WEEK 8 - Visual Cognition Pt.1

• What is light? An electromagnetic radiation differing in wavelengths to form different colours which we construct
• Which wavelengths are perceived as visible light? 400nm – 700nm
• Where does light reflecting off objects in the world enter through? The pupil
• What is the purpose of the lens? Changes its shape to focus the image on the retina
• What is the retina? Thin layer of tissue at back of eye containing photoreceptors
• Explain how the lens changes shape to focus near-objects on the retina? Light travels through lens on oblique angles

  
• Explain how the retina is laid out spatially? Objects that are adjacent in the real world will tend to be adjacent in images that land on the retina e.g. fingers on a hand are next to each other IRL and are also next to each other as they land on the retina. Photoreceptors next to each other on retina are most likely picking up objects next to each other IRL

  
• What are photosensitive cells? Photoreceptors which are able to absorb photons of light and produce action potentials thus detect light
• What are the 2 types of photoreceptors? Rods and cones
• How are cones shaped? Smaller and tapered

  How are rods shaped? Larger and cylindrical
• How do cones function? Higher in definition, better at detecting details but require more light photons in order to fire
• How do rods function? Have lower definition but do not require as much light in order to fire
• What do cones detect? Colour

  What can rods not detect? Colour
• Which photoreceptor underlies daytime focal colour vision? Cones

  Which photoreceptor underlies night vision? Rods
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  What is the distribution of the retina?

  -              Fovea - focal point consists solely of cones

  -              The peripheral retina - has both rods and cones but more rods than cones – increasingly more rods as we deviate more from the fovea

  
• What happens at the blind spot? As axons are exiting the eye where the blind spot is, there are no photoreceptors at the blind spot
• Explain why rods and cones differ in properties?

  -              Can be explained by the way cells are organised

  -              Bipolar cells can gather information from individual photoreceptors or multiple photoreceptors can converge information to a single bipolar cell

  -              Rods show much more convergence than cones

  -              As we deviate from the fovea, 120 photoreceptors per bipolar cell for rods

  -              Closer to the fovea, 6 photoreceptors per bipolar cell for peripheral cones

  -              At the fovea, 1 photoreceptor per bipolar cell for cones in the fovea

  
• Explain why rods are more sensitive but have low definition? Rods are better at detecting light due to high convergence where many rods are able to detect light in the area but not the exact location s they all converge onto few bipolar cells.
• Explain why cones have low sensitivity high definition? Cones have low sensitivity as they require more light photons to respond than rods, but singular cones/ fewer cones project information to bipolar cells giving more discriminating details
• What are the steps following projection of information from bipolar to retinal ganglion cells?

  1. retinal ganglion passes information to the optic nerve

  2. optic nerve projects back through optic chiasm to the lateral geniculate nuclei (LGN) of the thalamus

  3. LGN relays information to the primary visual cortex – V1

  
• Why is the perception of objects in the world difficult for our visual systems? Stimulus arriving on our retinas does not uniquely identify objects in the world as image on retina can be caused by infinite number of objects
• What is the inverse projection problem? An image on the retina can be caused by infinite number of objects as there is no 3rd dimension to our retinas

  
• What is viewpoint invariance? Our ability to recognise an object regardless of our viewpoint

  
• How do we recognise different viewpoints as the same object? The structuralist approach explains this.
• How does the structuralist approach explain this? States that perceptions are created by combining elements called sensations in flexible and cognitive ways – the action of combining is a cognitive process

   

  e.g. each dot is a sensation that we combine into a perception

  
• Why is the structuralist approach limited? We impose some cognitive organisation of our own in a top down manner - shown through apparent movement
• What is apparent movement? When we perceive movement even though there is none

  
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  What are illusory contours? We see a blue circle but the lines have just changed colour – we see edges even though no edges are present

  
• What is figure-ground segregation? Involves determining which part of an image is the foreground/ figure which is the background/ ground

  How does this show that our mind imposes structure? Our mind can switch between foreground and background without the actual image changing

  
• What is the Gestalt Approach? Explains how the whole is different from the sum of its parts thus perception not only built up from sensations but a result of perceptual organisation
• What are the Gestalt Laws?

  1. Law of Pragnanz
  2. law of proximity
  3. law of similarity
  4. law of good continuation
  5. law of common region
  6. law of connectedness
  7. law of closure
  8. law of synchrony
  9. law of common fate
• What is the Law of Pragnanz? States that stimulus patterns are perceived as simply as possible (Pragnanz = succinct) e.g. instead of perceiving 9 separate shapes, our minds will choose the simpler explanation of 5 separate shapes

  
• What is the law of proximity? States that nearby things are grouped together

  
• What is the law of similarity? States that similar things are grouped together

  
• What is the law of good continuation? States that connected points resulting in straight or smooth curves belong together

  -              Lines are perceived as following the smoothest path

  
• What is the law of common region? States that elements within the same region tend to be grouped together e.g. dots in b are considered as grouped together as defined by circle around them which overrides law of proximity

  
• What is the law of connectedness? States that elements that are connected together are grouped into a single object

  
• What is the law of closure? States that if there are breaks or occlusions in an object, we nevertheless perceive objects as continuing through those breaks or behind those occlusions – our mind uses law of closure to override breaks and put together a perception

  
• What is the law of synchrony? States that elements occurring at the same time are seen as belonging together

  
• What is the law of common fate? States that elements moving in the same direction are grouped together