Sensing the World

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  1. What are the general principles of the sensory systems?
    • We do not have access to the world, only the action potentials carried by sensory pathways to the brain
    • Sensory receptors are not evenly distributed
    • Some parts of the sensory surface (eg. the fovea in vision, the fingertips in
    • touch) have a higher density of sensory receptors, allowing high acuity
    • Sensory signals are sent to the cortex along parallel pathways
    • Anatomically distinct pathways from the sensory periphery to cortex carry different types of information in parallel
    • Cortex creates topographical maps of the sensory periphery
    • Cortical representations reflect the structure of the sensory periphery, but are plastic
  2. What is Müller's law?
    • We are not aware of objects themselves, but of the signals about them that are provided by our nerves
    • Müller (1935)
  3. What are sensory receptors?
    • Specialised nerve terminals 
    • Some pain receptors are just free nerve endings 
    • All nerves transduce (convert) the energy they are sensitive to into action potentials
  4. What is sensory acuity?
    • The minimum distance between two points of stimulation at which they can be perceived as different 
    • This is because sensory receptors tile the skin, with some overlap
  5. What are the major features of receptive fields?
    • Sensitivity is not equal across the receptive field; areas where it is maximally sensitive and thus doesn't need as much input to cross threshold potential 
    • All of the sensory surface is accounted for by at least one receptive field/sensory receptor
    • It is non-uniform; smaller on finger than on leg
  6. What are the main receptor pathways for touch?
    • Pacinian corpuscles: large, vague receptive fields. Fast-adapting. Detects vibrations.
    • Meissners corpuscles: small, defined RFs. Fast-adapting. Detects touch.
    • Merkels discs: small, defined RFs. Slow-adapting. Detects touch.
    • Ruffinis ending: large, vague RFs. Slow adapting. Detects stretch.
  7. Why is it advantageous to have multiple sensory pathways?
    • Due to the refractory period there's a limit to the amount of info that can be transmitted (e.g. neurons can only fire 500x /s).
    • Thus multiple pathways is helpful, because the brain needs to make quick decisions.
    • Parallel pathways allow signals to be sent to the relevant brain region, e.g. light intensity SRs go to the area of the brain which deals with pupil diameter.
  8. Draw a homunculus for the somatosensory cortex. Also label it.
    Image Upload 1
  9. What is acuity correlated with, and why?
    • Density of sensory receptors, and cortical magnification 
    • Each sensory receptor has an equal sized area of the cortex. So where there is more SRs, there will be more cortex dedicated to the part of the surface
    • For example, fingers and genitals are represented a lot more than the lower lip
  10. What are the main features of the magnocellular pathway?
    • Carries visual neural information along the upper, dorsal stream of the brain to help us understand motion.
    • This magnocellular visual pathway tells us all about the ‘where’ of things- WHERE objects exist in relation to ourselves and HOW we guide our movement in relation to those objects, but not what they look like.
    • The magnocellular visual stream signals us to an awareness of the time properties of objects. For instance, detection of the movement, distance, and speed of an object moving towards us.
    • Fire quickly then plateau 
    • Less dense than parvocellular 
    • Not colour sensitive
  11. What are the main features of the parvocellular pathway?
    • Parvus means small
    • Gets input from midget cells (retinal ganglion cell type)
    • Deals with fine detail, WHAT things look like
    • Fire more slowly and consistently 
    • More dense than magnoceullular 
    • Colour sensitive
  12. Which vision area is overrepresented in the visual cortex, and why?
    • The fovea 
    • Because the majority of the ganglion cells focus on the majority of photoreceptors that are there, and the most information comes from there
  13. Do you remember phantom limbs?
    If not, look at the powerpoint
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Sensing the World
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