Donders Institute for Brain, Cognition and Behaviour
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Thesis defense Georgios Spyropoulos (Donders series 442)

26 February 2021

Promotor: prof. dr. P. Fries

Aspects of variability in gamma oscillatory activity in awake macaque visual cortex

Gamma oscillations are a prominent type of collective neuronal activity. Similar to other types of collective activity, gamma oscillations display dynamic shifts in their properties at multiple timescales: This variability can be observed as a slow drift in gamma oscillatory power and/or frequency across seconds or minutes but also as a difference in the amplitude and/or the duration of neighboring gamma cycles. This dynamic modulation of gamma strength and frequency potentially reflects the influence of, both, extrinsic and intrinsic factors on gamma dynamics. Extrinsic influences include shifts between cognitive or behavioral states, whereas intrinsic factors may reflect subtle adjustments in the circuit underlying gamma generation. My doctoral work explored some of these aspects of gamma variability in the visual cortex of awake macaques. In the first part of my doctoral studies, I and my collaborators focused on the influence of extrinsic factors on gamma variability. We found that selective attention modulates fine temporal aspects of the gamma rhythm in macaque visual cortex. The absence of attention is accompanied by theta rhythmic gamma bursts, whereas the presence of attention is characterized by a more tonic gamma rhythm. This presumably accounts for a significant portion of the observed variability in the amplitude of visual-cortical gamma oscillations. Subsequently, we studied the relationship between the amplitude and duration of individual gamma cycles. We showed that these quantities are positively correlated, but when properly computed their correlation is weaker than estimated in previous studies. This correlation is intrinsic, i.e. it is not dependent on external factors such as eye movements. This strongly suggests that this type of short timescale variability is a property of the underlying neuronal circuit. In sum, my work explored the rich phenomenology of gamma oscillations and shed some light on both external and internal causes of this phenomenology.