The Neuroanatomical Organization of Intrinsic Brain Activity Measured by fMRI in the Human Visual Cortex

Abstract

The existence of intrinsic neuronal activity has been demonstrated at many scales using a variety of methodological approaches, yet its biological relevance is only beginning to be understood. One way to study intrinsic neuronal activity in humans is by using resting-state (RS) functional magnetic resonance imaging (fMRI). In this thesis, I investigated the neuroanatomical organization and the biological relevance of RS-fMRI activity recorded from the human visual cortex. Using novel analysis methods, I examined the spatial and temporal organization of intrinsic fluctuations in fMRI activity across striate (V1) and extrastriate visual cortex (V2 and V3). First, I asked whether cortico-cortical neuronal interactions between different cortical visual field maps could be characterized using RS-fMRI. I show that it is possible to map, based on RS-fMRI recordings, the cortico-cortical neuronal interactions between V1, V2 and V3. My results corroborate the view that intrinsic neuronal activity reflects underlying neuroanatomical organization. Next, I asked whether intrinsic fMRI activity was comparable to that evoked by visual field mapping (VFM) stimulation. By detecting patterns of spatially-localized synchronized activity in fMRI activity, I found spatial patterns of synchronized fMRI activity that were similar in RS and VFM. However, for the activity obtained during stimulation, synchronization was spatially more extensive, reflecting the stimulus driven interactions between neighboring locations in the visual cortex. The resemblance of the synchronization patterns derived from RS and VFM suggest that their underlying causes share common organizational principles. Building on these first two experiments, I also examined the feasibility of reproducing the findings of the previous two experimental chapters using data acquired with a 3T rather than a high-resolution 7T scanner. Despite the lower resolution and signal-to-noise ratio of the 3T data, I find that the results obtained with it are in agreement to those obtained previously with 7T data. I also made suggestions for how the quality of measurements at 3T could be further improved. Finally, I asked whether the propagation of fMRI activity within and between V1, V2 and V3 relates to structured neuronal activity. To explore this question, I implemented a descriptive model aimed at disentangling various contribu- tions to measured fMRI activity. Applying this approach to 7T fMRI data re- vealed changes in cortical excitability and directed interactions in RS and VFM. Moreover, my results pointed to a task-dependent reconfiguration of local, feed-forward and feedback interactions within and across V1, V2 and V3. Here, my main contribution is that I show that it is possible to separate the contribution of local cortical activity and directed influences at the scale of cortical visual field maps. I conclude my thesis by interpreting and discussing my findings in the light of various theoretical perspectives. I stress that —in addition to externally triggered responses— there is a constant stream of internal processes that we should take into account as well. Drawing from Shannon’s information theory, I propose that, for a signal to be truly informative, it has to make a difference to a receiver —in this case the brain. In most contemporary neuroimaging studies, brain signals are interpreted from the perspective of the experimenter as receiver. How- ever, the key question is how those signals are actually received and used by the rest of the brain. Retinotopic maps are a prime example of this. In summary, my studies show that RS-fMRI recordings can reveal a remarkable amount of detail regarding the functional neuroanatomical organization of the human visual cortex. Furthermore, my findings justify and facilitate the comparison of RS and stimulus-evoked activity. While I have focused on RS and VFM activity, the methods presented in this thesis can also be applied to study task-dependent changes in a variety of experimental and behavioral con- ditions, both in health and disease. From this, I conclude that the brain is not only concerned with the demands imposed by the environment but also with internally generated dynamics.