Of sounds, photons and maps: in vivo optical characterisation of the auditory thalamocortical system

Thesis

As information ascends up the sensory streams, the maps of receptor surface might be faithfully relayed from one brain structure to another, degraded or lost altogether. Such transformations can inform about the type of circuit computation carried out in each region. In vivo high-resolution imaging methods, like two-photon microscopy, are useful for characterising the functional architecture of neuronal circuits down to the micro-scale. To better understand the rules governing thalamocortical connectivity and the origin of cortical maps, I used in vivo two-photon calcium imaging to characterise the properties of thalamic axons innervating different layers and subfields of mouse auditory cortex. Although topographically organised at a global level, I found the frequency selectivity of individual thalamocortical axons to be surprisingly heterogeneous, even in the middle layers (L3b/4) of the primary cortical areas where the thalamic input is dominated by the lemniscal projection. Subsequently, I employed a dual-colour imaging approach to explore the spectral transformations taking place between the thalamocortical projection and granular and supragranular layers of auditory cortex. Some differences in local spectral properties between neurons in L4 and L2/3 were observed in awake, passively listening mice, confirming previous observations on the anaesthetised preparation. Finally, I present a demonstration of a novel fibre-based high-resolution fluorescence imaging method to optically investigate neuronal circuits in deep brain regions, with minimal invasiveness. In short, my work provides some new insights on the functional micro-organisation of the auditory thalamocortical system and constitutes a classical example of the power of optical methods for the study of neuronal circuits in vivo.

Authors: Vasquez Lopez, SA

• DOI: 10.5287/ora-z5x5wvo5p
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English