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Representation of visual information within the dendritic tree of pyramidal cells in primary visual cortex

Baragli, C; (2017) Representation of visual information within the dendritic tree of pyramidal cells in primary visual cortex. Doctoral thesis , UCL (University College London).

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Abstract

A fundamental aim in system neuroscience is to understand how sensory information is represented in the brain. While a wealth of studies investigated how visual stimuli are encoded in populations of neurons, little is known about how visual information is represented in a single cell at the level of the dendritic tree. This is due in part to difficulties in measuring dendritic activity within dendritic arbours, which are typically fine three dimensional structures. To overcome this, we have used a custom 3D two-photon acousto-optic lens microscope to measure visually-evoked dendritic activity in pyramidal cells in mouse primary visual cortex. Pyramidal neurons were sparsely co-labelled with the genetically-encoded calcium indicator GCaMP6f and with the red fluorophore TdTomato. Dendritic activity was monitored with GCaMP6f, and simultaneous recording of the activity-independent red TdTomato fluorescence was used to detect movement of the tissue. Random access point measurements were made at 80-120 Hz from multiple locations distributed in three dimensions, and this enabled me to monitor neuronal activity in a substantial fraction of the dendritic tree and in the soma near-simultaneously. Patterns of dendritic activity were characterized in layer 2/3 neurons and in the tuft of layer 5 pyramidal neurons during presentation of gratings in awake animals. In the tuft of layer 5 neurons, calcium transients tended to be either localized to 1-2 dendritic branches, or to occur in the full-dendritic tuft and in the soma. In contrast, in layer 2/3 neurons calcium transients were mainly local, and somatic calcium transients were rarely accompanied by global events. Visually-evoked dendritic activity was evident in layer 2/3 neurons, and I observed that dendrites tend to have similar orientation tuning curves as the soma. But several dendrites with different preferred orientations were present in all the cells imaged. Dendritic activity in layer 2/3 neurons was little affected by anaesthesia and locomotion. In contrast, apical dendrites of layer 5 neurons showed little response to visual stimuli, but they were strongly modulated by the state of the animal. Indeed, anaesthesia suppressed dendritic activity in these cells, while locomotion enhanced it compared to the resting state. These data suggest that dendrites in layer 5 and in layer 2/3 pyramidal neurons have different integration mechanisms in vivo, and that they process different types of synaptic inputs. While layer 2/3 neurons receive mainly feedforward (bottom-up) visual inputs, apical dendrites in layer 5 cells are driven primarily by top-down inputs (animal state).

Type: Thesis (Doctoral)
Title: Representation of visual information within the dendritic tree of pyramidal cells in primary visual cortex
Event: UCL (University College London)
Language: English
UCL classification: UCL > Provost and Vice Provost Offices > School of Life and Medical Sciences
UCL > Provost and Vice Provost Offices > School of Life and Medical Sciences > Faculty of Life Sciences
URI: http://discovery.ucl.ac.uk/id/eprint/1537297
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