The document discusses the physiological basis of electroencephalograms (EEG), focusing on the synchronization of neural activity in the cortex and the role of pyramidal neurons in generating detectable EEG signals. It outlines the history of EEG discovery, mechanisms of synchronization, and the significance of different rhythmic activities (alpha, beta, theta, and delta) in cognitive processes. Additionally, it emphasizes the importance of neuronal oscillations for memory, attention, and conscious awareness.

1. Electroencephalogram,
EEG generators

2. EEG Physiological Basis
I
II/III
IV
V
VI
•Sources:
• EPSPs
• IPSPs
• Sign of Signal:
• Cortico-Cortical inputs
• Thalamo-cortical inputs
• Signal Summation
• Synchrony
• Direction
• Action Potentials?

3. Outline for the session
• History and Introduction
• Concept of field potentials and dipoles
• The net result !!
• Mechanisms of Synchronization
• Modulation of Synchronized rhythms – Why ?
• Summary

4. Outline for the session
• History and Introduction

5. Discovery of Brain Electrical Activity
1875 - Richard Caton, a physician in England - Rabbit cortex electrical activity
1890 - Adolph Beck of Poland
1929 - Hans Berger (1873 - 1941), an Austrian psychiatrist
- The discoverer of human EEG (= ElectroEncephaloGram)
- Alpha and Beta waves, eye closed and open, mental task
- Sleep and Awake
- Illness and EEG
- Drug and EEG: Phenobarbital, morphine, cocaine
- Telepathic transmission

6. Outline for the session
• History and Introduction
• Concept of field potentials and dipoles

7. EEG
Many neurons need to sum their activity in order to be detected by EEG
electrodes. The timing of their activity is crucial. Synchronized neural activity
produces larger signals.

8. Layers of the cerebral cortex
The 2 mm thick
cortex can be
divided into six
distinct layers.
Each layer is
distinguished both
by the type of
neurons that it
contains and by the
connections that it
makes with other
areas of the brain.
It is believed that
the activation of the
large pyramidal
cells of layer V is
what is reflected in
most EEGs.

9. Cortical pyramidal neurons
Pyramidal cells (layer V) are the
primary output neurons of the
cerebral cortex.
Pyramidal neurons are
glutamatergic and compose
approximately 80% of the neurons of
the cortex.
They have a triangularly shaped
cell body, a single apical dendrite
extending towards the pial surface,
multiple basal dendrites, and a
single axon.
Pyramidal cells are grouped
closely together and organized in
the same orientation.

10. EEGs reflect synchronous firing of pyramidal cells
There are perhaps 105 neurons under a mm2 of cortical surface.
EEG electrodes measure the space-averaged activity of >107 neurons.
A µvolt signal will only be detected if pyramidal neurons are synchronously
activated and many small dipoles are combined.

13. The “equivalent dipole”
+
-
+ +
+
+
+
+
+
+
+ +
+
+
+
--
+
-
Equivalent
Dipole
Thalamo-Cortical fiber

15. Scalp EEG: positive polarity
EPSP
IPSP
+
EPSP deep layer
+
-
EPSP superficial layer
+
+
-
-
IPSP superficial layer
Scalp EEG: negative polarity
-
+
-
IPSP deep layer
-
+
Note:
- At a surface electrode, both positive and negative polarity may
indicate depolarization (EPSPs) depending on the orientation of
the dipole.
- EPSPs in superficial layers and IPSPs in deeper layers appear
(at a surface electrode) as a negative potential.

16. Synaptic Geometry
dipoles cancel
dipoles reinforce
dipoles cancel
dipoles cancel

17. Possible dipole orientations
In a sphere (the head), dipoles can be radial (perpendicular to the
surface of the skull), tangential (parallel to the surface of the skull)
or partially radial and partially tangential.

18. detectable
electrical field
undetectable
electrical field
undetectable
magnetic field
detectable
magnetic field
The activation of pyramidal neurons in layer V of the cortex that are oriented
perpendicular to the surface of the skull will contribute to an EEG signal.
It takes approximately 100,000 adjacent neurons acting in temporal synchrony
to produce a measurable change in electric field outside the head.

19. Outline for the session
• History and Introduction
• Concept of field potentials and dipoles
• The net result !!

20. The Electroencephalogram
Two ways of generating synchronicity:
a) pacemaker; b) mutual coordination
1600 oscillators (excitatory cells)
un-coordinated
coordinated

21. Normal EEG Rhythms
Alpha: 8-13 Hz
Beta: 14-30 Hz
Theta: 4-7 hz
Delta: <3.5
1 sec

22. Outline for the session
• History and Introduction
• Concept of field potentials and dipoles
• The net result !!
• Mechanisms of Synchronization

24. THALAMUS

25. Gyri orbitales
Gyri frontales
Co
rp
u
G. praecentralis
G. postcentralis
RE
Gyri
temporales
VA
VL
VP LP
Pu
LGN
MGN
Gyri
parietales
A
M
ss
tri
atu
m

