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Trial Lecture – time and place
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Adjudication committee
- First opponent: Associate Professor Hanne Stensola, University of Agder
- Second opponent: Associate Professor Vincent Bonin, KU Leuven, NERF Institute
- Third member and chair of the evaluation committee: Associate Professor Jørgen Afseth Sugar, University of Oslo
Chair of the Defence
Associate Professor Rune Enger, University of Oslo
Principal Supervisor
Associate Professor Koen Gerard Alois Vervaeke, University of Oslo
Summary
The ability to navigate the environment is essential for all mammals. In the rodent brain, this ability is supported by place cells in hippocampus, neurons that are active at an animal’s specific position and are thought to be the basis of spatial navigation and memory. However, how position tuning is generated is largely unknown, partially due to difficulties in studying deep brain circuits like hippocampus. The discovery that position tuning is also present in superficial areas such as retrosplenial cortex (RSC) provides an easier opportunity to study how position tuning comes about. In my thesis, I characterized the long-range projections of areas involved in spatial navigation, such as entorhinal cortex (EC) and RSC. Furthermore, we anatomically defined another area involved in spatial navigation, posterior parietal cortex (PPC). In the first study, I characterized the extrinsic projections of the principal neurons in layer II of EC that express calbindin (CB+), with the combined use of retrograde tracers and immunohistochemistry. We showed that EC CB+ layer II neurons mainly give rise to an excitatory projection, of which half of the neurons project intrinsically and commissurally. We also showed that the projections of medial and lateral subdivisions of EC (MEC, LEC) differ considerably: MEC CB+ neurons target mainly CA1 of hippocampus, whereas LEC CB+ neurons project to various forebrain targets. In the second study in this thesis, we provided an anatomical definition of PPC relative to extrastriate areas, as PPC of the mouse has been poorly defined in the literature and this can confound the interpretation of functional studies. We showed that PPC in the mouse brain is, similarly to the rat brain, subdivided into three anatomical areas, medial, lateral and posterior subdivisions (mPPC, lPPC, PtP), and that each of these subdivisions partially overlaps with anterior portions of extrastriate areas. In the third study in this thesis, I characterized the tuning properties of the main long-range inputs to RSC, an area that contains position tuned neurons, while mice performed a spatial task using two-photon in vivo axonal calcium imaging. I found that many of the inputs examined in my experiments conveyed position information to RSC, with substantial differences between areas. Axons from the secondary motor cortex (M2) contributed the most spatial information by far. Posterior parietal- (PPC), anterior cingulate- (ACC), orbitofrontal (OFC) cortex and thalamus also conveyed position information but substantially less. Surprisingly, primary (V1) and medial subdivision of secondary visual cortex (V2M) contributed negligibly. Altogether, these data show that spatial information is conveyed between a vast network of interconnected cortical and subcortical circuits. In summary, I have: (i) characterized the projections of CB+ neurons in EC layer II; (ii) defined the anatomy of PPC in the mouse brain; (iii) described the tuning properties of the principal inputs to RSC while mice performed a spatial task.
Additional information
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