High-frequency oscillations (20 to 120 Hz) and their role in visual processing

Oscillatory firing of neurons in response to visual stimuli has been observ ed to occur with different frequencies at multiple levels of the visual sys tem. In the cat retina, oscillatory firing patterns occur with frequencies in the range of 60 to 120 Hz (Omega-oscillations). These millisecond-precis e temporal patterns are transmitted reliably to the cortex and may provide a feed-forward mechanism of response synchronization. In the cortex, visual responses often show oscillatory patterning with frequencies between 20 an d 60 Hz (gamma-oscillations), which are not phase locked to the stimulus on set and therefore do not show up in regularly averaged evoked potentials. g amma-Oscillatory responses synchronize with millisecond precision over long distances and are mediated by the reciprocal corticocortical connectivity. Modulatory systems like the ascending reticular activating system facilita te synchronization and increase the strength of gamma-oscillations. During states of such functional cortical activation, the dominant frequency of th e EEG is shifted from lower frequencies in the delta-/Theta-range to higher frequencies in the gamma-range. Therefore, functional states indicate diff erent degrees of temporal precision with which large neuronal populations i nteract. Response synchronization also depends on relations of global stimu lus features. This suggests that millisecond-precise neuronal interactions serve as a fundamental mechanism for visual information processing.