W4 Flashcards
(25 cards)
How does the retina process colour?
The retina contains cones (for colour vision) and rods (for night vision).
Three types of cones: S-cones (blue), M-cones (green), L-cones (red).
Cones respond to a range of wavelengths but are most sensitive to a specific colour.
What happens in the LGN?
The Lateral Geniculate Nucleus (LGN) in the thalamus processes colour signals before sending them to the visual cortex.
It has colour-opponent cells, meaning it compares colours rather than detecting them individually.
Two major opponent systems:
Red-Green (L-cones vs. M-cones)
Blue-Yellow (S-cones vs. a combination of L- and M-cones)
What causes colour blindness?
Most people with colour blindness are not truly colour blind but have a deficiency in distinguishing certain colours.
Caused by missing or mis-tuned cones in the retina.
More common in men due to X-linked inheritance.
Types of colour blindness: Protanopia
Protanopia: No red cones
Types of colour blindness: Deuteranopia
Deuteranopia: No green cones
Types of colour blindness: Tritanopia
Tritanopia: No blue cones (rare)
Types of colour blindness: Monochromacy
Monochromacy: Only one or no cone types → true colour blindness
How is cerebral achromatopsia different from regular colour blindness?
Regular colour blindness is caused by issues in the retina.
Cerebral achromatopsia is due to damage in the V8 area of the visual cortex (e.g., from a stroke).
Patients cannot perceive colours at all, even though their eyes and LGN are functioning.
What are centre-surround receptive fields in colour processing?
Retinal ganglion cells and LGN neurons have a centre-surround structure for detecting colour contrasts.
Example:
Red-ON / Green-OFF → Red light excites, green light inhibits.
Blue-ON / Yellow-OFF → Blue excites, yellow inhibits.
This helps with edge detection and enhances colour contrast.
Where is colour processed in the brain?
V1 (Primary Visual Cortex): Initial processing of colour signals.
V2 & V4: More complex colour perception, detecting colour constancy.
V8: Critical for conscious colour perception (damage here causes cerebral achromatopsia).
What is colour constancy?
The ability to perceive colours consistently despite changes in lighting conditions.
Example: A white shirt looks white indoors and outdoors, even under different light sources.
How does the brain achieve colour constancy?
V4/V8 in the visual cortex adjust for lighting conditions.
The brain subconsciously discounts the light source to maintain stable colour perception.
What is Area V1, and why is it important?
Area V1, also called the primary visual cortex, is the first stage of cortical processing for visual information. It receives input from the retina via the lateral geniculate nucleus (LGN) and is responsible for detecting basic visual features like edges, lines, and orientations.
What are orientated edges and bars in the context of V1 simple cells?
V1 simple cells, due to their structured on/off receptive fields, act as orientation-selective filters, meaning they maximally respond to edges or bars of light presented at a specific angle within their receptive field.
How does the visual system process information from the LGN to V1, specifically regarding receptive fields?
The LGN contains neurons with center-surround receptive fields, responding to differences in light intensity.
Multiple LGN neurons with aligned receptive fields converge onto single simple cells in V1.
This convergence creates elongated receptive fields in V1, enabling the detection of lines and edges of specific orientations.
Essentially, the LGN provides the “pixel” level information, and V1 assembles it into “line” level information.
What is the functional difference between receptive fields in the LGN and V1?
LGN receptive fields primarily detect changes in light intensity within a small, circular area, emphasizing contrast.
V1 receptive fields, particularly those of simple cells, are specialized for detecting the orientation of lines and edges.
Therefore, the LGN focuses on basic light detection, while V1 begins the process of form and shape analysis.
In short LGN is basic light information, and V1 is edge detection
How do complex cells differ from simple cells?
Complex cells are also orientation-sensitive but have no distinct on/off regions. They respond to lines anywhere in their receptive field, making them less position-specific than simple cells.
What are hyper-complex (end-stopped) cells, and how do they function?
Hyper-complex cells build on complex cells but also respond to line length. They stop firing when the line extends beyond a preferred length, which helps in detecting object boundaries and texture segmentation.
What is inhibition over space, and how does it work in visual processing?
Inhibition over space means that certain neurons suppress others when detecting features like edges and textures. This helps define contrast and segmentation in visual scenes.
How does inhibition over space relate to texture segmentation?
Textures are defined by statistical differences in features like orientation, size, and density. The brain separates objects by detecting these differences, allowing us to distinguish surfaces and break camouflage.
What is the simultaneous tilt illusion?
A central line appears tilted in the opposite direction to surrounding lines due to orientation-based inhibition.
What is the simultaneous size illusion (Titchener Circles/Ebbinghaus Illusion)?
A shape appears larger or smaller depending on the surrounding objects due to size-based inhibition.
What is inhibition over time, and how does it affect perception?
Inhibition over time, also called adaptation, occurs when prolonged exposure to a stimulus reduces a neuron’s response. This creates after-effects, where a new stimulus is perceived differently.
What is the tilt after-effect?
After looking at tilted lines for a long time, neutral lines appear tilted in the opposite direction. This happens because orientation-sensitive neurons become fatigued, shifting the balance of activation.
