EEG - Electroencephalography Neuroanatomy and Physiology Questions and Answers 1 — Questions and Answers
Question 1: The primary generators of the electrical potentials recorded by scalp EEG are:
- Action potentials from deep cortical neurons
- Postsynaptic potentials of pyramidal cells in the cerebral cortex (Correct answer)
- Electrical activity from glial cells and astrocytes
- Axonal currents from white matter tracts
Correct answer: Postsynaptic potentials of pyramidal cells in the cerebral cortex
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?
- Hippocampus
- Basal Ganglia
- Thalamus (Correct answer)
- Cerebellum
Correct answer: Thalamus
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:
- A secondary burst of excitatory postsynaptic potentials (EPSPs)
- The summation of multiple action potentials
- A prolonged afterhyperpolarization and inhibitory postsynaptic potentials (IPSPs) (Correct answer)
- Artifact from patient movement following the sharp wave
Correct answer: A prolonged afterhyperpolarization and inhibitory postsynaptic potentials (IPSPs)
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?
- The primary visual cortex (Brodmann area 17)
- The primary auditory cortex (Brodmann areas 41, 42)
- The prefrontal cortex (Brodmann areas 9, 10)
- The primary motor and somatosensory cortex (Brodmann areas 4, 1, 2, 3) (Correct answer)
Correct answer: The primary motor and somatosensory cortex (Brodmann areas 4, 1, 2, 3)
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?
- The high electrical impedance of the cerebrospinal fluid
- The filtering effect of the skull and scalp, which attenuates small or asynchronous signals (Correct answer)
- The requirement for bilateral hemispheric involvement for any recordable potential
- The specific firing frequency of pyramidal neurons
Correct answer: The filtering effect of the skull and scalp, which attenuates small or asynchronous signals
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.
Question 6: The synchronization of cortical neurons necessary to produce normal resting rhythms and abnormal discharges like generalized spike-and-wave is heavily dependent on the integrity of:
- Corpus callosum pathways
- Corticospinal tracts
- Corticothalamic circuits (Correct answer)
- The fornix and limbic pathways
Correct answer: Corticothalamic circuits
Corticothalamic circuits, the reciprocal connections between the cortex and the thalamus, are fundamental for synchronizing neuronal activity across large brain regions. These circuits are responsible for pacing normal rhythms like sleep spindles and the alpha rhythm, and their dysfunction is implicated in the generation of widespread, synchronized discharges seen in generalized epilepsies.
EEG - Electroencephalography Neuroanatomy and Physiology Questions and Answers 1
The primary generators of the electrical potentials recorded by scalp EEG are: