Looking Beyond the Top-1: Transformers Determine Top Tokens in Order

Uncovering the inner mechanisms of Transformer models offers insights into how they process and represent information. In this work, we analyze the computation performed by Transformers in the layers after the top-1 prediction remains fixed, known as the "saturation event". We expand this concept to top-k tokens, demonstrating that similar saturation events occur across language, vision, and speech models. We find that these events occur in order of the corresponding tokens' ranking, i.e., the model first decides on the top ranking token, then the second highest ranking token, and so on. This phenomenon seems intrinsic to the Transformer architecture, occurring across different variants, and even in untrained Transformers. We propose that these events reflect task transitions, where determining each token corresponds to a discrete task. We show that it is possible to predict the current task from hidden layer embedding, and demonstrate that we can cause the model to switch to the next task via intervention. Leveraging our findings, we introduce a tokenlevel early-exit strategy, surpassing existing methods in balancing performance and efficiency and show how to exploit saturation events for better language modeling.