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. 2004 Apr 13:5:13.
doi: 10.1186/1471-2202-5-13.

Memory-matches evoke human gamma-responses

Christoph S Herrmann  1 , Daniel Lenz, Stefanie Junge, Niko A Busch, Burkhard Maess
Affiliations

Memory-matches evoke human gamma-responses

Christoph S Herrmann et al. BMC Neurosci. .

Abstract

Background: Human brain activity in the gamma frequency range has been shown to be a correlate of numerous cognitive functions like attention, perception and memory access. More specifically, gamma activity has been found to be enhanced when stimuli are stored in or match with short-term memory (STM). We tested the hypothesis that gamma activity is also evoked when stimuli match representations in long-term-memory (LTM). EEG was recorded from 13 subjects performing a choice reaction task. Visual stimuli were either known real-world objects with a memory representation or novel configurations never seen before.

Results: All stimuli evoked an early gamma response which was maximal over occipital electrodes. This evoked gamma activity was significantly larger for items that matched memory templates.

Conclusions: Therefore, we argue that gamma activity results from the feedback from memory into perception systems. This assumption seems to be true for STM as well as LTM.

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Figures

Figure 1
Figure 1
Examples of the stimuli used in the experiment. Two objects with a representation in LTM (red) and the corresponding non-objects which are composed of the same parts but have no such representation in LTM (blue). Subjects were to differentiate round (top row) from edgy figures (bottom row) in order to keep the results free of confounds through their responses.
Figure 2
Figure 2
Average time-frequency representation of the EEG activity at electrode O2 (averaged across all 13 subjects). A clear peak of evoked gamma activity is visible shortly before 100 ms in the frequency range of 30 to 40 Hz. This peak is significantly stronger for objects (top) than non-objects (bottom).
Figure 3
Figure 3
Individual time-frequency representations of the EEG activity at electrode O2 for two subjects. The plots reveal different amplitudes and frequencies of the evoked gamma response (subject 1: 35 Hz, subject 2: 40 Hz). Both subjects show a stronger activation for objects than non-objects.
Figure 4
Figure 4
The evoked peak of gamma activity shows a clear difference between known objects (red) and non-objects (blue).
Figure 5
Figure 5
The difference between objects and non-objects is also clearly visible in the topographic maps. The bilateral occipital distribution indicates that the gamma activity results from extrastriate visual cortices.
See this image and copyright information in PMC

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