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

Abstract

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.

Introduction

Synchronized oscillatory activity in the gamma range (30–100 Hz) could play a major role in linking the different areas involved in the same object representation into a unified, coherent percept. Indeed, the distributed nature of the network coding for a given object in a particular task is now well documented, in particular by many neuroimaging studies (for review see Courtney et al., 1997). Following the theoretical proposal from Milner, 1974, von der Malsburg and Schneider, 1986, the neural structures participating in this network might be dynamically linked by oscillatory synchronous firing. Over the last decade, this hypothesis has been supported by a growing body of experimental findings in anesthetized and awake animals (Gray et al., 1989, Kreiter and Singer, 1996; for review see Singer and Gray, 1995).

The inspection of the human EEG/MEG literature since the 1950s reveals numerous reports of gamma oscillatory activities in various sensory modalities and tasks (Galambos et al., 1981, Pantev et al., 1991, Pfurtscheller et al., 1994, among many others: Sem-Jacobsen et al., 1956, Sheer, 1989; for review see Tallon-Baudry and Bertrand, 1999). At first glance, these observations may appear rather disparate. Two major factors contribute to this impression: (1) different electrophysiological phenomena have been very often gathered under the same term ‘40-Hz activity’ in an undifferentiated manner; and (2) a common framework for interpretation is somehow lacking.

The three different types of 40-Hz activities observed in humans (namely, evoked, induced and steady-state responses) (Galambos, 1992) will be presented. Experimental evidence showing that induced gamma activity can be related to the generation of unified object representation will be given. The functional specificity of induced gamma oscillations, as compared to the evoked gamma response or classical evoked potential, will be assessed.