RODATTN · Mechanisms of attentional modulation of neural responses in visual cortex of mice

Mechanisms of attentional modulation of neural responses in visual cortex of mice

This project was aimed at understanding certain fundamental questions about how information is represented in the brain. In particular, we wanted to study how the visual cortex in mice is activated by specific visual stimuli, and critically, how does that activation change over the course of learning, that is, as the animal learns the relvance of one particular stimulus. This is a very difficult question to address because it requires measuring the activity of the same population of neurons over many days, as an animal is awake and actively learning new associations. To perform such measurements we needed to first develop the tools for this. The solution was an elaborate one, which involved holding a mouse in place using an implant on its head, while it ran in a virtual-reality environment where it was given the illusion of running through a corridor as it ran on a wheel kept under its feet. We previously had to inject calcium sensitive dye into the brain of the mouse and then implant a small glass window on the brain to allow direct viewing of the brain surface. Then while the mouse was running in the virtual reality, we positioned a laser-scanning microscope on top of the mouse's head, and were able to observe the activity of neurons through the implanted window. Once we developed the ability to record from the same population of neurons over multiple days, we developed a simple behavioural task in which the mouse learns to associate one stimulus with a reward and another with absence of reward, and these stimuli were presented to them in the virtual reality. Mice learnt this task within a week, and as they were learning we recorded the activity of hundreds of neurons simultaneously. We were able to complete an entire set of experiments and on analysing the data have found some very interesting results. Most strikingly, there is a large increase in the fraction of cells that represent the rewarded stimulus, and the population of neurons encode the information about the stimulus in a way such that as the animal learns the relevance of the stimulus, there a progressive enhancement in the amount of information encoded about the rewarded stimulus. Figure 3. Neuronal population information about the rewarded stimulus increases as the animal learns the task over a few days We expect to develop these findings further by studying other ways in which we can modulate the attentional context while presenting the same visual stimulus. For this we have made the mouse learn to switch attention from a visual task to an olfactory task while we imaged its brain. We expect to analyse that data soon. The results of that experiment will be crucial in understanding the nature in which the brain switches attention from one modality to another. These results and the expected results together throw light on some of the most important questions about how the brain works, such as the neural dynamics underlying memory and attention. This has been possible by bringing together some of the most cutting edge technologies together and will allow us to further our understanding of the nature of brain function and disease.

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