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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
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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)
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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
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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
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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
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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.
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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.
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Draw a homunculus for the somatosensory cortex. Also label it.
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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
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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
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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
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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
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Do you remember phantom limbs?
If not, look at the powerpoint
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