Electro-Encephalogram (Eeg)

ELECTRO-ENCEPHALOGRAM (EEG)

Q. 1

An adolescent is brought to the emergency department following an episode of myoclonic jerks at morning after waking up. His consciousness was not impaired. His EEG shows generalized 3-4 Hz spike and slow wave complexes. Most probable diagnosis is?

 A

Generalized tonic clonic seizure

 B

Absent seizure

 C

Temporal lobe epilepsy

 D

Juvenile myoclonic epilepsy

Q. 1

An adolescent is brought to the emergency department following an episode of myoclonic jerks at morning after waking up. His consciousness was not impaired. His EEG shows generalized 3-4 Hz spike and slow wave complexes. Most probable diagnosis is?

 A

Generalized tonic clonic seizure

 B

Absent seizure

 C

Temporal lobe epilepsy

 D

Juvenile myoclonic epilepsy

Ans. D

Explanation:

Juvenile myoclonic epilepsy is a subtype of idiopathic generalized epilepsy with onset usually between 8 and 20 years of age. Myoclonic jerks, especially in the morning, are of variable intensity ranging from simple twitching (“flying saucer syndrome”) to falls; consciousness is not impaired in it. It is precipitated by alcohol and sleep deprivation. Patients will have normal intelligence. The typical interictal EEG abnormality consists of a generalized 4- to 6-Hz spike or polyspike and slow-wave discharges lasting 1-20 seconds. Usually, 1-3 spikes precede each slow wave.

Also know:

  • In Generalized tonic clonic seizure EEG shows a normal background with generalized epileptiform discharges such as spike or polyspike wave complexes at 2.5 to 4 Hz.
  • During absence seizures there is an abrupt onset of bilaterally synchronous and symmetrical 3 Hz spike-wave discharge, irrespective of whether typical absences are simple or complex.
  • Ictal recordings from patients with typical temporal lobe epilepsy usually exhibit 5-7 Hz, rhythmic, sharp theta activity, maximal in the sphenoidal and the basal temporal electrodes on the side of seizure origin.

Ref: A-Z of Neurological Practice: A Guide to Clinical Neurology By Andrew J. Larner, Alasdair J Coles, Neil J. Scolding, Roger A Barker, 2011, Page 368 ; Clinical Electroencephalography by Misra,2005, Page 188


Q. 2

EEG taken during which of the following sleep stages shows sleep spindles?

 A

Stage 1

 B

Stage 2

 C

Stage 3

 D

Stage 4

Q. 2

EEG taken during which of the following sleep stages shows sleep spindles?

 A

Stage 1

 B

Stage 2

 C

Stage 3

 D

Stage 4

Ans. B

Explanation:

Stage 2 sleep, a component of Non REM sleep is characterised by the presence of sleep spindles and K complexes. Sleep spindles appears as sinusoidal waves occurring at a frequency of 12–14 Hz. K complexes are occasional high voltage biphasic waves.
  • Stage 1 sleep is characterised by the presence of theta waves 4-Hz).
  • In stage 3 of NREM sleep, a high-amplitude delta rhythm (0.5–4 Hz) are seen.
  • Stage 4 of NREM sleep is characterised by the presence of slow waves.
Ref: Barrett K.E., Barman S.M., Boitano S., Brooks H.L. (2012). Chapter 14. Electrical Activity of the Brain, Sleep—Wake States, & Circadian Rhythms. In K.E. Barrett, S.M. Barman, S. Boitano, H.L. Brooks (Eds), Ganong’s Review of Medical Physiology, 24e.

Q. 3

A 72-year-old woman with insomnia participates in a sleep study . As part of the study protocol, she has EEG leads attached, then goes to sleep. At one point during the evening, 12-16 Hz sleep spindles and K-complexes are observed. Which of the following stages of sleep is associated with this pattern?

 A

REM

 B

Stage 1

 C

Stage 2

 D

Stage 3

Q. 3

A 72-year-old woman with insomnia participates in a sleep study . As part of the study protocol, she has EEG leads attached, then goes to sleep. At one point during the evening, 12-16 Hz sleep spindles and K-complexes are observed. Which of the following stages of sleep is associated with this pattern?

 A

REM

 B

Stage 1

 C

Stage 2

 D

Stage 3

Ans. C

Explanation:

Stage 2 has more theta waves than stage 1 and is associated with sleep spindles (short bursts of 12-16 Hz activity) and K-complexes (high amplitude slow waves with superposed sleep spindles) on the electroencephalogram.

