Mode shifting between storage and recall based on novelty detection in oscillating

hippocampal circuits

M. MeeterJ. M. J. MurreL. M. Talamini

Date of Presentation: 05/09/2012

Introduction

Role of Acetylcholine in Mode Shifting

• Hippocampal novelty detection may regulate levels of acetylcholine– Shifts hippocampal dynamics between encoding or

retrieval• Information with high novelty content would

induce a learning state• Input similar to already stored patterns would

induce a retrieval state– Little learning takes place during retrieval to protect

existing patterns from modification.

Role of Acetylcholine in Mode Shifting

• Hippocampal novelty detection may regulate levels of acetylcholine– Shifts hippocampal dynamics between encoding or

retrieval• Retrieval mode: low ACh levels• Learning mode: high ACh levels

Role of Acetylcholine in Mode Shifting

• Hippocampal novelty detection may regulate levels of acetylcholine– Shifts hippocampal dynamics between encoding or

retrieval• Information with high novelty content would

induce a learning state• Input similar to already stored patterns would

induce a retrieval state– Little learning takes place during retrieval to protect

existing patterns from modification.

Learning State

Role of Acetylcholine in Mode Shifting

• Hippocampal novelty detection may regulate levels of acetylcholine– Shifts hippocampal dynamics between encoding or

retrieval• Information with high novelty content would

induce a learning state• Input similar to already stored patterns would

induce a retrieval state– Little learning takes place during retrieval to protect

existing patterns from modification.

Retrieval State

Opposition to ACh’s Role in Mode Shifting

• ACh has a slow and sustained influence on hippocampal activity– Depolarization develops a few seconds after ACh

release– Lasts 10 s or more

• Dynamics are too slow to underlie mode shifting.• Opposers support a fast mode shift– On the order ot 10s or 100s of milliseconds

Opposition of ACh’s Role in Mode Shifting

• ACh has a slow and sustained influence on hippocampal activity– Depolarization develops a few seconds after ACh

release– Lasts 10 s or more

• Dynamics are too slow to underlie mode shifting.• Opposers support a fast mode shift– On the order of 10s or 100s of milliseconds

Opposition to the Opposition of ACh’s Role in Mode Shifting

• Time scale at which natural learning and retrieval take place is slow– Rats take minutes to familiarize themselves to new

environments– Fear conditioning takes several seconds to take (in

rats)– In humans, long-term recall deteriorates when

study times are less than 2 s

The System

Structural and functional properties of the subregions and connections

Feedback and feedforward inhibition

Oscillatory population dynamics

Theta (4-10 Hz) and Gamma (20-40 Hz)

Features

Integrate-and-fire nodes (Sodium, potassium, chloride, and leak currents

Hebbian learning (LTP) and negative Hebbian learning (LTD)

Entorhinal Cortex

• Main cortical input structure of the hippocampus

• Considered as one single input layer that propagates the same information to the DG, CA3, and CA1

Dentate Gyrus

• Receives the majority of the input from the EC• Sparseness of activation

– High number of granule cells but very few fire at a given moment– Divergent input

• Orthogonalizes input

• Orthogonalization– Patterns that are correlated in a given layer (i.e. EC) generate

uncorrelated representations in the projection field (i.e. DG)

• No Hebbian plasticity between the DG and CA3• Feedforward inhibition to CA3

CA3

• Involved in either:– Autoassociative learning and pattern completion– Heteroassociative learning and sequence recall

• Both cases are inferred from extensive recurrent connections among CA3

• Difference in time scale:– Autoassociation: learning over short intervals– Heteroassociation: learning over longer intervals

CA1

• Receives a direct projection from the EC (Yeckel and Berger, 1990)

• Indirect projection from tri-synaptic loop

• Proposed to be a translator between the code of the CA3 and the cortical code

• Model associates the CA3 pattern with the EC pattern

Medial Septal Nuclei

• Fibers from the CA1 and CA3 target GABAergic septal projection neurons and ACh neurons– Hippocampal activity inhibits the septum

• Modulates the hippocampus using ACh– Nonspecific target– Seems to affect the entire hippocampus

Hippocampus

Summary

EC:- Major input

DG:- Orthogonalizer

CA3:- Storage

CA1:- Translator

Septum:- ACh modulation

Acetylcholine as a Novelty Signal

• Experiments– During exploration of a new environment, ACh is

increased relative to baseline– ACh levels decrease during consecutive

explorations of the same test enviornment

Effects of Acetylcholine

• Dampens transmission between CA3 and CA1

• Slow, subthreshold depolarization of hippocampal principal neurons lasting several seconds

• Enhancement of LTP at CA3, CA1, and DG

• Reduction of adaptation in CA3, CA1, and DG

• Suppressed inhibition of DG and pyramidal cells (supression of basket cells)

Effects of Acetylcholine

• Learning mode– High ACh– CA3-CA1 transmission dampened– Activity in CA1 is dominated by EC– Allows CA3-CA1 connection to store the association

between the EC pattern and CA3 pattern• Retrieval mode– Low ACh – CA1 relays the reinstated CA3 pattern to the EC and

other output structures

Results

Storage

“Correct CA1 Nodes”

