Circuit mechanisms underlying simple forms of visual memory

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Animals must navigate a constantly changing environment and learn which aspects of that environment are important and which are not. To allow this, the brain must adjust how visual information is processed to suppress neural responses to features that are of no revelance to the animal while amplifying information that is useful, for instance indicating food or danger. Such "plasticity" is a general property of the brain and can occur on different time-scales. Changes over seconds, such as adjusting visual sensitivity to a darker room, is called adaptation. Other processes can occur over days. Habituation, for instance, can inhibit neural responses to a repeated stimulus when it does not provide useful information. But if the same stimulus is paired with food or drink, habituation is blocked and responses can instead be enhanced. This project aims to understand how fast adaptation and these simple forms of long-term memory are linked and whether the same neural circuits can control both. We will do this by studying how visual stimuli generate neural responses in the cerebral cortex using awake mice and specialized optical microscopes that allow the electrical activity of large numbers of neurons to be observed in real-time as changes in light emission from each neuron. An important general mechanism of plasticity in the brain is to change the strength of the connections between neurons. These neurons can be either excitatory or inhibitory and it is changes in the balance between the excitation and inhibition that a neuron receives that determines whether its response is increased or decreased. Our particular focus will be on how inhibitory neurons of different types are connected into different circuits that determine the responses of the excitatory neurons. We have recently obtained new information about how inhibitory neurons can increase or decrease responses on time-scales of seconds and our general hypothesis is that the connections within these same circuits are either strengthed or weakened to establish simple forms of visual memory such as habituation or Pavlovian learning. We will test ideas about the underlying circuits by observing the activity of different components of these circuits (that is to say different types of neurons) as well as artificially increasing or decreasing their activity on a second-to-second basis. The information from this project will help us understand how neurons in the brain adjust sensory processing on different time-scales to allow a mammal to operate in a changing world.