line endings, and crossings are fundamental for texture segmentation. Textures differ in the number and type of textons, and segmentation occurs when these elements differ between regions.
What did Nothdurft’s (1990/1991) research show about texture segmentation?
Nothdurft showed that texture segmentation relies on orientation and size contrasts. Even when two regions have the same mean orientation, they can be segmented if there is local orientation contrast. Size contrast also aids in differentiating textures.
How does Orientation Contrast contribute to texture segmentation?
Orientation contrast helps to segment textures when two regions have the same mean orientation but differ in their local orientation. This ability to segment based on orientation contrast develops around 3 months of age in infants.
What is Size Contrast and how does it affect texture segmentation?
Size contrast refers to the ability to segment objects based on differences in the size of textures or features. Local size differences allow regions to be separated and identified as distinct textures.
What are centre-surround cells in V1?
Orientation-sensitive cells that detect edges and contrasts.
What are single-opponent cells?
Cells that respond to a single specific orientation (e.g., vertical).
What are double-opponent cells?
Cells that respond to two orientations and contrast with surroundings.
How do centre-surround cells detect orientation?
They fire when the centre contrasts with the surrounding area.
How does the Centre-Surround Mechanism in V1 explain the Tilt Illusion?
The tilt illusion occurs because of the inhibitory effects from the surrounding area of a stimulus. When vertical lines are surrounded by lines tilted in one direction (e.g., left), the centre-surround mechanism causes the vertical lines to appear tilted in the opposite direction. This is due to the inhibitory contrast between the center and surrounding stimulus, leading to the illusion of the lines being tilted in the opposite direction.
What is the Simultaneous Tilt Illusion, and how does it work?
The simultaneous tilt illusion occurs when you view vertical lines surrounded by lines tilted in one direction (e.g., leftward). The surrounding tilted lines cause the central vertical lines to appear tilted in the opposite direction (e.g., rightward). This illusion is due to spatial inhibition from the surrounding lines, which distort the perception of the central lines’ orientation. When the surrounding lines are removed, the vertical lines appear perfectly straight.
