۱- Introduction

۲- Materials and methods

۳- Results

۴- Discussion

۵- Conclusion

References

Abstract

Gamma rhythms (~20–۷۰ ​Hz) are abnormal in mental disorders such as autism and schizophrenia in humans, and Alzheimer’s disease (AD) models in rodents. However, the effect of normal aging on these oscillations is unknown, especially for elderly subjects in whom AD is most prevalent. In a first large-scale (236 subjects; 104 females) electroencephalogram (EEG) study on gamma oscillations in elderly subjects (aged 50–۸۸ years), we presented full-screen visual Cartesian gratings that induced two distinct gamma oscillations (slow: 20–۳۴ ​Hz and fast: 36–۶۶ ​Hz). Power decreased with age for gamma, but not alpha (8–۱۲ ​Hz). Reduction was more salient for fast gamma than slow. Center frequency also decreased with age for both gamma rhythms. The results were independent of microsaccades, pupillary reactivity to stimulus, and variations in power spectral density with age. Steady-state visual evoked potentials (SSVEPs) at 32 ​Hz also reduced with age. These results are crucial for developing gamma/SSVEP-based biomarkers of cognitive decline in elderly.

Introduction

Gamma rhythms are narrow-band oscillations often observed in the electrical activity of the brain, with center frequency occupying ~20-70 Hz frequency range. Previous studies have proposed involvement of these rhythms in certain higher cognitive functions like feature binding (Gray et al., 1989), attention (Chalk et al., 2010; Gregoriou et al., 2009) and working memory (Pesaran et al., 2002). Further, some studies have shown that these rhythms may be abnormal in neuropsychiatric disorders such as schizophrenia (Hirano et al., 2015; Tada et al., 2014), autism (An et al., 2018; Uhlhaas and Singer, 2007; Wilson et al., 2007) and Alzheimer’s disease (Mably and Colgin, 2018; AD; Palop and Mucke, 2016). Gamma rhythms can be induced in the occipital areas by presenting appropriate visual stimuli such as bars and gratings, and their magnitude and center frequency critically depend on the properties of the stimulus such as contrast, size, orientation, spatial frequency and drift rate (Jia et al., 2013; Murty et al., 2018; Ray and Maunsell, 2015). Recently, we showed that large (full-screen) gratings induce two distinct narrow-band gamma oscillations in local field potentials (LFP) in macaque area V1 and posterior electrodes in human EEG, which we termed slow (~20-40 Hz) and fast (~40-70 Hz) gamma (Murty et al., 2018).