Cortical Rhythms (L12-13)

Question 1

Name the machine that studies the stages of sleep.

Answer 1

The Electroencephalogram (EEG).

Question 2

What is sleep characterised by?

Answer 2

• Loss of behavioural control

- Loss of consciousness

Question 3

Which 2 types of memory is sleep especially important for?

Answer 3

• Declarative memory

- Procedural memory

Question 4

Name the types of sleep and the stages at which they occur.

Answer 4

Light sleep - stage 1 and 2

Slow wave sleep (SWS) – stage 3 and 4

Rapid eye movement sleep (REM)

Non-REM

Question 5

Describe the oscillations that occur during slow wave sleep and the Hz of each.
(3)

Answer 5

• Slow oscillations – in humans ~0.8 Hz.
• Spindles (10-15 Hz), waxing and waning oscillation – arise from the thalamus – stage 2 and SWS.
• Hippocampal sharp waves – depolarisation with very fast 100-300 Hz

Question 6

Describe the oscillations that occur during REM and the Hz of each.
(3)

Answer 6

• Ponto-geniculo-occipital (PGO) waves that propagate from the
  pontine nucleus in the brainstem to lateral geniculate nucleus and visual cortex
• Theta oscillations – hallmark of REM sleep – mainly in the hippocampus
• Gamma 30-80 Hz oscillations

Question 7

Read:

What is sleep for?

Answer 7

• Consolidation of memories = active re-processing of new memories within the networks that encoded them.
• Occurs best during sleep so consolidation and encoding don’t interfere with each other.

Question 8

During consolidation newly encoded memory traces are
reactivated in the fast learning store and that then drives the slow learning store.

Representations in the slow learning store are gradually strengthened.

Fast learning store = ___1___

Slow learning store = ___2___

Answer 8

1. Hippocampus

2. Neocortex

Question 9

Read:

Place cell firing.

Answer 9

Place is partly coded by an increase in firing rate. Different hippocampal neurons respond to different parts of a spatial field.

Question 10

_____ means replaying during sleep the sequence of firing in hippocampal cells that occurred while encoding. Episodic events need a sequential structure.

Answer 10

Reactivation.

Question 11

Which parts of the brain are activated as a human navigates through a maze?

Answer 11

Occipito-parietal and hippocampal regions.

Question 12

Read:

Reactivation.

Answer 12

• Occurs during SWS – only in a minority of neurons recorded
• Reactivations occur in the order they were experienced
• Reactivations are noisier and less accurate
• Happens at a faster firing rate
• Promotes the gradual redistribution and reorganisation of memory to a site for long-term storage (system consolidation)
• Promotes synaptic changes that are necessary to stabilize memories (synaptic consolidation) which occurs during the subsequent REM sleep

Seen in the thalamus, striatum and neocortex.

Question 13

SWS = consolidation of ___1___ memories – hippocampus dependent.

REM – consolidation of ___2___ – hippocampus independent.

Answer 13

1. Declarative

2. Non-declarative

Question 14

_____ hypothesis – optimum benefit to memory consolidation when SWS and REM take place in succession.

Answer 14

Sequential.

Question 15

Read:

How do sleep oscillations promote synaptic plasticity?

Answer 15

• Spindles and action potentials associated cause increases in calcium
• Repeated spindles associated with LTP
• Spindles could therefore strengthen synapses

Question 16

Name the 2 main theories of memory formation.

Answer 16

• Synaptic homoeostasis

- System consolidation

Question 17

True or false?
- During waking and encoding synapses become potentiated

• During SWS there is a global downscaling – ~1 Hz SW ideal for LTD

Answer 17

True.

Question 18

Describe System consolidation.

3

Answer 18

• Events encoded in hippocampus and neocortex during awake
• Slow oscillations in SWS drive reactivations
• Synchronization of spindles and slow waves drives plasticity

Question 19

Read:

Odour memory reactivation experiment.

