Oscillatory gamma activity in humans: a possible role for object representation.

The coherent representation of an object has been suggested to be established by the synchronization in the gamma range (20-100 Hz) of a distributed neural network. So-called '40-Hz' activity in humans could reflect such a mechanism. We have presented here experimental evidence supporting this hypothesis, both in the visual and auditory modalities. However, different types of gamma activity should be distinguished, mainly the evoked 40-Hz response and the induced gamma activities. Only induced gamma activities seem to be related to coherent object representations. In addition, their topography depends on sensory modality and task, which is in line with the idea that they reflect the oscillatory synchronization of task-dependent networks. They can also be functionally and topographically distinguished from the classical evoked potentials and from the alpha rhythm. It was also proposed that the functional role of gamma oscillations is not restricted to object representation established through bottom-up mechanisms of feature binding, but also extends to the cases of internally driven representations and to the maintenance of information in memory.