1. Neural Coding and Function of Hippocampus Flashcards
patient H.M.
- started the investigation into different brain regions playing different roles
- bilateral removal of medial temporal lobes and hippocampus
- anterograde and slight retrograde (before) amnesia
- lots of cortex affected as well
what memory are we focusing on?
declarative memory
declarative memory
past events - experience it once and are able to remember again
- medial temporal lobe - diencephalon
can we study cognitive abilities in animals?
- many people have argued the importance of the cortex for cognition (Kirsch et al, 2008) - complex make up with 6 layers of neurons all projecting to other layers (PFC too)
- animals have impressive cognitive abilities even without a cortex - ‘insight related cognition’
- animals may show simple precursors to higher order cognition and episodic memory
animal models
- cases like H.M. are rare and we cant study in humans because its unethical
- can use more invasive techniques in animals/human studies methodology is limited
scala naturae
- ladder of being (simple > complex)
- similar in terms of evolution in the brain
- birds and mammals 300 million years apart/birds don’t have cortex but we do
- to see if the cortex is responsible for higher cognitive function, test that in an animal with no cortex (bird)
Prior (2008) - concept of themselves
does a magpie recognise its self in the mirror?
- the bird acts aggressive thinking it is another rival bird
- suggests no theory of mind
- however, other magpies look behind the mirror knowing it is not another bird - similar to child’s ToM
contingency testing
some birds make specific movements to see how the mirror responds
- knows its not a rival bird
tool use
Weir et al., (2002) - bird presented with a straight wire and out of reach grub > bird bends wire to hook it out of tube
Bird et al., (2009) - uses stones to displace water so floating grub becomes in reach
hippocampus
- cells might do different things depending on sub region
curved shape from central to lateral part of brain - described as a tri-synaptic pathway as it contains the perforant pathway, mossy fibres, Schaffer collaterals
- subregions: dentate gyrus, CA3, CA1, Subiculum
- contains projection and inhibitory interneurons
Dentate gyrus/Perforant pathway
- input path/location of the hippocampus from dentate gyrus
- densely packed with granule cells
- many different interneurons
- prominent in neurogenesis (new cell formation) - function of new memories?
are granule cells excitatory or inhibitory?
excitatory projection neurons
projection neurons
across subregions
project to next part (e.g. dentate gyrus to CA3)
inhibitory interneurons
within a subregion
- connect neurons within one of the subregions (e.g. within dentate gyrus)
CA1 and CA3
- hippocampus proper (main area) - complex structure
- complex network of of pyramidal neurons and inhibitory interneurons (differ in type of dendrites, where they project to, what they project to)
- there is a continuous path of pyramidal neurons (multipolar neurons) > primary excitation units of mamalian PFC - dense recurrent network in CA3
- lots of interneurons at each region (CA3 - pyramidal neurons that connect to each other - formation of memories?)
- overall you see a tri-synaptic pathway
- many cannabinoid receptors - link between weed and poor memory
tri-synaptic pathway
input from perforant pathway through dentate gyrus - axons of these neurons (mossy fibres) project to CA3 - pyramidal neurons here project to CA1 - continues to subiculum
subiculum
- output region of the hippocampus
- two types of principle neurons:
1) regular firing = consistent firing of AP’s
2) bursting neurons = fire AP’s in bursts - very central brain location so hard to study
hippocampal formation
- part of the limbic system
- main inputs and outputs are in terms of entorhinal cortex which sits underneath hippocampus
- perforant pathway = layer II of entorhinal cortex to CA3
- tempero-ammonic pathway = layer III of entorhinal cortex to CA1
- entorhinal cortex projects to different regions (temporal lobe > entorhinal cortex > hippocampus - towards centre of brain)
- have dendritic trees that extend to both sides of input location
- gives you an idea of the complexity when trying to study specific neurons in this location
electrophysiology
- measure the flow of ions in the brain with electrodes
- from recorded signal we can determine when an AP was elicited by a neuron
- results in spike trains (identifying neural activity) - key to linking neuronal activity with behaviour
neuronal coding
AP reflect sensory perceptions, cognitive functions and behaviour
- RATE CODING = number of spikes in a time window (usually per second - Hz)
rate coding
cognitive function represented in AP’s
- can look at the neural coding of individual neurons and compare them (could be related or random)
- can also see whether different neurons fire simultaneously or are correlated (e.g. one fires and another fires slightly after/is excited)
- could fire when animal in specific locations of sees a stimulus of a specific orientation (Hubel and Wiesel)
- Hafting et al (2005)
Hafting et al (2005)
- rat in a box
- black lines showed its walking pattern around the box
- electrode placed in entorhinal cortex
- red dots resemble where this neuron elicited an AP
- shows clusters of activity in specific locations
- if you know the neural activity you can identify where the rat is located in the box (DECODING FROM AP’S)