Two new studies by faculty at Georgetown University Medical Center and colleagues shed new light on the brain mechanisms underlying conscious awareness. The studies are published in the current issue of the journal Neurology. "Despite many advances in our understanding of the brain, the mechanisms of conscious awareness are still poorly understood," said Kimford J. Meador, MD, professor and chair of neurology at Georgetown University Medical Center and the senior author of both articles. "As our understanding of the processes underlying conciousness advances, so will our ability to help patients suffering from a variety of neural impairments."
The first article, "Gamma coherence and conscious perception," adds to the growing body of knowledge about the role of brain waves, or electrical pulses in the brain. Gamma waves are fast electrical waves that have been hypothesized to be involved in conscious perception. However, scientists still do not fully understand their role.
In this study, researchers looked at electrical activity from a grid of electrodes implanted in the brains of six patients who were undergoing surgery to treat their epileptic seizures. The implantation was part of a medical procedure that is routinely conducted before brain surgery in some patients to determine the precise location in the brain causing the seizures.
While the electrodes were implanted, researchers used this opportunity to study how the patients' brains responded to a painless stimulus to one of their fingers. Although the patients were conscious each time they received the stimulus, they did not always notice it, even though the stimuli were identical. The initial electrical response of the brain was similar for both the perceived and non-perceived stimuli, but only the perceived stimuli were associated with synchronized gamma waves in the hand sensory region. The results suggest that synchronized fast neural activity in the sensory area may be necessary to perceive simple stimuli.
The second study, "Pathophysiology of altered consciousness during seizures," used single-photon computed tomography (SPECT) scans to show blood flow changes in the brain when patients with epilepsy lost consciousness during seizures. It involved 71 patients with epilepsy who received the SPECT scans while having seizures, or very shortly after the seizures, as part of a diagnostic evaluation, to determine the parts of their brains affected during seizures. The researchers looked particularly at the thalamus, a walnut-sized mass located deep within the brain, and the midbrain which is a region just below the thalamus. Previous studies have shown that the thalamus and midbrain play an important role in consciousness, but it was unclear how focal seizures impair consciousness.
Of the 49 patients who experienced complete loss of consciousness during seizures, 92% showed an increased blood flow to the thalamus or midbrain on their SPECT scans. Eight of the nine patients who experienced no loss of consciousness during their seizures also had no increased blood flow to the thalamus or midbrain.
"These findings are consistent with a role for the thalamus and upper brainstem in conscious mechanisms," the authors wrote. Impaired consciousness from seizures adversely impacts the quality of life of patients with epilepsy. It appears that distant effects or actual spread of the seizure to the thalamus or midbrain produces loss of awareness in patients with epilepsy. Techniques that block this effect on the thalamus/midbrain would likely benefit patients with epilepsy.
This research was funded by the Medical College of Georgia Research Foundation, and was conducted at the Medical College of Georgia, where the senior author was a faculty member before coming to Georgetown. Part of these findings were presented at the 2001 annual meeting of the American Academy of Neurology in Philadelphia.
Materials provided by Georgetown University Medical Center. Note: Content may be edited for style and length.
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