Transient large amplitude potentials in the occipital areas (ponto-geniculo-occipital [PGO] spikes) are associated with REM sleep.
 
Stage 1 or drowsiness, is characterized by the attenuation of alpha rhythm (8-13 Hz) and the appearance of 4-7-Hz theta waves.
 
Stages 3 and 4, or slow wave sleep, are characterized by high amplitude slow waves, especially in the delta (< 4 Hz) frequency range.

Q. 4

Alpha block on EEG is seen during which of the folllowing activities?

 A

Sleep

 B

Seizures

 C

Concentrating

 D

None of the above

Q. 4

Alpha block on EEG is seen during which of the folllowing activities?

 A

Sleep

 B

Seizures

 C

Concentrating

 D

None of the above

Ans. C

Explanation:

 
When attention is focused on something, the alpha rhythm is replaced by an irregular 13–30 Hz low-voltage activity, the beta rhythm.
 
This phenomenon is called alpha block and can be produced by any form of sensory stimulation or mental concentration, such as solving arithmetic problems.
Ref: Ganong’s Review of Medical Physiology 23rd edition, Chapter 15.

 


Q. 5

Slow waves In EEG activity seen in :

 A

Depression

 B

Delirium

 C

Schizophrenia

 D

Mania

Q. 5

Slow waves In EEG activity seen in :

 A

Depression

 B

Delirium

 C

Schizophrenia

 D

Mania

Ans. B

Explanation:

B i.e. Delirium


Q. 6

EEG rhythm recorded from the surface of the scalp during REM sleep –

 A

Alpha

 B

Beta

 C

Delta

 D

Theta

Q. 6

EEG rhythm recorded from the surface of the scalp during REM sleep –

 A

Alpha

 B

Beta

 C

Delta

 D

Theta

Ans. B

Explanation:

B i.e. Beta


Q. 7

The frequency of beta waves (per sec) in EEG is

 A

0-4

 B

4-7

 C

7-13

 D

13-30

Q. 7

The frequency of beta waves (per sec) in EEG is

 A

0-4

 B

4-7

 C

7-13

 D

13-30

Ans. D

Explanation:

D i.e. 13-30


Q. 8

EEG rhythm having lowest frequency is‑

 A

Alpha

 B

Beta

 C

Delta

 D

Theta

Q. 8

EEG rhythm having lowest frequency is‑

 A

Alpha

 B

Beta

 C

Delta

 D

Theta

Ans. C

Explanation:

C i.e. Delta


Q. 9

Buerger waves (alpha waves) of EEG have the rhythm per sec of –

 A

0-4

 B

4-7

 C

8-13

 D

13-30

Q. 9

Buerger waves (alpha waves) of EEG have the rhythm per sec of –

 A

0-4

 B

4-7

 C

8-13

 D

13-30

Ans. C

Explanation:

C i.e. 8-13


Q. 10

An EEG –

 A

Provides indication of intelligence

 B

Tends to show waves of smaller amplitude during deep sleep than of alert state

 C

Tends to show waves of smaller amplitude during deep sleep than of alert state

 D

Is bilaterally symmetrical

Q. 10

An EEG –

 A

Provides indication of intelligence

 B

Tends to show waves of smaller amplitude during deep sleep than of alert state

 C

Tends to show waves of smaller amplitude during deep sleep than of alert state

 D

Is bilaterally symmetrical

Ans. D

Explanation:

D i.e. Is bilaterally symmetrical


Q. 11

The normal adult human electroencephalogram (EEG):

 A

Will not show high frequency waves during Stage 3 sleep

 B

Shows alpha rhythm when a person is awake but inattentive

 C

Has lower frequency waves during mental activity

 D

A & B Both

Q. 11

The normal adult human electroencephalogram (EEG):

 A

Will not show high frequency waves during Stage 3 sleep

 B

Shows alpha rhythm when a person is awake but inattentive

 C

Has lower frequency waves during mental activity

 D

A & B Both

Ans. D

Explanation:

A i.e. Will not show high frequency waves during stage -3 sleep; B i.e. Shows alpha rhythm when a person is awake but inattentive.


