appears that sensory input may be blocked or gated from interfering with the intrinsic activity of the brain during REM. Measures of bulk electrical signalling in the brain by MEG show no impact of auditory stimuli on the gamma wave patterns; measurements on conscious subjects show a distinct modulation due to the auditory input. In this way, the thalamocortical system acts to gate the brain from external stimuli during REM. Non-rapid eye movement (NREM) sleep differs from REM in that gamma activity is no longer prominent, stepping aside for lower frequency oscillations. While electrical activity at gamma frequencies can occasionally be detected in NREM, it is infrequent and comes in bursts. The exact purpose of its appearance in NREM is not understood. In NREM sleep, thalamocortical oscillatory activity is still present, but the overall frequencies range from the slow (<1 Hz), to the delta (1–4 Hz), and theta (4–7 Hz) range. Synchronized theta oscillation has additionally been observed in the hippocampus during NREM. Alpha oscillations and attention Gamma-range oscillations are not the only rhythms associated with conscious thought and activity. Thalamocortical alpha frequency oscillations have been noted in the human occipital-parietal cortex. This activity could be originated by the pyramidal neurons in layer IV. It has been shown that alpha rhythms seem to be related to the focus of one's attention: external focus on visual tasks diminishes alpha activity while internal focus as in heavy working memory tasks show an increase in alpha magnitudes. This is contrary to gamma wave oscillatory frequencies which emerge in selective focus tasks. Thalamocortical dysrhythmia Thalamocortical dysrhythmia (TCD) is a proposed explanation for certain cognitive disorders. It occurs upon the disruption of normal gamma-band electrical activity between the cortex and thalamic neurons during awakened, conscious states. This disorder is associated with diseases and conditions such as neuropathic pain, tinnitus, and Parkinson's disease and is characterized by the presence of unusually low-frequency resonance in the thalamocortical system. TCD is associated with disruption of many brain functions including cognition, sensory perception, and motor control and occurs when thalamocortical neurons become inappropriately hyperpolarized, allowing T-type calcium channels to activate and the oscillatory properties of the thalamocortical neurons to change. A repeated burst of action potentials occurs at lower frequencies in the 4–10 Hz range. These bursts can be sustained by inhibition from the thalamic reticular nucleus and may cause an activation of cortical regions that are normally inhibited