Answer 19

Re-exposure to an odour during SWS leads to enhanced retention performance.

Subjects sleep in the fMRI scanner - re-exposure to the odour during SWS activated left anterior hippocampus (left) and retrosplenal cortex (right).

No activation was seen when learning not associated with the odour.

This and other studies support the system consolidation theory.

Question 20

Abnormalities of sleep are associated with most _____ disease.

Answer 20

Neuropsychiatric.

Question 21

Read:
The Magnetoencphalogram (MEG).

Answer 21

Voltage changes are accompanied
by magnetic field changes.

MEG - measures the magnetic
field rather than electric fields.

Tiny fields but no distortion.

Keeps temporal resolution and
improves the spatial resolution - < 1cm.

Question 22

Describe the main limitation of the EEG.

Answer 22

Poor spatial resolution (10 cm2):

• Signals distorted by blood vessels, dura, skull, scalp muscles and skin.
• Mainly samples activity from the superficial layers of the cortex.

Question 23

List the pros of the EEG.

Answer 23

• Technically easy
• Non-invasive
• High temporal resolution

Question 24

Read:

Alpha oscillations.

Answer 24

• 9 – 11 Hz, large amplitude.
• Associated with relaxed wakefulness.
• Seen in primary sensory neocortical areas.
• Origin – locally generated or thalamo-cortical.
• Function – Idling rhythm – preserves the pattern of connectivity formed by previous sensory inputs.

Question 25

The local mean field potential of the EEG reflects the _____ behaviour of a large number of interacting neurons.

Answer 25

Average.

Question 26

How do we process perceptions, emotions, memories that
are distributed across different brain regions so easily?
(2)

Answer 26

• Local interactions - ~ 1cm -
  monosynaptic connections.

Large-scale synchrony
>1 cm - e.g. across
hemispheres or cortical
regions.

Question 27

_____ _____ involved in executive functions such as memory, attention, planning.

Answer 27

Prefrontal cortex.

Question 28

The _____ in humans and animals is critical for memory functions.

Answer 28

Hippocampus.

Question 29

Read:

Different EEG signals and their respective Hz.

Answer 29

Slow waves < 1 Hz

Delta rhythms 0.5 - 4 Hz

Theta 3-7 Hz

Alpha 9 -11 Hz

Spindles 8-15 Hz

Beta 15 - 30 Hz

Gamma 30 - 60 Hz

High gamma > 60 Hz

Ultrafast > 100 Hz

Question 30

Read:

Theta Oscillations.

Answer 30

4 – 7 Hz (theta 1), 7 – 12 Hz (theta 2) medium amplitude.

Theta 1 is associated with active exploratory movement.

Theta 2 is associated with passive exploratory movement.

Seen in most neocortical area, and hippocampus.

Origin – locally generated – some projections from medial septum to hippocampus.

Function – primary sensory processing and learning and recall.

Also occurs with gamma oscillations.

Question 31

Read:

Beta Oscillations.

Answer 31

Beta 1 ( 11-20 Hz) and beta 2 ( 21-29) Hz, low amplitude.

Beta 1 - associated with intense mental activity, attention.

Seen in primary sensory areas and central fronto-parietal area.

Origin – local neocortical generator.

Function – long-range synchronisation, learning and memory.

Associated with gamma oscillation.

Question 32

Read:

Gamma Oscillations.

Answer 32

Gamma slow 30-80 Hz, low amplitude.

Associated with sensory processing – occurs during REM sleep.

Origin – local neocortical generator.

Function – temporal code for primary sensory information.

Question 33

Read:

Abnormal network synchrony in AD.

Answer 33

Rodent study suggests:

• Deficits in PV interneurons => deficits in gamma => deficits in memory.

Human post-mortem studies have shown decreased PV expression in humans
so same thing applies.

Means that targeting interneurons and increasing their activity could restore
function.