Q. 12

Which of the following in anesthesia will produce decreased EEG activities

 A

Hypothermia

 B

Early Hypoxia

 C

Ketamine

 D

N20

Q. 12

Which of the following in anesthesia will produce decreased EEG activities

 A

Hypothermia

 B

Early Hypoxia

 C

Ketamine

 D

N20

Ans. A

Explanation:

A i.e. Hypothermia

Hypothermia causes progressive slowing of brain activity at core temperatures below 35°C. Severe hypothermia (<28°C) results in loss of cerebrovascular auto regulation, decrease in cerebral blod flow, coma, loss of ocular reflex, and progressive decrease in EEGQ.

Situation

Effect On EEG

– Hypothermia (<35°C)

Progressive slowing of EEGQ

Profound hypothermia (7- 20°C)

Complete electrical silent

CO2 level

 

– Hypocarbia

– Slowing of EEG

– Hypercarbia (5-20%)

– Decreased cerebral excitability & increased electroshock seizure

threshold

– Hypercarbia (>30% above normal)

– Increased cerebral excitability & epileptiform discharge

– Hyper carbia (>50% above normal)

EEG depression.

02 levels

 

– Hyperoxia

– Cerebral excitation 1/t low amplitude fast frequency EEG

– Early thypoxia

– Cerebral excitability (d/t peripheral cemoreceptor stimulation)

– Persistent Hypoxia

Vt. acute increase in amplitude of EEGQ

Late Hypoxia approching anoxia

Diffuse slowing of EEGQ Vt marked reduction in amplitude with

appearance of slow wavesQ

 

EEG silence

Sensory stimulation (in awake)

Desynchronization of EEG patterns, increased amplitude, and

increased frequency.

Narcotic Analgesics

 

– Low dose

– Increase amplitude of both a & p bands.

– High dose

– Theta & delta frequencies develop heralding the onset of sedation

Enflurane

Dose dependent depression of EEG activity, but at moderate high

concentration (>3%), it produces epileptiform paroxymal spike

activity and burst suppressionQ. It should be avoided in epileptic

patientsQ.

 

No seizure activity on EEG & excitatory effects (as seen in

Isoflurane, Desflurane and

enflurane)

)

– Dose dependent depression of EEG activity

Sevoflurane

– Does not cause seizure activity at any level of anesthesia with or

without hypocapnia.

Propofol

– EEG frequency decreases & amplitude increase

Ketamine

– Unlike those seen with other IV. anesthetics, and consists of loss

of alpha rhythm and predominant theta activityQ.



Q. 13

Absence seizures are characterized on EEG by:

 A

3 Hz spike & wave

 B

1-2 Hz spike & wave

 C

Generalized poly spikes

 D

Hypsarrythmia

Q. 13

Absence seizures are characterized on EEG by:

 A

3 Hz spike & wave

 B

1-2 Hz spike & wave

 C

Generalized poly spikes

 D

Hypsarrythmia

Ans. A

Explanation:

Answer is A (3 Hz spike and wave):

EEG in absence seizures shows bursts of bilaterally synchronous and symmetric 3 Hz spike and wave activity.

Characteristic features of Absence seizures / Petitmal seizures:

Clinical Hallmarks

  • Almost always begin in childhood and cease by age of 20.
  • Sudden brief lapses of consciousness without loss of postural control
  • Seizure typically lasts only for seconds. Impairment is so brief that patient may be unaware of it.
  • Consciousness returns as suddenly as it was lost i.e. Inset and termination of attack, both are abrupt.
  • Usually accompanied by subtle bilateral motor signs such as

–            Rapid blinking of eye lids

–            Chewing movements

–            Small amplitude, clonic movements of hands

  • There is no postictal confusion
  • The seizure can occur hundreds of times per day.
  • First clued to absence seizure is often unexplained ‘day dreaming’ and a decline in school performance recognized by a teacher.

Electrophysiological hallmark

  • Electrophysiological hallmark of typical absence seizure is a generalized symmetric, 3 Hz spike and wave discharge that begins and ends suddenly superimposed on a normal EEG background.
  • Hyperventilation tends to provoke these electrographic discharges and even the seizures themselves.

Q. 14

EEG is usually abnormal in all of the following, except :

 A

Subacute sclerosing panencephalitis

 B

Locked – in state.

 C

Creutzfoldt – Jackob disease

 D

Hepatic encephalopathy

Q. 14

EEG is usually abnormal in all of the following, except :

 A

Subacute sclerosing panencephalitis

 B

Locked – in state.

 C

Creutzfoldt – Jackob disease

 D

Hepatic encephalopathy

Ans. B

Explanation:

Answer is B (Locked in state):

`Normal a- activity on the EEG in a patient of coma alerts the clinician to the locked in syndrome or to hysteria or catatonia.’ – Harrison 16th/1629

All conditions other than the ‘locked in state’ present characteristic EEG patterns.

