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
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
