Beta Activity

Description

As was initially identified and named by Hans Berger after he named alpha activity, beta activity comprised waves with durations of 30 to 40 milliseconds (Berger, 1969). This equates to rhythms of 25 to 33 Hz, and describes the activity that Berger was able to detect with the use of one EEG channel and subjects who often had skull defects. The fast activity that is commonly seen in modern, routine EEG includes a broader range of frequencies, and beta activity is now defined as activity with a frequency of 13 Hz or greater. However, its occurrence across the frontal and central regions tends to have a frequency within the more narrow range of 20 to 30 Hz. Although frontal–central beta activity rarely has a frequency below 18 Hz or above 35 Hz, it may occur with frequencies in the range of 14 to 40 Hz (Kellaway, 1990; Niedermeyer, 1999). Frontal–central beta activity is state dependent, and occurs most commonly with drowsiness and sometimes continues through stage 2 of non–rapid eye movement (NREM) sleep. In drowsiness and sleep, it occurs as bursts with an amplitude that may reach a maximum of about 60 μV and a rhythmicity that may be out of phase between the two hemispheres (Kellaway and Fox, 1952; Kozelka and Pedley, 1990; Santamaria and Chiappa, 1987). Normal frontal–central beta activity is symmetric in its amplitude, and an amplitude asymmetry greater than 35% is abnormal (Fisch, 1999; Kellaway, 1990). The distribution depends on age. When frontal–central beta activity first develops, which is usually between the ages of 6 months and 2 years, it is over the central and posterior head regions (Fisch, 1999). During childhood, it gradually migrates anteriorly and becomes frontally predominant by early adulthood.

Distinguishing Features

• Compared to Muscle Artifact

Since the frontalis muscle runs over the frontal–central region, muscle artifact often co-localizes with the region of maximum frontal–central beta activity. This artifact characteristically has frequency components of 25 Hz and greater, so it therefore can resemble the frequency of frontal–central beta activity. Waveform differences are the principal means to distinguishing between these two patterns. Electromyographic (EMG) artifact has a sharper contour with less rhythmicity, especially when the high-frequency filter is set at 70 Hz or higher. A high-frequency filter set to 40 Hz or lower can give EMG artifact a smoother contour, more rhythmicity, and general waveform features of beta activity. When EMG artifact occurs as a rhythm within the beta frequency range without a low setting to the high-frequency filter, it occurs as individual EMG potentials that each have durations of less than 20 milliseconds but are separated by a repeating interval that produces the beta frequency rhythm. A variation in this interval between the repeating EMG potentials provides a distinguishing feature, especially when the interval becomes so brief that the potentials appear continuous. Activity that is this fast is beyond the upper frequency of typical, cerebrally generated beta frequency activity and usually indicates muscle artifact.

• Compared to Paroxysmal Fast Activity

Normal beta activity differs from paroxysmal fast activity (PFA) by typically beginning and ending gradually, even if over only a second. PFA’s abrupt occurrence in the EEG, which is due to a sudden change in both amplitude and frequency components, makes PFA more distinct as an identifiable pattern amid ongoing background activity.

• Compared to Sleep Spindles

Only when beta activity is localized to the vertex or midline frontal region does it appear similar to spindles, but beta activity with this location is associated with drowsiness, which compounds the challenge of identification. However, midline beta activity differs from sleep spindles by not having an abrupt beginning and ending. Midline beta activity does not typically occur in bursts and instead usually builds up over seconds and persists for seconds before attenuating over seconds. Therefore, it does not have the characteristic steeper, spindle-like waveform. Furthermore, midline beta activity usually has a predominant frequency greater than 15 Hz, which is faster than the oscillation within sleep spindles.

Co-occurring Patterns

Other signs of drowsiness always accompany frontal–central beta activity. These signs may include decreased muscle artifact, slow roving eye movements, intermittent alpha rhythm, slowed alpha rhythm, rhythmic temporal theta, and a Cigánek rhythm.

Clinical Significance

Frontal–central beta activity is a normal variant that most commonly is a sign of drowsiness or sleep onset, but it is present in full wakefulness in some individuals. More rarely, it accompanies anxiety and can be a sign of vigilance. Its overall functional significance has been interpreted as indicating cognitive processing and maintenance of the current sensorimotor and cognitive system state, and relates to the mu rhythm in this regard (Engel and Fries, 2010; Miller, 2007; Mundy-Castle, 1951; Pfurtscheller et al., 1997). As such, its detection in drowsiness may be due to its increased visibility with decreases in other EEG activities present in wakefulness. Simultaneous EEG and functional MRI studies depict an anatomic correlate that is precentral for frontal–central beta activity, which suggests a greater role for the beta activity in motor processing (Ritter et al., 2009).

Abnormally asymmetric frontal–central beta activity may indicate cortical dysfunction beneath the region with the lower amplitude (Fisch, 1999; Kellaway, 1990

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