EEG Characteristic;

Subacute sclerosing panencephalitis (SSPE)

Creutzfoldt Jackob disease (CJD)

Hepatic encephalopathy

Characteristic     periodic    pattern    with   bursts

Characteristic stereotype periodic bursts of < 200

Characteristic      Symmetric

every 3 to 8 seconds of high voltage, sharp

ms duration occurring every I to 2 sec. Makes the

high voltage triphasic slow

slow waves followed by periods of attenuated

diagnosis of CJD very likely

wave infrontal region

(falt) background

(Repetitive high voltage, triphasic and polyphasic

sharp discharges are seen in most advanced cases)

 

 Locked in state :

  • This represents a pseudocoma in which an awake patient has no means of producing speech or volitional movement in order to indicate that he awake
  • Patient may signal with vertical eye movement and lid elevation which remain unimpaired.
  • It usually results from infarction or haemorrhage of the ventral pons which transects all descending corticospinal and corticobulbar pathways.
  • EEG is normal

Q. 15

A symmetric high-voltage, triphasic slow wave pattern is seen on EEG in the following :

 A

Hepatic encephalopathy

 B

Uremic encephalopathy

 C

Hypoxic encephalopathy

 D

Hypercarbic encephalopathy

Q. 15

A symmetric high-voltage, triphasic slow wave pattern is seen on EEG in the following :

 A

Hepatic encephalopathy

 B

Uremic encephalopathy

 C

Hypoxic encephalopathy

 D

Hypercarbic encephalopathy

Ans. A

Explanation:

Answer is A (Hepatic encephalopathy)

`Symmetric, high voltage, triphasic slow wave (2 – 5 /second) pattern on Electroencephalogram (EEG) is characteristic (but non specific) of Hepatic encephalopathy’

The diagnosis of hepatic encephalopathy should be considered when four major factors are present

  1. Acute or chronic hepatocellular disease and / or extensive portal systemic collateral shunts.
  2. Disturbance of awareness and mentation which may progress from forgetfulness and confusion to stupor and finally coma.
  3. Shifting combinations of neurological signs including asterixis, rigidity, hyperreflexia, extensor plantar signs and rarely seizures.
  4. A characteristic but (nonspecific), symmetric, high voltage, triphasic slow wave (2 to 5 /second) pattern on electroencephalogram.

Q. 16

Burst Supression pattern on EEG is typically seen in:

 A

Anoxic Encephalopathy

 B

Absence seizures

 C

SSPE

 D

Herpes Simplex Encephalitis

Q. 16

Burst Supression pattern on EEG is typically seen in:

 A

Anoxic Encephalopathy

 B

Absence seizures

 C

SSPE

 D

Herpes Simplex Encephalitis

Ans. A

Explanation:

Answer is A (Anoxic Encephalopathy):

Burst Supression Pattern on EEG is typically seen in Anoxic Encephalopathy or Hypoxic Ischemic Encephalopathy.

Burst Supression Pattern on EEG

  • The burst suppression pattern on EEG is typically charachterized by an isoelectric background interrupted by periodic / pseudoperiodic or non-periodic bursts of abnormal activity.
  • The suppression period is of either no or low amplitude cerebral activity while the bursts can be a mixture of a variety of all types of wave forms including sharp, spike, alpha, beta, theta and delta activities.
  • Burst suppression implies the deepest level of coma state before brain death and carries a poor prognosis. `This pattern is commonly seen in patients with an acute and severe degree of cereberal insult most commonly in severe anoxic encephalopathy, acute intoxication with CNS depressant drugs and severe hypothermia’.
  • This is generally an invariant pattern that does not change with the state of sleep-wakefulness or in response to stimuli.

Q. 17

Delta waves on EEG are recorded in:

September 2007

 A

REM

 B

01 NREM

 C

02 NREM

 D

Deep sleep

Q. 17

Delta waves on EEG are recorded in:

September 2007

 A

REM

 B

01 NREM

 C

02 NREM

 D

Deep sleep

Ans. D

Explanation:

Ans. D: Deep sleep

Alpha waves generally are seen in all age groups but are most common in adults.

They occur rhythmically on both sides of the head but are often slightly higher in amplitude on the nondominant side, especially in right-handed individuals.

A normal alpha variant is noted when a harmonic of alpha frequency occurs in the posterior head regions.

