Thesis defense Eelke Spaak, (Donders Series 177)
9 July 2015
Promotor: Prof.dr. O. Jensen, copromotor: dr. F. de Lange
On the role of alpha oscillations in structuring neural information processing
In this thesis, I have investigated the role that alpha oscillations in the visual system play in structuring neural information processing. Modulations of alpha activity are associated with the direction of visuospatial attention. On the basis of this, it has been suggested that alpha fulfils an important role in gating information within and between different regions of the cortical hierarchy. I have attempted to shed light on several aspects of this hypothesis. First, I demonstrate that alpha activity can be externally induced (i.e., entrained), that this entrainment follows a similar pattern to endogenous alpha oscillations, and that externally entrained alpha has consequences for visual perception. This demonstrates that alpha activity is causally involved in processing visual information, and not an epiphenomenon. Second, I show that one way in which alpha might modulate processing in visual regions is through an interaction with the cortical gamma rhythm. This interaction is present on both a slow timescale (amplitude correlation) and a much faster one (phase-amplitude coupling). Third, from hippocampal theta and gamma oscillations, for which we know more about the underlying physiology than about cortical alpha activity, I derive a biophysical model of the coupling between low-frequency oscillations (like theta or alpha) and higher-frequency activity (gamma). Using this computational model, I show that fast-spiking interneurons are likely responsible for such a cross-frequency interaction. In the final two empirical chapters, I demonstrate that spatially specific alpha modulation is not limited to explicitly cued covert spatial attention, but is also present in more ecologically valid conditions. Thus, fourth, I show that the modulation of visual alpha activity is one of the mechanisms by which the brain prepares itself for upcoming stimuli through which it needs to conduct a visual search. Fifth, and finally, I show that spatially specific alpha activity also occurs during free visual exploration of natural scenes. To conclude, in this thesis, I have built upon the fruitful middle ground that neural oscillations occupy between markers of the cognitive on the one hand, and physiological phenomenon on the other, to add to our understanding of the mechanistic basis of cognition.