• Correct CA1 Nodes: number of firing CA1 nodes that receive a one-to-one connection from an EC node

• Incorrect CA1 Nodes: number of firing CA1 nodes that are not connected to an EC node

• Missing CA1 nodes: number of CA1 nodes that receive a one-to-one connection from an EC node that does not fire

Retrieval Retrieval mode (ACh = 0.1)

Pattern Completion

• After storing one pattern, a variable number of EC nodes associated with the pattern are deactivated

• Test in retrieval mode (ACh = 0.1) and learning mode (ACh = 0.75)

• Pattern completion measured as the maximum proportion of correct CA1 nodes that were simultaneously active during the theta cycle

Pattern Completion

CA1 activity is strongly correlated with DG activityPattern completion occurs in DG

Number of correct nodes increasesNumber of incorrect nodes increasesACh depolarizes all cells making activation easier

Pattern Completion

• With small cue-sizes, more of the pattern is completed in learning mode than in retrieval mode but comes at a price of a compromised integrity of retrieval

• Hypothesis: During effortful retrieval, input cues do not lead to an instatement of a stored pattern, so the hippocampus shifts to learning mode

• Consistent with the “retrieval practice effect”– Implies that effortful and successful retrieval constitutes a

power learning method– Effort retrieval has a stronger effect on retention than fast,

easy recall

Novelty Detection and Dentate Gyrus

• After acquisition of one pattern, the network was cued with patterns that were either the same as the stored pattern (old), completely different (new), or consisted of a variable ratio between old and new EC nodes

• DG same: Same DG nodes activated as during the stored pattern

• DG diff: Different DG nodes activated than with stored pattern

• Correct CA1 nodes

Novelty Detection and Dentate Gyrus

DG and CA1 liked old stuff, not new stuffLittle overlap under areas of same and diff

All types of input elicit strong activity in DG and CA1Larger range of mixed input activates mixture of old and new DG nodes

Effects of ACh on Pattern Storage-New pattern was presented during one theta cycle with different levels of ACh modulation

- Acquisition was evaluated by presenting the pattern during one theta cycle with ACh modulation set at 0.1

- Maximum number of correct CA1 nodes simultaneously active during retrieval phase was used to assess ACh effects on learning performance

- If ACh was too high during learning phase, then learning was inhibited

- If ACh is too high, then EC alone can cause CA1 to fire before CA3 can stimulate CA1

- CA1 cells that do not receive EC input will form stronger connections with their CA3 input, which does not reflect EC firing for diminished retrieval performance

Effects of ACh on Novelty Detection

• Stored one pattern and presented either the “old” pattern or a randomly selected new pattern while varying ACh modulation

• Look at difference in activity from the old pattern versus the new pattern

Effects of ACh on Novelty Detection

Novelty detection decreases as ACh increasesDifference in activity is first seen in DG

Insights from the Model

• Prominent role of DG in novelty detection• Reasoning for the existence of separate learning and

retrieval modes– Retrieval in learning mode is unreliable through the activation

of features that were no part of the original memory• Separation of learning and retrieval modes enhances

accurate retrieval• Hippocampus incorporates a low band filter to ensure

that ACh modulation fluctuates with novelty and not with theta or gamma rhythms

Slow Shift vs. Fast Shift

• Slow Shift– Mediated by ACh

– Mode shifting induced by novelty of input

– Most learning takes place when a pattern is new

• Fast Shift• Mediated by theta modulation

where LTP and LTD dominate in different phases of the cycle

• Occurs automatically due to differing learning and retrieval dynamics on different phases of theta

• Old and new patterns are continuously learned, unlearned, and relearned

Predictions of the Model

• Learning should occur more quickly in experimentally induced high ACh states versus low ACh states

• Novel configurations lead to increased activation of cholinergic cells leading to a surge in hippocampal ACh release

• Novel configurations produce enhanced synaptic plasticity in the hippocampus

• New patterns should elicit little activity in the hippocampus in the absence of ACh

• Model suggests that balance of strength between direct perforant path and trisynaptic input to CA1 is essential to pattern encoding

Other Possible Novelty Signals

• Novelty signal over the CA3 through intermediaries of lateral septum, and raphe nuclei, reticular formation– Could influence levels of arousal

• Novelty signal via the EC to the ventral striatum– Could influence chain of events that facilitate a

change of behavioral strategy

Uses of the Model

• Explore the significance of different parallel inputs (input from different layers of the EC) to the hippocampus for memory processing

• Explore how autoassociation and heteroassociation may be implemented in circuitry

• How suppression of familiar objects in parahippocampal cortex affects configuration novelty detection in hippocampus

• How hippocampal subdivisions differentially contribute to neuropsychological constructs such as recall, recognition, and familiarity processing

Problems

• “Therefore, at present there is no direct evidence for a hippocampal influence on ACh release during behavioral learning.” !!!!!!!!!

• “That is, the GABAergic cells would disinhibit ACh cells in response to hippocampo-septal stimulation.”– Opposite of what the model says it should do– “However, the inhibitory hippocamp-medioseptal

projection directly onto ACh neurons may be sufficient to regulate activity of ACh neurons during mode-shifting.”

Extra Stuff