They tend to be present posteriorly more than anteriorly and are especially prominent with closed eyes and with relaxation.

Alpha activity disappears normally with attention (e.g., mental arithmetic, stress, opening eyes).

In most instances, it is regarded as a normal waveform.

An abnormal exception is alpha coma, most often caused by hypoxic-ischemic encephalopathy of destructive processes in the pons (e.g., intracerebral hemorrhage).

In alpha coma, alpha waves are distributed uniformly both anteriorly and posteriorly in patients who are unresponsive to stimuli.

  • Beta waves are observed in all age groups.

They tend to be small in amplitude and usually are symmetric and more evident anteriorly.

Drugs, such as barbiturates and benzodiazepines, augment beta waves.

  • Theta waves normally are seen in sleep at any age. In awake adults, these waves are abnormal if they occur in excess. Theta and delta waves are known collectively as slow waves.
  • Delta waves are slow waves have a frequency of 3 Hz or less.

They normally are seen in deep sleep in adults as well as in infants and children.

Delta waves are abnormal in the awake adult.

Often, they have the largest amplitude of all waves.

Delta waves can be focal (local pathology) or diffuse (generalized dysfunction).


Q. 18

The EEG cabins should be completely shielded by a continuous sheet of wire mesh of copper to avoid the picking up of noise from external electromagnetic disturbances. Such a shielding is called as:

 A

Maxwell cage

 B

Faraday cage

 C

Edison’s cage

 D

Ohms cage

Q. 18

The EEG cabins should be completely shielded by a continuous sheet of wire mesh of copper to avoid the picking up of noise from external electromagnetic disturbances. Such a shielding is called as:

 A

Maxwell cage

 B

Faraday cage

 C

Edison’s cage

 D

Ohms cage

Ans. B

Explanation:

Ans. Faraday cage


Q. 19

In EEG type of wave seen in metabolic encephalophathy

 A

Alpha

 B

Beta

 C

Gamma

 D

Delta

Q. 19

In EEG type of wave seen in metabolic encephalophathy

 A

Alpha

 B

Beta

 C

Gamma

 D

Delta

Ans. D

Explanation:

Ans. is ‘cl’ i.e., Delta

E.E.G. changes in metabolic encephalopathy

In metabolic encephalopathy changes are typically nonfocal

  • E.E.G. has been widely used to evaluate metabolic encephalopathy.
  • The E.E.G .findings are abnormal in acute encephalopathic stages.
  • It is difficult to establish a diagnosis of metabolic encephalopathy with certainity through E.E.G.
  • There is generalized slowing of the E.E.G with an excess of the delta and theta waves with suppression of normal alpha and beta wave activity and occasionally bilateral spikes and waves complexes occurring in absence of seizure activity”.
  • In metabolic encephalopathies, the E.E.G evolution correlates well with the severity of encephalopathy. o However EEG has little specificity in differentiating etiologies in metabolic encephalopathy.
  • For example, though triphasic waves are most frequently mentioned in hepatic encephalopathy, they can also be seen in uremic encephalopathy or even in aged psychiatric patients treated with lithium. o Spikes and waves may appear in hypo or hyperglycemia uremic encephalopathy or vitamin deficiencies.

Common principles of EEG changes in metabolic encephalopathy are : – 

  •  Varied degrees of slowing
  • Associated mixtures of epileptic discharge
  • High incidence of triphasic waves
  • Reversibility after treatment of underlying causes
Metabolic encephalopathy

EEG rythm

Grade I (almost normal)

Dominant activity is alpha rhythm with minimal teta activity

Grade II (mildly abnormal) Dominant teta background with some alpha and delta activities.
Grade II (moderately abnormal) Continuous delta activity predominates, little activity of faster frequencies
Grade IV (severely abnormal) Low-amplitude delta activity or suppression-burst pattern
Grade V (extremely abnormal)

Nearly”flat” tracing or electrocerebral inactivity.


Q. 20

Characteristic change in EEG is found in ? 

 A

Prion disease.

 B

Epilepsy.

 C

Hypothermia

 D

Herpes.

Q. 20

Characteristic change in EEG is found in ? 

 A

Prion disease.

 B

Epilepsy.

 C

Hypothermia

 D

Herpes.

Ans. C

Explanation:

Ans:C.)Hypothermia.

Change seen in the picture above represents burst suppression on EEG.

