Alcoholism and Human Electrophysiology : Part 4

by National Institute of Health

(Page 4 of 5)

Theta/Delta Underlying Visual P3

As discussed above, P3 has multiple sources, with contributions from the frontal cortex and hippocampus. P3 consists of delta and theta oscillations with a higher proportion of delta oscillations from the posterior regions of the brain, and theta occurring in the frontal and central regions. Synchronization occurs in the theta range between the hippocampus and frontal and parietal regions in the brain (these regions communicate with each other) during attention tasks.

Slow EEG oscillations (delta and theta) depend on the activity level of specific receptors (muscarinic cholinergic receptors) for the brain chemical acetylcholine; the level of acetylcholine in the cortex and hippocampus is reduced during delta oscillations and elevated during theta activity. Recent findings from the COGA project indicate that the cholinergic receptor genes are involved in theta and delta production and that these genes also are associated with alcohol dependence. The production of theta and delta rhythms also involves interactions between the GABA and cholinergic neurotransmitter systems; the frequency of theta is controlled by the GABA system, whereas its power is controlled by the cholinergic system.

In a visual oddball paradigm, evoked delta and theta power while processing the target stimuli is significantly decreased among alcoholics compared with control subjects, indicating that the reduced P3 amplitudes reported in alcoholics are caused by deficits in theta and delta oscillations that underlie P3. In a Go/No-Go paradigm, alcoholics also manifested significantly decreased delta and theta power, particularly during No-Go processing. An increase in theta power is related to an increase in theta in the hippocampus, known to be an inhibitory rhythm associated with GABAergic activity. An increase in theta is associated with inhibition of nonrelevant information while attending to relevant information. As most information is irrelevant and must be suppressed, this yields the high amplitude of P3 to relevant stimuli. Hence the deficit in inhibitory theta oscillations underlying P3 in alcoholics suggests deficient inhibitory control during information processing (attention and memory mechanisms) in alcoholics. This finding provides further support for the hypothesis that CNS disinhibition is involved in alcoholism.

Frontal Midline Theta

Frontal midline theta observed in humans during attention-related tasks is integral to understanding brain functioning during information processing. In one study, researchers examined event-related EEG changes during the performance of mental calculations in alcoholics and control subjects. The study was based on the hypothesis that performance deficits would be indicated by reduced frontal theta power in alcoholic patients. Researchers recorded EEGs in adult alcoholics and normal volunteers during the performance of a simple addition problem and during a resting interval. The difference between resting and active theta power is a measure of processing capacity - the lower the resting theta power and the higher the active theta power, the more efficient the brain processing. The difference in theta power between resting and problem-solving conditions was significantly lower in alcoholics compared with control subjects at the anterior frontal electrodes. Alcoholics manifested increased resting theta and decreased active theta, indicating decreased and inefficient processing capacity. These deficits in performance, indexed by low evoked (active) theta power during mental effort, reflect frontal lobe dysfunction in alcoholics. These deficits involve inhibitory processes and are manifested as impairments in working memory and sustained attention.

Gamma

Numerous studies indicate that gamma oscillations serve as a mechanism for binding features of an object (feature binding) - for example, the shape and color of an object. Early phase-locked gamma is involved in selective attention and is larger in response to attended stimuli than unattended stimuli, particularly over frontal regions. Several investigators have reported an association between gamma oscillations and P3 components obtained in response to target stimuli in an oddball task. One recent study found that alcoholics manifest lower gamma power than control subjects during target processing between 0 and 150 ms in a visual oddball paradigm. This effect was strongest frontally and lateralized to the left side. No group differences in gamma were observed for nontarget and novel stimuli. Control subjects manifested significantly higher gamma power in the processing of the target relative to the processing of the nontarget stimulus, whereas alcoholics did not manifest higher gamma power during target processing. Increased evoked gamma is thought to reflect a matching process between the template in working memory and the current stimulus. These findings of gamma deficits in response to target stimuli in alcoholics, particularly in frontal regions, provide further evidence for deficits in cognitive processes (attention allocation, working memory) in alcoholics.

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