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On computations and their maintenance in the mouse visual system
On computations and their maintenance in the mouse visual system
The mammalian visual system is composed of several stages of stimulus feature extraction, distributed across multiple visual areas. Brain areas of the visual system are hierarchically organized from retina, to thalamus and superior colliculus, to primary visual cortex and higher visual areas. Each of these areas is uniquely adapted to perform specific computations on the visual information it receives. These computations arise through the anatomical arrangement of axons and dendrites, as well as experience dependent plasticity mechanisms, which form specific circuit motifs over the course of development. However, once these circuits are established, parts of this system remain plastic, allowing for some degree of adaptability. In this thesis, I will pose the following two questions. First, what are the connectivity rules in the visual thalamus, which result in visual channel segregation? I will focus on the convergence of eye-specific inputs to thalamic neurons as my model. Second, what is the stability of visual feature tuning in the primary visual cortex, and what are the factors that modulate their stability? Here, I will use orientation preference of layer 2/3 neurons as my model. In the first study, I evaluated the convergence of retinal ganglion cell (RGC) inputs from the two eyes onto thalamocortical (TC) neurons in the dorsal lateral geniculate nucleus (dLGN) of the mouse. The canonical view of this brain region in mammals is that it maintains the separation of distinct visual channels. This includes the separation of information from the two eyes. In the past, several conflicting reports have been published on the level of such binocular convergence in the mouse. I employed a dual colour optogenetic input mapping approach and demonstrated that the level of binocularity of TC neurons is relatively low. This is because individual TC neurons receive disproportionately stronger input from one eye compared to the other. I next tested whether limited axodendritic overlap, between RGC axons and TC neuron dendrites, could explain this low level of binocular convergence. Although the segregation of RGC projections from the two eyes into two distinct zones does result in regions where ipsilateral dominant neurons are more numerous, limited axodendritic overlap cannot explain the low level of convergence onto individual neurons. Instead, synaptic selection and refinement prevents the mixing of information from the two eyes at this level of the mouse visual system. In the second study, I investigated representational drift in the mouse primary visual cortex (V1), using chronic two-photon calcium imaging. Representational drift, the time dependent decrease in the similarity of neuronal responses to sensory stimuli, has been observed across multiple brain regions, including V1. However, so far, a specific tuning feature that undergoes such time dependent drift has not been demonstrated. Furthermore, a recent study showed that the frequency of exposure to an odour correlates with the stability of its representations in the olfactory cortex. My results demonstrate that the preferred orientation of neurons is one visual feature that undergoes time dependent representational drift. I then used cylinder lens goggles to alter the range of orientations a mouse experiences for several weeks, and found that this did not alter the drift rate of preferred orientation. Nevertheless, the distorted visual experience altered the direction of preferred orientation drift in favour of the experienced orientation, resulting in a shift of the overall distribution of preferred orientations. This suggests that ongoing representational drift may allow the visual system to adapt to changes in the statistics of the visual environment. Taken together, in this thesis I explore how both anatomy and experience shape computations and their maintenance in the mouse visual system.
mouse, visual cortex, dorsal lateral geniculate nucleus, binocular integration, representational drift
Bauer, Joel
2023
English
Universitätsbibliothek der Ludwig-Maximilians-Universität München
Bauer, Joel (2023): On computations and their maintenance in the mouse visual system. Dissertation, LMU München: Faculty of Biology
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Abstract

The mammalian visual system is composed of several stages of stimulus feature extraction, distributed across multiple visual areas. Brain areas of the visual system are hierarchically organized from retina, to thalamus and superior colliculus, to primary visual cortex and higher visual areas. Each of these areas is uniquely adapted to perform specific computations on the visual information it receives. These computations arise through the anatomical arrangement of axons and dendrites, as well as experience dependent plasticity mechanisms, which form specific circuit motifs over the course of development. However, once these circuits are established, parts of this system remain plastic, allowing for some degree of adaptability. In this thesis, I will pose the following two questions. First, what are the connectivity rules in the visual thalamus, which result in visual channel segregation? I will focus on the convergence of eye-specific inputs to thalamic neurons as my model. Second, what is the stability of visual feature tuning in the primary visual cortex, and what are the factors that modulate their stability? Here, I will use orientation preference of layer 2/3 neurons as my model. In the first study, I evaluated the convergence of retinal ganglion cell (RGC) inputs from the two eyes onto thalamocortical (TC) neurons in the dorsal lateral geniculate nucleus (dLGN) of the mouse. The canonical view of this brain region in mammals is that it maintains the separation of distinct visual channels. This includes the separation of information from the two eyes. In the past, several conflicting reports have been published on the level of such binocular convergence in the mouse. I employed a dual colour optogenetic input mapping approach and demonstrated that the level of binocularity of TC neurons is relatively low. This is because individual TC neurons receive disproportionately stronger input from one eye compared to the other. I next tested whether limited axodendritic overlap, between RGC axons and TC neuron dendrites, could explain this low level of binocular convergence. Although the segregation of RGC projections from the two eyes into two distinct zones does result in regions where ipsilateral dominant neurons are more numerous, limited axodendritic overlap cannot explain the low level of convergence onto individual neurons. Instead, synaptic selection and refinement prevents the mixing of information from the two eyes at this level of the mouse visual system. In the second study, I investigated representational drift in the mouse primary visual cortex (V1), using chronic two-photon calcium imaging. Representational drift, the time dependent decrease in the similarity of neuronal responses to sensory stimuli, has been observed across multiple brain regions, including V1. However, so far, a specific tuning feature that undergoes such time dependent drift has not been demonstrated. Furthermore, a recent study showed that the frequency of exposure to an odour correlates with the stability of its representations in the olfactory cortex. My results demonstrate that the preferred orientation of neurons is one visual feature that undergoes time dependent representational drift. I then used cylinder lens goggles to alter the range of orientations a mouse experiences for several weeks, and found that this did not alter the drift rate of preferred orientation. Nevertheless, the distorted visual experience altered the direction of preferred orientation drift in favour of the experienced orientation, resulting in a shift of the overall distribution of preferred orientations. This suggests that ongoing representational drift may allow the visual system to adapt to changes in the statistics of the visual environment. Taken together, in this thesis I explore how both anatomy and experience shape computations and their maintenance in the mouse visual system.