depolarization by these many different inputs. TC neurons exhibit gamma oscillation when depolarized to greater than -45 mV, and the frequency of oscillation is related to the degree of depolarization. This oscillation is caused by the activation of leaky P/Q-type calcium channels found in the dendrites of the cells. Because of the leaky channel properties, spontaneous, inherent oscillation can also occur independent of any rhythmic input as well, though the ramifications of this capability are not entirely known and may add nothing but background noise to the thalamocortical loop. The cortex provides feedback to the thalamus through links to dendrites of these thalamocortical cells and serves as the source of constant thalamic oscillation. Oscillatory behavior depends on the conscious/unconscious state of the brain. During active thinking, electroencephalography reveals a strong appearance of gamma range oscillation from around 20–50 Hz. Thalamocortical circuits Thalamic cells synapse on apical dendrites of pyramidal cells in the cortex. These pyramidal cells reciprocally synapse back on thalamic neurons. Each loop is self-contained and modulated by sensory input. Inhibitory interneurons both in the cortex and the reticular nucleus of the thalamus regulate circuit activity. Inputs to thalamocortical system The thalamus gates information into thalamocortical loops based on the source of the signal. There are two major sources for TC input: sensory perception and information about the current mental state. Cortical structures of external events or sensory data are referred to as specific inputs and enter into the ventrobasal thalamus at the "specific" thalamic nuclei. These neurons project to layer IV of the cortex. Similarly, nonspecific inputs provide context from internal state of the brain and enter into intralaminar "non-specific" nuclei in the centrolateral thalamus with axons in layers I and VI. Both types of TC neurons synapse on the pyramidal cortical cells which are thought to integrate the signals. In this way, outside sensory information is introduced into the current context of cognition. Resonant columns Studies involving manipulation of slices of visual cortex have shown that thalamocortical resonance from stimulated TCs induces the formation of coherent regions of similar electrical activity through vertical layers of the cortex. In essence this means that groupings of activated cortical cells form from the activation of these thalamic cells. These regions are columnar and are physically separated from adjacent resonance columns by areas of inhibited cortex between them. It is not known what the exact function of these columns is, although