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Optogenetic examination of the role of feedback on visual processing and perception

Final Report Summary - NEUROOPTOGEN (Optogenetic examination of the role of feedback on visual processing and perception)

This ERC starting grant funded 5-year project began in July 2012. The goal was to advance our understanding of the role of feedback on visual processing and perception.

What we see is influenced by behavioral and sensory context, learning and prior experience. Although this general insight dates back over 100 years to Helmholtz’s theory of visual perception as ‘subconscious inference’ it is still poorly understood how this influence is achieved at a neuronal level. While it is increasingly recognized that in part it occurs as early as the initial stage of the visual processing hierarchy in the cerebral cortex, visual area V1, the mechanisms are largely unclear. Feedback connections, i.e. projections from areas higher in the processing hierarchy back to areas lower in the processing hierarchy are thought to play an important role to such contextual modulation, likely by interacting with a diverse population of inhibitory interneurons and neuromodulatory influences.
We used several lines of attack towards the goals of this project, including multichannel electrophysiology, causal manipulations, advanced psychophysical techniques and statistical modeling. Using recent techniques to selectively manipulate neuronal activity with light (optogenetics) we identified differential roles on context modulated visual processing played by two types of interneurons in the primary visual cortex. Contextual influences on visual processing such as spatial attention typically modulate the gain of neuronal responses to visual stimuli. Our results point to a novel mechanism for such gain modulation. To link these neuronal mechanisms to visual perception we explored the visual responses during visually guided behavior in animals trained to perform visual discrimination tasks. Activity in sensory neurons that correlates with an animal’s perceptual decision is widely believed to provide insights into how the brain uses information from sensory neurons. Recent theoretical work developed simple predictions to differentiate decoding schemes, and found support for optimal linear read-out of early sensory populations. Even for tasks in a fixed task context that are optimized to isolate feed-forward sensory processing, signatures of the neural activity of the task-relevant visual neurons are incompatible with optimal linear read-out. Instead, they show signatures of feed-back modulation including their temporal dynamics and profile across cortical layers. These suggest that the task relevant early visual signals are modulated by top-down feedback reflecting the animal’s moment-by-moment beliefs. However, contrasting with important theoretical proposals on the role of feedback our results also support the view that some beliefs based on behavioral context, even when these beliefs have demonstrable and substantial behavioral consequences, are, surprisingly, not fed back onto task-relevant early sensory neurons.