Trends in Cognitive Sciences
Cognitive functions of gamma-band activity: memory match and utilization
Section snippets
Stimulus features
A wide variety of stimulus features have been shown to modulate gamma-band responses in animals and humans. This is to be expected because, in early sensory cortices, neuronal responses are largely determined by physical stimulus properties. Recently, it has been demonstrated that the size, eccentricity and duration of visual stimuli influence the strength of evoked and induced gamma responses in humans [11] (for a comparison of evoked and induced responses, see Box 1). Visual stimuli evoke
Attention
Several recent studies show that, in addition to bottom-up factors like the ones described above, top-down influences can also strongly modulate activity in the gamma-band. Most notably, the influence of attention has been demonstrated both in animal and in human experiments. These data show that coherent activity in the gamma-band is enhanced during attentional selection of sensory information. In cats, gamma activity has been found to accompany the attention-related P3 component in the
Integrating bottom-up with top-down processing
As discussed in the preceding sections, bottom-up and top-down factors both have an influence on synchronization of neural activity in the gamma band. We suggest that they do not just relate to these influences per se but, more prominently, to the interaction between these influences. On closer inspection, all of the tasks mentioned above contain as a crucial element the comparison of stimulus-related information with memory contents. Although also possible in modes of more automatic
Conclusions
The data summarized here demonstrate how low-level (bottom-up) as well as high-level (top-down) brain factors modulate gamma-band activity. The model we have suggested explains these effects by assuming that match and utilization processes are directly related to comparison, readout and storage of synchronized neural signals. Several predictions can be derived from this model that could be tested to validate MUM and to further elucidate the relation between oscillatory activity and memory-based
Acknowledgements
We would like to thank Niko Busch and Stefan Debener for data recording and analysis. We are obliged to Stefan Debener for critical reading and comments on the manuscript. A.E. and M.H.J.M. are supported by a grant from the Volkswagen Foundation, Hannover, Germany.
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