EEG - Electroencephalography Neuroanatomy and Physiology Questions and Answers

Question 1

The primary generators of the electrical potentials recorded by scalp EEG are:

Accepted Answer

Postsynaptic potentials of pyramidal cells in the cerebral cortex

Answer

Scalp EEG primarily reflects the summation of excitatory (EPSPs) and inhibitory (IPSPs) postsynaptic potentials from large, vertically oriented pyramidal neurons in the cerebral cortex. Action potentials are too brief and their electrical fields are too localized to be reliably detected at the scalp. Glial cells and white matter tracts do not generate the synchronized electrical fields necessary for scalp EEG detection.

Question 2

A patient undergoing an EEG displays rhythmic 8-13 Hz activity over the occipital regions that attenuates with eye-opening. This activity is primarily modulated by which subcortical structure?

Accepted Answer

Thalamus

Answer

The posterior dominant alpha rhythm, an 8-13 Hz activity seen in relaxed wakefulness, is generated through corticothalamic circuits. The thalamus acts as a pacemaker and gatekeeper, synchronizing cortical activity to produce these characteristic rhythms. The hippocampus is involved in memory, the basal ganglia in motor control, and the cerebellum in coordination; they are not the primary modulators of the alpha rhythm.

Question 3

During a recording, an epileptiform discharge characterized by a sharp wave followed by a slow wave is observed. The slow wave component is physiologically understood to represent:

Accepted Answer

A prolonged afterhyperpolarization and inhibitory postsynaptic potentials (IPSPs)

Answer

The slow wave following an interictal spike or sharp wave represents a period of profound inhibition. This is caused by the activation of inhibitory interneurons, leading to inhibitory postsynaptic potentials (IPSPs) and a prolonged afterhyperpolarization in the surrounding neuronal population, which temporarily suppresses excitability.

Question 4

An EEG electrode placed at C3, according to the International 10-20 System, primarily records activity from which cortical region?

Accepted Answer

The primary motor and somatosensory cortex (Brodmann areas 4, 1, 2, 3)

Answer

The central electrodes (C3, Cz, C4) are situated over the Rolandic fissure (central sulcus). This area encompasses the precentral gyrus (primary motor cortex, Brodmann area 4) and the postcentral gyrus (primary somatosensory cortex, Brodmann areas 1, 2, and 3).

Question 5

Which of the following best describes the neurophysiological basis for why a minimum cortical area of approximately 6-20 cm² must be synchronously activated to generate a detectable scalp EEG signal?

Accepted Answer

The filtering effect of the skull and scalp, which attenuates small or asynchronous signals

Answer

The skull, dura, and scalp act as low-pass filters and have high electrical resistance, a process known as volume conduction. This significantly attenuates the electrical potentials generated by the brain. Therefore, a large area of cortex with thousands of neurons firing synchronously is required to produce a signal strong enough to overcome this impedance and be detected by scalp electrodes.

EEG - Electroencephalography Practice Test

EEG - Electroencephalography Practice Test

EEG - Electroencephalography Neuroanatomy and Physiology Questions and Answers