During memory retrieval a substantial fraction of neurons ...



Nick L. Theodorou

Department of Physics & Astronomy

Nicholas.Theodorou.09@ucl.ac.uk

Human Memory Retrieval is predicted by Theta Frequency Phase locking of Single Neurons.

Based on Rutishauser et al. Nature 464 903- 7

Hypothesis

During memory retrieval a substantial fraction of neurons produce spikes which are phase-locked with the 3 – 8 Hz range of the Local Field Potential.

Can an increase of phase-locked spikes during learning predict an increase in memory retrieval strength?

Method I

A memory test with two parts was devised.

Learning: Patients were given a set of 100 previously unseen images.

Recognition: Patients viewed a set of 100 where 50 were previously unseen and 50 were part of the previous set. Patients decided on a six point confidence scale whether they had seen the image before or not.

To segregate the data from the recognition part of the test, the 50 images previously seen were characterised into remembered or forgotten; true positive or false negative. Then the neuronal activity was compared between true positive and false negative answers.

Method II

To record the single neuronal activity and the local field potential (LFP), microwires were implanted into the hippocampus and amygdale, part of a technique known as Electrophysiology.

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To compute results, data analysis was done on: behaviour; spike sorting; estimation of phase-locking; phase-reset analysis; and the Spike-Field Coherence.

If a difference between True Positive and False Negative was found it was verified using a control that had randomly assigned labels TP or FN.

Motivations

The purpose of this investigation was to investigate the formation and retrieval of memory by recording the activity of a population of single neurons. More specifically this study chose to isolate brain oscillations in the theta frequency range (3 – 8 Hz) as synaptic plasticity is induced, and analyse the synchronisation in terms of phase between the local theta oscillation and the coordination of ‘spike’ timing. Spikes of individual neurons timed against the local field potential (LFP) – the ‘coordinated action-potential timing across populations of neurons’. The LFP is the favoured inducer of synaptic plasticity, and synaptic plasticity is thought to underlie memory formation.

The investigation is important because a fuller understanding of memory formation would have many medical applications, especially in developing treatments for brain trauma or epilepsy.

Results I

The behavioural results of the test showed that the Patients had a good sense of the quality of their memories as confidence mapped well on to performance.

The figure on the left shows the receiver operating characteristics were asymmetrical which is signature of recognition memory dependent on the Medial Temporal Lobe.

Right is a histogram of the preferred phase of all the neurons that were phase-locked to an LFP oscillation in the in the theta range. This result was the motivation for the hypothesis. (n = 51 of 246, 21%)

Results II

The timing between single neuron spikes and the ongoing theta oscillation was quantified by the Spike-Field Coherence.

If the SFC is larger, the more accurate it can be said that a spike follows a particular phase.

A striking difference was found:

The theta range Spike Field Coherence for Remembered was ~50% higher than that for Forgotten.

Results III

Further data analysis was done using the Spike-Triggered Average which is constructed by averaging LFP segments of ± 400 ms centred on every spike. If a logical relationship between the spike timing and LFP exists, the resulting trace will deviate from zero.

Strong oscillations in the theta range were seen in each of the significantly phase locked neurons.

Yet the power of the LFP when the spike occurred did not distinguish between Remembered and Forgotten results.

Conclusion

The results indicate further evidence for the association of the theta range with synaptic plasticity.

There are many factors such as novelty, attention, and arousal influence the effectiveness of memory formation. However these results show that the Spike Field Coherence may distinguish between learning trials that were later remembered and those that were forgotten.

Further Reading:

Martin, S et al. Synaptic plasticity and memory: an

evaluation of the hypothesis. Annu. Rev. Neurosci. 23, 649–711 (2000).

Buzsa´ki, G. Theta oscillations in the hippocampus. Neuron 33, 325–340 (2002).

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