Psycholinguists refer to two types of process involved in comprehension. These processes are not independent but interact with each other:

• Bottom-up processes refer to speech perception and word recognition, ie what happens around Levels 1 and 2 when we identify units in the stream of sound and try to access their lexical meaning. These processes provide the 'data' for comprehension. (Rost, 2002: 96)
  
  
• Top-down processes involve interpretation of the 'data', based on our knowledge of the world, for example, when we predict the words we think we will hear from our knowledge of context, or interpret a statement based on our social expectations.

The terms 'bottom-up' and 'top-down' imply dynamic processes. In comprehension, we are constantly moving from the data we perceive to finding an interpretation for it, while simultaneously using our interpretations to predict data we expect to meet.

In second language learning, top-down strategies, such as guesswork from background knowledge, can be very useful since they can compensate for restricted linguistic knowledge. However, they are insufficient on their own for effective comprehension.

Bottom-up skills are also inadequate on their own since they may prevent the listener from keeping up with the stream of speech. We may need to miss out some of the bottom-up processes in order to process speech quickly in real time, particularly under difficult conditions.

To compensate we can rely on top-down processes of guess-work and inference. However, fluent and effective listening still requires us to monitor the bottom levels in the background. This is a particular difficulty for second language learners who, by definition, require longer to process the incoming sounds. Learning to combine the two types of processing is a real challenge for learners.