What does an empty seat represent in early Buddhist iconography?

In early aniconic Buddhist art (such as at Sanchi, Bharhut, and Amaravati), the Buddha was not represented in human form; instead, symbols were used to depict events in his life. An empty seat (vajrasana or asana) under the Bodhi tree represents the Buddha's meditation and his attainment of enlightenment at Bodh Gaya. Other symbols include the wheel (Dharmachakra) for the First Sermon, the stupa for the Mahaparinibbana, and a horse without a rider for the Great Renunciation (Mahabhinishkramana). The empty seat thus specifically represents the meditative presence of the Buddha.