26. Thalamo-cortical reentrant loops.
Steriade, M. (1999). Coherent oscillations and
short-term plasticity in corticothalamic networks. TINS, Vol. 22 (8), 337-344.
Basic Circuitry:
Cortex
RE
Cortex
Dorsal Thal. = Relay Nuclei
RE
L-circ
Th-cx
Dendro-dendr.
Th-cx
RE
L-circ
Aff
‚Secondary neurons‘

27. 1,2
Afferent brainstem input to Th-cx (1),
Activation of RE and Cortex (2)
Cortex
L-circ
RE
Th-cx
Dendro-dendr.
Th-cx
RE
L-circ
Aff

28. 3 Excitatory processes in Cortex;
Inhibition of primary L-circ;
Inhibition of other RE cells
Cortex
L-circ
RE
Th-cx
Dendro-dendr.
Th-cx
RE
L-circ
Aff

29. 4
Excitatory feedback response from cortex.
Disinhibition of primary L-circ neurons.
Inhibition of secondary Th-cx neurons.
The resulting effect is that during time 4, Th-cx are again under inhibitory control from L-circ neurons and, at the same time are activated
from cortico-thalamic cells. Thus, only strong (converging and/or amplified) cortical feedback will trigger another excitatory activation
wave into the cortex in time 5.
Cortex
The strong
inhibition of the
secondary Th-cx
cell may lead to low
threshold spikes
(LTS) and, thus, to
a 10 Hz oscillation.
L-circ
RE
Th-cx
Th-cx
RE
L-circ
Aff

30. 5 The primary Th-cx cell may start a new excitatory burst into the cortex. At this stage
(because released from the L-circ inhibition), a new afferent input will have a strong effect.
The secondary Th-cx cells remain under inhibition
RESULT: Center-surround ‚on-off‘ effect with a resulting strong focal activation of
cortical target neurons.
Cortex
RE
L-circ
Th-cx
Th-cx
RE
L-circ
Aff

31. Summary of findings:
Afferent brainstem activation is missing and cortical activation is strong:
- Th-cx cells are hyperpolarized and oscillate with spindle frequency Note that a depol.
current pulse during maximal hyperpol. leads to high frequency bursts. The result is
increased oscillatory cortical activation leading to Delta activity.
- The effect of increased cortical activation is even larger if stimulation patterns are
oscillatory
SLEEP: Spindles and Delta
RE
Cortex
Th-cx hyperpolarized,
Sleep spindles
L-circ
Th-cx
Missing brainstem afferents

33. Izhikevich Spiking Neuron
Model (2003)
• Claimed to be as realistic as
Hodgkin-Huxley neurons.
• As computationally efficient as
simple integrate-and-fire.
Izhikevich, E. M. (2003). "Simple model of spiking
neurons." IEEE Transactions on Neural Networks
14(6): 1569-1572.

35. Variables
v: Membrane potential.
u: Membrane Recovery variable, giving -ve feedback to v (higher value
means neuron less likely to fire), representing Na+ and K+ ionic
currents.
4 Parameters
c: Reset value for v; higher value means easier / more likely to fire
again.
a: Speed of recovery of u; higher value means faster recovery.
b: Sensitivity of u to v; higher values tightly couple the 2 variables
resulting in
possible sub-threshold oscillations and low threshold
spiking.
d: Additive reset value for u; higher value means harder / less likely for
neuron to
fire again.

36. Figure 3.15 Two Representations of Neural Circuitry (Part 2)

37. Outline for the session
• History and Introduction
• Concept of field potentials and dipoles
• The net result !!
• Mechanisms of Synchronization
• Modulation of Synchronized rhythms – Why ?

38. Why do we need so many
oscillations ?

39. Fourier Transform: The inner product
Rodrigo Quian Quiroga
Sloan-Swartz Center for Theoretical Neurobiology
California Institute of Technology
http://www.vis.caltech.edu/~rodri

40. Frequency spectrum of a normal EEG.

41. Temporal spectra from frontal scalp
From Freeman et al. 2003

42. Roles of Neuronal Oscillation
• Memory processes are most closely related to theta and
gamma oscillations;
• Attention seems closely associated with alpha and gamma
oscillations;
• Conscious awareness may arise from synchronous neural
oscillations occurring globally throughout the brain;
• Gamma wave and epileptics seizure.
Neuronal Oscillations represent a dynamical interplay
between cellular and synaptic mechanisms.
- Lawrence M. Ward, TRENDS in Cognitive Sciences Vol.7 No.12 December 2003,
553-559

43. Rhythmic activity appears to play a major role in
information processing in the brain
Object recognition
Feature extraction/abstraction
Associative learning
Selective attention
Novelty detection

44. Outline for the session
• History and Introduction
• Concept of field potentials and dipoles
• The net result !!
• Mechanisms of Synchronization
• Modulation of Synchronized rhythms – Why ?
• Summary

45. To summarize…
• Alpha rhythm
• Spindles
• Delta rhythm
• Theta rhythm
• Beta, gamma rhythm