Observational learning, defined as learning by observing others' actions, is a fundamental behavior of human and animals and is impaired in patients with autism spectrum disorders (ASDs). How the neurons and circuits in the brain accomplish this remarkable function is unknown. In observational learning, the observer needs to actively understand and store others' actions from his own personnel perspective, and then utilize the information in later self-actions. This proposal studies the role of hippocampal neurons in this process of action understanding and how they mediate the learning effect of observation. We will test a hypothesis that observing others' actions triggers the hippocampal neuronal activity patterns that encode the observer's own execution of the same actions. According to the hypothesis, observation leads to learning by facilitating and strengthening the activity patterns encoding self-actions. Our main approach is to simultaneously record a large number of neurons in freely moving rats performing observational tasks. We will determine what activity patterns occur during observation and how they compare with the patterns during the observer's own actions. We will then determine whether the observation-induced activity patterns enhance the observer's learning behavior and whether the disruption of these patterns reduces the enhancement. In addition, we will determine whether the observation-induced neural activity patterns and their learning effects are impaired in a transgenic rat model with social dysfunction. The outcome of this proposal may reveal a novel neural circuit mechanism of observational learning and generate insights into how observational learning is impaired in ASDs at the neural circuit level in vivo.