Alpha Spectra

Figure 3 shows a 4-sec period of alpha rhythm recorded from four scalp locations. The subject is a healthy waking adult, relaxed with eyes closed. Amplitude spectra recorded from left frontal (top left), right frontal (top right), left posterior (bottom left),

Figure 3 Alpha rhythm recorded from a healthy relaxed subject (age 25) with closed eyes using an electrode on the neck as reference. Four seconds of data are shown from four scalp locations (left frontal channel 30, right frontal channel 26, left posterior channel 108, and right posterior channel 100). The amplitude is given in microvolts. This EEG was recorded at the Brain Sciences Institute in Melbourne, Australia, using the electrode cap shown in Fig. 2 (right).

Figure 3 Alpha rhythm recorded from a healthy relaxed subject (age 25) with closed eyes using an electrode on the neck as reference. Four seconds of data are shown from four scalp locations (left frontal channel 30, right frontal channel 26, left posterior channel 108, and right posterior channel 100). The amplitude is given in microvolts. This EEG was recorded at the Brain Sciences Institute in Melbourne, Australia, using the electrode cap shown in Fig. 2 (right).

and right posterior (bottom right) scalp based on 5 min of EEG are shown in Fig. 4. The amplitude spectra show mixtures of frequencies that depend partly on scalp location; however, alpha rhythm is dominant at all locations in this typical example. This subject has two frequency peaks near 10 Hz, a relatively common finding. These two alpha oscillations are partly distinct phenomena as revealed by their separate distributions over the scalp and their distinct behaviors during mental calculations.

E. Alpha, Theta, Cognitive Tasks, and Working Memory

Two prominent features of human scalp EEG show especially robust correlation with mental effort. First, alpha band amplitude normally tends to decrease with increases in mental effort. Second, frontal theta band amplitude tends to increase as tasks require more focused attention. In addition to amplitude changes, tasks combining memory and calculations are associated with reductions in long-range coherence in narrow (1 or 2 Hz) alpha bands, whereas narrowband theta coherence increases. Large alpha coherence reductions at large scalp distances (e.g, > 10 cm) can occur with no appreciable reduction in alpha amplitude and simultaneously with increases in short-range (o 5 cm) alpha coherence.

Figure 4 Amplitude spectra for the same alpha rhythms shown in Fig. 3 but based on the full 5-min record to obtain accurate spectra. Frequency resolution is 0.25 Hz. The double peak in the alpha band represents oscillations near 8.5 and 10.0 Hz.

Figure 5 A single column of cerebral cortex with height equal to cortical thickness (2-4 mm) and diameter in the approximate range 0.03-1 mm (the mesoscopic scale). The volume microcurrent sources s(r0, w, t) (mA/mm3) are generated by synaptic and action potentials at cell membrane surface elements, located by the vector w inside the column. A part of the column (e.g., the center) is located at r0. The microcurrent sources integrated over the volume of the column produce a dipole moment per unit volume of the column P(r0, t), given by Eq. (2) and expressed in mA/mm2. In Eq. (3), the electric potential V(r, t) at any tissue location r external to columns (including scalp) is due to the summed (cortical volume or surface integral) contributions from all column sources P(r0, t).

Figure 5 A single column of cerebral cortex with height equal to cortical thickness (2-4 mm) and diameter in the approximate range 0.03-1 mm (the mesoscopic scale). The volume microcurrent sources s(r0, w, t) (mA/mm3) are generated by synaptic and action potentials at cell membrane surface elements, located by the vector w inside the column. A part of the column (e.g., the center) is located at r0. The microcurrent sources integrated over the volume of the column produce a dipole moment per unit volume of the column P(r0, t), given by Eq. (2) and expressed in mA/mm2. In Eq. (3), the electric potential V(r, t) at any tissue location r external to columns (including scalp) is due to the summed (cortical volume or surface integral) contributions from all column sources P(r0, t).

F. The Rhythmic Zoo

Human EEG exhibits many waveforms, especially in the experience of clinical electroencephalographers (neurologists with specialized training). Some EEGs have known clinical significance and some do not. Any complicated waveform can be described as a mixture of oscillations with different frequencies and amplitudes, as indicated by Eq. (1). However, more picturesque descriptions are often preferred by electroencephalographers to characterized the "zoo" of EEG waveforms. Such labels include paradoxical alpha, spike and wave, delta focus, sharp transient, sleep spindle, and nonspecific disrhythmia.

Cortical EEG (ECoG) typically consists of complex waveforms composed of rhythms with different frequencies, locations, and spatial extent. This normal ECoG differentation between cortical areas is eliminated by anesthesia, suggesting a transition from more locally to more globally dominated brain dynamics. Highly localized cortical rhythms are not recorded on the scalp. Cortical beta rhythms are often strongly attenuated between cortex and scalp because they are more localized than some of the alpha band activity. EEG during sleep, coma, and anesthesia typically exhibits large scalp amplitudes, implying widely distributed cortical source activity.

A common but oversimplified view of alpha rhythms is one of brain idling. However, upper and lower alpha band amplitudes may change independently, depending on scalp location and task. Some tasks cause lower alpha band amplitude to decrease while upper alpha band amplitude increases. Alpha amplitude reductions may be local cortical phenomena occurring in task-relevant brain areas, whereas task-irrelevant regions may be unchanged or even produce larger alpha amplitudes. Another (possibly complementary) hypothesis is that increases in higher frequency alpha amplitudes reflect a specific memory processing function and not simple idling.

Generally, intracerebral electrodes record a variety of alpha rhythms. Some intracerebral alpha rhythms are blocked by opening the eyes and some are not. Some respond in some way to mental activity and some do not. The alpha band rhythms recorded on the scalp represent spatial averages of many alpha components. For an alpha rhythm of a particular type to be observed on the scalp, it must be synchronized (roughly in phase) over a large cortical area.

Was this article helpful?

0 0
Conquering Fear In The 21th Century

Conquering Fear In The 21th Century

The Ultimate Guide To Overcoming Fear And Getting Breakthroughs. Fear is without doubt among the strongest and most influential emotional responses we have, and it may act as both a protective and destructive force depending upon the situation.

Get My Free Ebook


Post a comment