Contextual modulation of stimulus processing in the mouse visual thalamus

Crombie, Davide (2024): Contextual modulation of stimulus processing in the mouse visual thalamus. Dissertation, LMU München: Graduate School of Systemic Neurosciences (GSN)

Abstract

In order to appropriately utilize visual signals from the environment, the brain needs to place this information into a context. Contexts are multifaceted, and can refer to the overall nature of the visual landscape, as well as the goals and behaviours of the animal. The behavioural context can determine the type of visual signals that the animal is likely to encounter, and, simultaneously, the type of behaviours the animal is likely to perform on a moment-to-moment basis. These behaviours may be used to explore the visual scene, and can themselves generate certain types of visual signals. It is therefore unsurprising that visual systems in the brain account for context at the earliest stages of sensory processing. One of these initial processing stages in the mammalian brain is the visual thalamus. This dissertation presents the results of three investigations on the contextual modulation of neural activity in the dorsal lateral geniculate nucleus (dLGN) of the thalamus. To conclude, novel results involving the ventral lateral geniculate nucleus of the thalamus in setting a behavioural context are synthesized. Taken together, it is hoped that these works will contribute to an acknowledgement of the diversity of contextual modulations in vision, and lead to a better understanding of how the brain constructs its own sensory world. Until the last decade, investigations of sensory processing in the mammalian brain were typically carried out in anaesthetized animals using simple artificial stimuli. However, brains have evolved process complex natural environments while the animal is awake and behaving. These factors must therefore be taken into account when investigating the function of a certain piece of neural circuitry. The research presented in Chapter 2 examines the role of feedback from the visual cortex to the dLGN in awake animals viewing naturalistic video stimuli. The work reveals a robust effect of this feedback on dLGN neurons in the presence of naturalistic stimulation. Furthermore, it was found that cortical feedback produces effects that are similar to locomotion, but that the two influences are statistically independent, suggesting that locomotion operates on dLGN neurons through a different mechanism. Researchers use behavioural signals like locomotion to infer that the animal is in a behavioural state termed "arousal". Another external marker for arousal is pupil dilation. Previous work examined the effects of arousal on sensory systems by partitioning neural activity into two mutually exclusive groupings based on the level of arousal. The work in Chapter 3, however, extends the notion of behavioural state by showing relationships between dLGN activity and pupil dilation over multiple, nested temporal scales. Some of these modulations, especially at faster timescales, were related to specific behaviours such as locomotion and eye movement. Overall, despite the presence of a robust neural activity pattern characterizing these modulations across temporal scales, there was a diversity in the strength with which individual neurons coupled to a certain temporal component. This pattern of results indicates that, rather than operating in two distinct modes, dLGN neurons are under the continuous influence of a multitude of arousal-related factors. Modelling plays an important role in neuroscience, allowing the translation between conceptual and quantitative understanding. In Chapter 4, a descriptive model is introduced to assess the additive contributions of locomotion, pupil size, and cortical feedback to the stimulus responses of individual dLGN neurons. This work not only utilizes a novel efficient and robust method for assessing responses to naturalistic visual stimuli, but also provides a unified quantitative account of the various modulatory influences acting on dLGN neurons. The enclosed research primarily provides descriptive accounts of contextual modulation of activity in the early visual system of mammals. Where it is limited is in the discussion of mechanisms and functional roles. Chapter 5 reviews two findings from other research groups uncovering a novel mechanism, involving the ventral lateral geniculate nucleus (vLGN) of the thalamus, by which mammals might control behavioural responses to visual stimuli. These novel findings highlight the usefulness of asking "how" and "why" in neurobiological research. Thus, in the final chapter, the potential mechanisms underlying dLGN modulations uncovered in Chapters 2 through 4 are discussed, and various proposals for their teleological functions in vision are made.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 570 Biowissenschaften, Biologie
• Fakultäten: Graduate School of Systemic Neurosciences (GSN)
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 11. Juli 2024
• 1. Bericht­erstatter:in: Busse, Laura