Summary Task 5 - Brain Computer Interface

BRAIN COMPUTER INTERFACE (BCI)
NETWORK MODELS OF BRAIN FUNCTION (MCLEOD)

MEMORY FORMATION IN THE HIPPOCAMPUS

 Hippocampus consists of 2 interlocking sheets of cells: cornu ammonis (CA) & dentate
gyrus (DG)
 Involved in learning of new information
 Example: patient HM – bilateral removal of parts of temporal lobe including
hippocampus  developed anterograde amnesia

ROLE OF HIPPOCAMPUS IN MEMORY FORMATION

 Damage affects ability to form new memories
 Within new learning, the effects are selective – damage in humans:
 Failure to form new episodic memories
 Recoding of events which make up day to day experience – formation of
specific memory which can later be recalled by cueing with part of original
memory
 New procedural memories can still be formed
 Gradual development over many related experiences – example: skill
acquisition
 Hippocampus involved in formation of certain sorts of memories rather than being
actual site of storage

INFORMATION FLOW TO & FROM THE HIPPOCAMPUS

 Hippocampus receives input from parahippocampal gyrus & entorhinal cortex
 These areas receive input from virtually all association areas
 Suitable place for combining information about different aspects of an
experience
 Output from hippocampus to cortical areas which provide input to the hippocampus
 Does not need to be long-term depository for memories which were formed there
initially
 Connections with sublimbic structures – provide general regulation of activity in
hippocampus, making it more or less likely to operate at any particular time

INTERNAL STRUCTURE OF THE HIPPOCAMPUS

 Input to hippocampus comes on perforant path
 Information processing within hippocampus occurs in 3 sequential stages – dentate gyrus, 2
areas of cornu ammonis (CA3 & CA1)


Dentate  Perforant path synapses in the dentate gyrus

, gyrus  Output from dentate gyrus carried by mossy fibres to cells in CA3 region
 Typical mossy fibre connects about 15 different CA3 pyramidal cells
 Each pyramidal cell receives about 50 mossy fibre inputs

CA3  Direct input from perforant path – much weaker than dentate granule cell inputs
via mossy fibres
 Output branches
 one forms a set of recurrent connections, synapsing back to dendrites
of other CA3 cells
 Schaeffer collateral – carries output from CA3 cells to CA1
 Can transmit info to most other CA3 cells within 3 synaptic connections
 Intrinsic, recurrent, excitatory connections are the dominant source of
input

CA1  Input from Schaeffer collaterals
 Output to neocortical areas which provided hippocampal perforant path input

COMPUTATIONAL THEORY OF HIPPOCAMPAL OPERATION

 Hippocampus involved in formation of episodic memories
 Formation must be very quick
 Internal structure of DG  CA3  CA1 suggests basis for computational theory of how
episodic memory might be formed BUT theory is still tentative


Sparse input  Factor that limits number of memories that can be stored in
from dentate associative networks
gyrus  Number of input per neuron
 Best way to maximise capacity of associative memory  sparse
representation at input
 Hippocampus does exactly that – any input pattern excites
relatively few CA3 cells
 Different input patterns are likely to activate different sets of
CA3 neurons
 Hypothesis: perforant path – DG cell system acts as competitive learning
network
 Removes redundancy – output from DG system less correlated &
more categorised than inputs from perforant path
 Role of DG – mossy fibre system: maximise separation of patterns
reaching CA3 autoassociation system
 Representations of different events do tend to be uncorrelated

Autoassociation  Ability to recall complex memory with a cue which is a sub-
in CA3 component of the whole