• Theta waves oscillate about 3.5 – 7.5 times per second (Hz).
• Some consider Theta to be from 4 – 8 Hz.
• Theta is normal in small very amounts in the healthy waking adult EEG.
• Theta in the adult waking EEG should be symmetrically distributed.
• Theta is more prominent and considered normal in the raw EEG’s of children & adolescents unless there is clear indication of pathology.
• Theta at the scalp surface in the adult EEG is normal as part of the early stages of drowsiness.
• Theta may occur in some individuals who are day dreaming.
• Some individuals produce a frontal-midline Theta (FZ) under task conditions that can be recorded at the scalp and is dependent on task difficulty and reflects transfer of information to long-term memory / memory functions

Abnormal Occurrence of Theta Waves

• Excessive Theta activity in the waking raw EEG of adults is considered abnormal. It can represent reduced metabolism cortical grey matter (too little oxygen uptake).
• Theta that is asymmetrically distributed in the waking adult EEG is considered abnormal (more on one side than the other)
• Spike and slow wave complexes that occur in seizure disorder often occur in the Theta frequency range.
• Excessive Theta on quantitative analysis is often seen in conditions such as…
  • ADHD
  • Learning disabilities
  • Head injuries or brain lesions
  • Certain neurological disorders

Physiological Origin of Theta Wave Rhythms

• Cortical Theta generation in humans is projected to the cortex via Thalamo-cortical projections and is influenced by GABAergic / Cholinergic inhibitory & excitatory inputs from the Reticular Activating System.
• Cortical Theta is essentially a slowing of the Alpha rhythm but may also be influenced by limbically generated (septo-hippocampul) Theta as described below.
• Sub-cortical Theta Rhythms are prominent in the Hippocampus & Limbic system and result from alternating inhibition / disinhibition between the septal nuclei & hippocampal nuclei.
• Limbic Theta oscillations are caused by alternating inhibition / disinhibition of GABAergic & Cholinergic neurons.
• The hippocampus is part of the “Papez Circle” including the Hippocampus (HIP), Entorhinal Cortex in Temporal Lobes, Sensory areas of the Temporals & Parietals, the Mammillary Bodies of the Hypothalamus, and the Thalamus that are involved in memory consolidation.

Role of Theta Waves in Having a Healthy Brain

• Hippocampal & Limbic Theta are highly involved in the encoding of meaningful events into long-term memory.
• Cortical Theta likely represents a “binding rhythm” that ties functionally related neural networks together in time for coordinated processing of memory storage functions.
• Inducing a Theta state via hypnosis or neurofeedback may aid in anxiety reduction and the processing of traumatic memories such as may occur in PTSD

Images of Theta Waves

Single channel;

19 Channel: Linked Ear Montage;

28 year old male with ADHD, Inattentive Type. Examples Highlighted in Red