Our brain constantly receives sensory cues from the environment and uses this information to construct a representation of reality. How we perceived the world, however, does not only rely on these sensory cues, but is also influenced by internal factors such as our past experiences or current emotional state. Our brain is thought to provide this “internal” information through so-called “top-down” input, which can affect sensory information processing at very early stages. How top-down inputs exactly modulate sensory processing remains for the most part unknown.
Fascinated by this basic yet complex neuroscientific question, I joined the research group of Markus Rothermel in April 2016 first as a master and now as a PhD student. In order to investigate how early sensory processing is shaped by top-down input, I am using the mouse’s olfactory system as a model. The olfactory bulb (OB), the first station of synaptic processing of odor information, is not only easily accessible experimentally, but also receives diverse and broad top-down input from both neuromodulatory and cortical sources.
My experiments focus on projections from the anterior olfactory nucleus (AON), an important component of the olfactory cortex. The AON represents the largest source of cortical input and is the only top-down system that targets both the ipsi- and contralateral OB. These characteristics and its extensive connections within and outside the olfactory system, make it an ideal candidate to study early modulation of olfactory information by top-down systems.
Shining light on the role of AON projections to the OB
In my first experiments I stimulated AON neurons electrically, while simultaneously conducting extracellular recordings of mitral/tufted (M/T) cells activity, the main OB output neurons. Electrical stimulation was also performed while visualizing M/T cells activity using widefield imaging. While these experiments give us a first idea of how the AON affects bulbar output, electrical stimulation lacks selectivity. This selectivity was achieved by specifically targeting AON axonal projections innervating the OB using a transgenic mouse line. Moreover, the use of optogenetics not only allows a selective activation but also inhibition of a predefined neuronal population. In the future, I am planning to conduct behavioral experiments, in order to investigate in which behavioral context these top-down systems are intrinsically activated. Furthermore, I am establishing a method that will allow us to combine optogenetic activation of top-down systems and 2-Photon imaging.
Taken together, my research aims to broaden our understanding of how the brain actively and selectively filters sensory information in order to produce adequate behavioral responses. In the long run, these data will hopefully open up new treatment options for sensory disorders that are characterized by abnormal reactions to particular sensory stimuli (e.g. autism spectrum disorder).