week 3 Flashcards
(27 cards)
What role do cone photoreceptors play in colour vision, and how many types do humans have?
Cones enable daylight colour vision by detecting different wavelengths. Humans have three types—S (blue), M (green), and L (red)—which allow us to perceive the full range of colours
What is the visible spectrum in the context of human colour vision?
The visible spectrum is the range of wavelengths to which our cone photoreceptors are sensitive, allowing us to perceive light as different colours
Define colour opponency
Colour opponency is a neural mechanism wherein centre-surround receptive fields in retinal ganglion cells create opponent channels (red-green and yellow-blue) that enhance colour contrast and edge detection
How does centre-surround organization in retinal cells affect colour perception?
It produces spatial inhibition by having an excitatory center and an inhibitory surround, which enhances local differences in colour and brightness, forming the basis for simultaneous colour contrast
What is colour constancy, and where in the brain is it primarily processed?
Colour constancy is the ability to perceive object colours as stable despite changes in lighting. It is chiefly processed in cortical areas, particularly in V4 and V8
How do the visual system’s compression mechanisms optimize colour processing?
Compression mechanisms (spatial inhibition, temporal inhibition, and filling-in) reduce redundant information, emphasizing important changes—like edges and shifts in light—while omitting constant data to save energy and processing power
What is cerebral achromatopsia (cortical colour blindness)?
Cerebral achromatopsia is a loss of colour perception due to damage in cortical areas (often V8), resulting in an inability to see colour despite normal functioning of the retina, LGN, and V1
What characterizes the common forms of colour blindness in humans?
Most common colour deficiencies—such as protanopia, protanomaly, deuteranopia, and deuteranomaly—involve either the absence or mis-tuning of one cone type, primarily affecting red-green discrimination
How do dichromats differ from individuals with milder forms of colour blindness?
Dichromats are missing one entire cone type, leading to a more severe loss of colour discrimination, whereas many with colour deficiencies have mis-tuned cones that permit partial colour vision
Describe the two main colour systems in the retina
The retina has an older luminance plus yellow-blue system (common in most mammals) and a newer red-green system unique to primates, evolved from the yellow branch to refine colour discrimination
What factors contribute to the differing perceptions of the dress illusion (#TheDress)?
Variability in perception arises from over-exposure of the image, differing viewing conditions (angles, screens), individual differences in colour processing and experience, and variations in colour constancy mechanisms
How do individual differences affect colour naming and perception in tasks like naming the colours of The Dress?
Though observers may see similar retinal colours, differences in processing—particularly in the blue-yellow system and the application of colour names—lead to divergent verbal labels
Outline the pathway of colour processing from the retina to the cortex
Colour information starts with cone photoreceptors and is organized by centre-surround receptive fields in retinal ganglion cells. It is then relayed through the LGN and processed in visual cortical areas (V1, V2, then specialized regions such as V4/V8 for colour constancy).
In what way do rods differ from cones regarding colour vision?
Rods function in low-light conditions and do not provide colour information, which is why vision under very dark conditions appears monochromatic
What role do filling-in mechanisms, such as the water-colour illusion, play in colour perception?
Filling-in mechanisms allow the brain to reconstruct or “fill in” the colour across an object by using edge information provided by centre-surround cells, thus completing the perception of an object’s overall colour
How do mammals differ in their number of cone types?
Most mammals have only two cone types, while humans, apes, and some monkeys have three. Some species, like birds and insects, have even more cone types, enabling broader spectral sensitivity
Why don’t humans see infrared or ultraviolet light?
Infrared detection isn’t necessary for humans because we’re warm-blooded. Ultraviolet light is harmful over time, so humans evolved to filter it out, whereas birds and insects retain UV sensitivity
What is the genetic basis of red-green colour blindness, and why is it more common in men?
The genes for the red-green cone pigments are carried on the X chromosome. Since men have only one X chromosome, a defective gene immediately leads to colour deficiency, while women (with two X chromosomes) are less frequently affected
How do tetrachromatic individuals differ from trichromats?
Some women have a fourth cone type due to genetic variation, allowing for finer colour discrimination beyond the typical human spectrum
What is rod monochromacy, and how does it differ from cone monochromacy?
Rod monochromacy (achromatopsia), is a rare genetic condition where an individual completely lacks functional cone photoreceptors, relying entirely on rod cells for vision. Rods are specialised for low-light, so people are totally colour blind, with extreme light sensitivity (photophobia).
Cone monochromacy occurs when only one type of cone photoreceptor is functional, leading to limited color discrimination rather than complete color blindness. Perceive the world largely in shades of one dominant hue, depending on the functioning cone type.
How does cerebral achromatopsia affect colour perception despite intact retinal function?
A stroke or brain injury affecting V8 in the visual cortex can cause complete colour loss, even though the retina, LGN, and early visual areas remain functional
Why is cerebral achromatopsia evidence that V8 is crucial for conscious colour perception?
Patients with V8 damage lose colour perception entirely, demonstrating that this area is responsible for interpreting and perceiving colour consciously
How does spatial inhibition contribute to simultaneous colour contrast illusions?
Centre-surround receptive fields exaggerate colour differences at edges, making adjacent colours appear more distinct than they actually are
What is the water-colour illusion, and how does it demonstrate filling-in?
The brain reconstructs colour across surfaces using edge information, filling in the perceived colour between boundary lines rather than directly detecting it