The pattern is found in patients with inactivated brain states, such as from general anesthesia, coma, or hypothermia. This pattern can be physiological, as during early development, or pathological, as in diseases such as Ohtahara syndrome.
Bursts are identifiable on EEG readings by their high amplitude (75-250μV), typically short period of 1–10 seconds, and have frequency ranges of 0–4 Hz (δ) and 4–7 Hz (θ).
Suppression episodes are identifiable by their low amplitude (< 5μV) and typically long period (> 10s).


Q. 21

EEG in anesthesia is useful in:

 A

Depth of general anesthesia

 B

Depth of local anesthesia

 C

Depth of neuromuscular block

 D

Depth of analgesia

Q. 21

EEG in anesthesia is useful in:

 A

Depth of general anesthesia

 B

Depth of local anesthesia

 C

Depth of neuromuscular block

 D

Depth of analgesia

Ans. A

Explanation:

Ans. a. Depth of general anesthesia

  • EEG in anesthesia is useful in monitoring the depth of anesthesia (GA) with 16-lead.

Uses of EEG in anesthesia

  • To assess the sedation and awarenessQ
  • To monitor epileptic activityQ, and to control antiepileptic drugs infusion, especially in paralyzed patients
  • To monitor the changes in conscious levelsQ
  • During cerebrovascular surgery, to confirm the adequacy of cerebral oxygenationQ
  • Monitoring the depth of anesthesiaQ(GA) with 16-lead

Have look on some new and mportant techniques

Bispectral Index Scale (BIS

  • It is based on the principle of EEGQ
  • It uses 3 EEG electrodes placed on frontal, parietal and temporal lobesQ

BIS is used to monitor

  • Intraoperative awarenessQ
  • Depth of anesthesia
  • Its value ranges from 0-100 (0: coma;100: awake)
  • Target intra-operative BIS: 40-60Q
  • Target sedation BIS: 60-80Q

Entropy

  • It is based on the principle of EEG and EMG

Q. 22

EEG waves are called ‑

 A

Delta waves

 B

Berger’s waves

 C

REM rhythm

 D

NERM rhythm

Q. 22

EEG waves are called ‑

 A

Delta waves

 B

Berger’s waves

 C

REM rhythm

 D

NERM rhythm

Ans. B

Explanation:

Ans. is ‘b’ i.e., Berger’s waves

  • EEG rhythms are called Berger’s rhythm.

Q. 23

Alpha waves in EEG represent ‑

 A

Eye closed with active mind

 B

Eye open, fully awake and alert

 C

Deep sleep

 D

None

Q. 23

Alpha waves in EEG represent ‑

 A

Eye closed with active mind

 B

Eye open, fully awake and alert

 C

Deep sleep

 D

None

Ans. A

Explanation:

Ans. is ‘a’ i.e., Eye closed with active mind

EEG rhythms and sleep wake cycle

  • Following EEG rhythms (Berger’s rhythm) are important in relation to sleep wake cycle : ‑
  1. Alpha rhythm : – In adult humans who are awake but at rest with mind wandering and the eye closed, oc­rhythm is prominent. It is a regular rhythm with a frequency of 8-13 Hz and amplitude of 50-100 V. It is most marked in parietal and occipital lobe. It is associated with decreased level of attention, i.e., person is awake but has decreased attention (relaxed) -> Person is thinking but with decreased attention (subconscious thinking).
  2. Beta rhythm : – When attention is focused on something, the alpha rhythm is replaced by Beta rhythm. It is an irregular 13-30 Hz low voltage activity. It is most evident on frontal lobe and occurs when patient is fully awake and alert. Therefore this replacement of beta rhythm for alpha rhythm is called arousal or alerting response or alpha block. This phenomenon can be produced by any form of sensory stimulation or mental concentration such as solving arithmetic problems. Person is thinking with a maximum concentration      Conscious thinking.
  3. Theta rhythm (4 – 7 Hz) : – When person with alpha rhythm becomes slightly more relaxed (as occurs when there is transition from wakefullness to sleep, i.e., stage 1 of REM), alpha rhythm is replaced by theta rhythm. Thinking is present but with even less attentiveness than which was present in alpha rhythm stage (Deep subcsoncious thinking). Two types of theta rhythm have been described : –
  • Hippocampal (occurs in mammals other than men, i.e., cat dogs, etc).
  • Cortical (occurs in men)

4. Delta rhythm (3-5 Hz) : – It is present when the person is in deep sleep (NREM stage 3 & 4) and there is no thinking.



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